Whither vapor barrier?

[Poindexter]

recent quote from @woodgeek in a different thread:

"'Vapor barriers' (like poly or visqueen) are frowned upon in the lower 48, often causing more problems than they supposedly solve. You do need a air barrier (which is vapor permeable to allow drying as needed), but a single one on the outside is almost certainly all you need. "

I cannot argue with this. I have been watching some videos from the "pretty good house" crowd lately, and I am kinda on board.

Imagine it is +90dF inside my home, and -45dF outside my home. Somewhere in my insulation blanket the ambient temperature is at dewpoint, and I am going to have condensation. I know this to be true.

Now imagine my vapor barrier (6mil plastic directly under the drywall) had been installed perfectly (it wasn't) and the house didn't settle at all since 1980 (it did settle), then I would not have any mold issues inside my 1980 build. Where reality meets theory, I find black mold in my fiberglass batt in every stud cavity I have opened, and the openings in drywall and vapor barrier for my receptacles and light switches were cut by a drunk back before meth was a thing.

And there is the changing weather to consider. If I build a house (currently at the southern edge of pretty good house zone 8) in the next 5 years or so I need to accept that within the say 100-200 year lifespan of the building, cooling is likely to become more important than heating.

Certainly we may be looking at societal collapse and famine and catastrophic sealevel rise in the life of the building. If it gets bad enough in this realm I am perfectly happy to just go home to be with Jesus. But I don't want cold feet or sweaty gonads while I am waiting for Him to take me home.

What is our current best practice to allow walls to dry faster than they get wet in a mixed climate where both heating and AC are essential? For the sake of this exercise you may assume I have access to infinity cord wood for heat, and infinity PV solar for cooling and dehumidification. I would like the build to have minimal carbon footprint for construction and minimal carbon foot print for operation, but I also want to retire without a mortgage payment.

 

[woodgeek]

OK, there have been lots of computer models that look at this stuff.

The basic idea you are missing is that if the air is stagnant in the cavity, vapor motion is by diffusion, not convection. In that case, just like there is a thermal gradient in the insulation (which we can imagine to be linear) there will ALSO be a gradient in water vapor content from humid (on the inside) to dry (on the outside). This linear gradient of temperature is intuitive for folks. A linear gradient of vapor content (grams per liter, not local relative humidity) forms because of a diffusive equilibrium. If you now assume that both vapor and temperature vary linearly with position, you could then compute that at any point in the cavity the local humidity is above the dewpoint.

QED.

Many models in many climates show that the above plan is just great, in that there is no condensation in the cavity anywhere, and if the dewpoint is ever reached (in heating or cooling) it is only at the outer plane, and then only when the outdoor relative humidity is 100%.

These models can be made more complicated by adding vapor buffering (the ability of insulation to absorb H2O without becoming wet, by hydration) AND making the temps time dependent. This also works just fine.

There is an obvious problem with the above... the assumption of a stagnant air cavity. If there are air leaks, then convention can locally (near the leak) carry a lot more vapor, and some of that vapor can come in contact with cold surfaces and condense liquid water. This situation describes most older construction in the US.

IF you try (70s style) to add a poly vapor barrier, and IF you got the airsealing perfect it would work. But ofc it only works with the poly on the warm side (no good for heat and AC). But worse, if there were any gaps in the poly, you would still get condensation and the situation would be WORSE, bc that condensation couldn't dry out bc of the poly barrier. The poly barriers installed in the 70s often were not taped, and so made the situation worse.

SO, the solution is an air impermeable, vapor permeable barrier film, e.g. Tyvek, and carefully taping all the seams with approved tape (that is tested to stay adhered for decades in service). This prevents the airflow, creating a static/diffusive cavity AND still allows the formation of the water vapor gradient needed to avoid condensation (with a VERY low diffusive flux of water vapor). Since the 'wettest' part of the cavity, that is closest to local condensation is the outside plane, you just put the Tyvek there.

And that is how we build a mold free cavity in the 21st century.

If alternatively you decided to use (vapor and air) impermeable foam on the outside of your framed cavity, the inside surface of the foam would still get cold, and bc there is vapor diffusion in the cavity (but no flux through the foam), now the vapor content of the cavity air will rise to match the interior value, and you will get condensation on the interior of the foam, making water deep in the cavity, than can only slowly dry to the interior.

The foam guys then say to avoid this, you need to make the foam THICKER than the cavity insulation, and you need to make this thickness higher the colder your climate. A 'minimum safe foam thickness' to avoid mold. In cold climates, you get numbers like 6 or 8+ inches of foam, and still a big risk of condensation if the owner is a little aggressive with the indoor humidification (due to a sick family member).

Skip the foam and vapor barriers, make a vapor permeable but air sealed cavity and it will be just fine.

 

[woodgeek]

And there is the changing weather to consider. If I build a house (currently at the southern edge of pretty good house zone 8) in the next 5 years or so I need to accept that within the say 100-200 year lifespan of the building, cooling is likely to become more important than heating.

Certainly we may be looking at societal collapse and famine and catastrophic sealevel rise in the life of the building. If it gets bad enough in this realm I am perfectly happy to just go home to be with Jesus. But I don't want cold feet or sweaty gonads while I am waiting for Him to take me home.

For the purposes of argument, lets say that crazy advances in medical technology allows both of us to live for another 77 years, to 2100. Current projections for climate change indicate an increase in global average temps of 2.5°C above the 20th century mean. Since we are already 1°C above that mean temp, this is an additional 1.5°C. Let's round up and call it +3°F when we are well over 100 years old. In the boreal region, the warming will be higher, probably +6-8°F above current levels on an annual average.

I'm sorry, but +7°F is unlikely to reduce your corner of Alaska to a sun-baked arid hellscape by 2100. Hell, you won't even be able to plant palm trees or grow bananas outside! But here in PA, I doubt I will get to do that either (some of my neighbors already DO grow bananas, but they take them inside for a couple months in winter). If you are not far inland, your climate will probably, gasp, resemble that of 2023 Vancouver.

If you are worried about sea level rise, be above 50' elevation and you won't have to worry about that either.

Re Famine, we can imagine a loss of agricultural productivity in current global 'breadbaskets', and can note the fact that current arctic/subarctic regions have poor topsoil (i.e. peat or rock) so they are unlikely to be big producers.

But, there is the fact that current agricultural productivity is very low bc of 75% of the plant calories being actively cultivated being filtered through animals on factory farms. And the calories added by animal grazing on uncultivated land are low single digits (despite >30% of the earth's ice free area currently being grazed). Adding to the fact that most continents (other than Africa) are projected to reach peak population and decline well before 2100. This gives us a good amount of margin on food... just requiring some greater substitution of plant based products into our diet, which might be a good thing anyways.

Perhaps you subscribe to some 'tipping point' climate catastrophe models. I think there ARE climate tipping points re ecology, where a whole habitat collapses/burns and get replaced with a new much less diverse ecology. But for temperature, I don't believe in tipping points.

Even in the case of more dire loss of agricultural productivity bc of unknown unknowns, there is always 'air protein'. But that might be too techno-optimist for some.

 

[EbS-P]

This is coming from a southerner so it could be off. Have you researched the insulated ZIP system exterior sheeting? Seems like a no brained here. Quick look it goes up to R12. Do that really well and 2x6 or feeling new age look into engineered studs (for the thermal break).

ZIP System® Insulated R-Sheathing | Huber Engineered Woods

ZIP System® R-sheathing is the simple all-in-one structural panel with built-in exterior insulation with integrated moisture, air and thermal protection.

www.huberwood.com

I’d totally pay for the insulated zip system. Then maybe insulated siding???

Edit… yeah probably would skip the insulated siding. At -40 I’m not sure I want vinyl.

 

[peakbagger]

My head is swirling over the various approaches to vapor barrier locations. In my current and expected northern New England climate I can expect both summertime hot/high humidity outdoors in the summer and cold bone dry in the winter. I see Zip system being used by lot of contractors which is putting the vapor barrier on the outside of the shell but that is almost a guarantee that in the winter, the moisture gradient will yield a dewpoint somewhere inside the wall cavity (not good). Note, there are modeling programs that are very good at modeling moisture gradient through a wall section that will take into account climate extremes so its a design step that has to be added. Most of the ultra low energy designs seem to call for a barrier immediately inside the interior wall. Insulation folks claim that a closed cell spray job will seal the entire wall so that its acts like a monolithic vapor barrier but that assumes that all gaps are sealed and that is tough with rough framing where studs are going to end up in parallel to another stud (windows and door opening on load bearing walls will typically have headers supported by jacks on the inside of the opening with a stud on either side). Gable walls can do away with the jacks but there is still a lot of potential framing leakage. One dirty secret of spray foaming is it may look perfectly bonded to the studs when sprayed but there could be shrinkage and pullback, months and even years after it was sprayed and long after the interior walls are in place.

At one point SIPs (Structurally Insulated Panels) were the latest and greatest usually used to wrap timberframes but detailing and shortcuts in installations led to unhappy folks. Roofs seemed to have the biggest issue as the panels expanded and contracted seasonally leaving distinct seams visible in shingles. I think a lot of builders just sold the owners on standing seam on purlins to bridge the gaps. Unfortunately there is new issue where moisture is slowly making its way through the foam and collecting under the outer skin near the ridge pole and causing rot. There is an interesting video done by third party firm showing this issue and how to mitigate it with modified ridge pole venting.

The Passive home course I took was advocating building and empty shell with no interior walls, then installing poly and sheetrock in the interior before building partition walls. That works except for electrical penetrations in the exterior walls. NEC outlet and switch locations mean that there will be penetrations in the exterior walls in most cases. So one approach is build a false wall inside the sheetrocked vapor barrier exterior walls and go with shallow conduit on top of the vapor barrier wall and put in second interior wall. This lines up well with the John Seigenthaler hidden radiant wall design where Iso board is placed in the interior of zip system type wall on the interior and then tubing with reflectors facing inwards is mounted behind the finish sheetrock. A channel can be left at the base of the emitters for the conduit. (conduit is needed as the wires are too shallow in the wall). Note the not so subtle concept that the house is built with the plywood facing in and the various zip type tapes are used to seal the interior with the stud cavities facing out. This is an interesting concept except that there is potential structural loss as plywood is usually used on the exterior to bridge wall sections and corners. Arguably someone will need to review lateral bracing without these bridged corners.

Having the vapor barrier on the interior is not an issue as long as the appropriate membranes and insulation is used to allow vapor transmission while retaining thermal characteristics outside the barrier. Essentially rain is kept out at the outer wall while vapor can still go through the water barrier. All sorts of Tyvek like material will fill the bill. To really make it work. a lot of designs install the siding to vertical furring strips that are intentionally vented at the top and bottom. This encourages air flow inside the exterior siding which means good vapor exchange while protecting the membrane (Tyvek and most other vapor permeable membranes degrade in the sun). It also reportedly is the way to keep natural wood siding from curling. The trade of is the top and bottom venting has to be pest proof as it is a tempting place for insects to setup a home.

The energy efficient homes with zip system on the outside can be successful if exterior insulation is thick enough so that the moisture gradient stays outside the face of the plywood. That means lots of insulation on the exterior and that is where the vapor permeability modeling comes in. A new product to the US is this vapor permeable insulation https://timberhpassets.s3.amazonaws...3/22_0803_TimberHP_Cut_Sheets_TimberBoard.pdf. Rockwool Comfortboard is also vapor permeable and is R 4. I have not found any foam systems that make the claim of vapor permeability. Basically, once the calcs are done then the boards are attached on the exterior of the zip system with a siding treatment discussed above using long screws and detailing to prevent the insulating boards from dropping. I think in many cases, the concept of 2x6 walls may go away and be replaced with 2x4s, as there is no need for the extra exterior wall depth as the R value for solid softwood is less than half of the Timberboard or 1/3 of Rockwool. With the exception of the window frames which can be a thermal bridge it makes a very high performing wall but the trade off is its not cheap.

Speaking of windows, window framing and installation is lot more a challenge than previously and trimming out the deeper window wells is pretty well custom work unless someone is a major optimist.

Lots of things to think about, the Passive home approach does not leave anything to trust, its all got to be documented and audited by a third party before the building is started. It adds cost and time but it assures its going to work right in the long run. The alternative is hire a contractor or an architect and hope that their education, background and interest means that they cover all the bases. The problem is issues like mold in walls may take years to appear and most homeowners are not going to be in the house that long. I know someone doing his own "pretty good house" type home and realistically he is 69 and at best might be in the place for 20 years. His theory is do it good enough that its not his problem. I will be looking at the same thing, I am 63 and the next house will be the last house. Do I sink in an extra 33% to build super energy efficient or do I build conventional and have to use more wood or to heat the house? The cost to heat my current somewhat efficient home is about 3.5 cords a year. The interest on the extra cost to go super efficient (My guess is 50 to 100K is going to be far more than the energy savings.

 

[EbS-P]

It really feels to me the science at this point should be settled on the best construction method. Whether it’s affordable or practical might not be. We have plenty of building science researchers across the globe and I feel someone should have answered this already with SCIENCE!! I Will hit the stacks are report back.

 

[peakbagger]

Good luck, lot of folks claim to have the ideal system but listen in on modern low energy forums by the guys doing the work and they are still innovating.

 

[Poindexter]

I am pretty sure vapor handling is in its infancy, compared to say foundations like under the pyramids or the Acropolis.

What if we get a bunch of 10 foot lengths of half inch PVC, and then plug one end of each pipe. Then we get a team of laborers who are allowed to be impaired with recreational pharmaceuticals. One of them has a big paint roller and smears roofing tar all along a wall. A group of laborers come behind and stick the pipes vertically to the wall. The foreman has to be sober enough to make sure all the plugged ends are up and the open ends are down. Layers and layers, maybe 18 inches thick.

We should have dead air space as long as the plugs at the top hold, and collected condensate within each tube just drips out the bottom.

I see a lot of "good" ideas about vapor management, but they all more or less require perfect installation.

 

[Poindexter]

I should add that building a foundation that might last for 3-4 houses on top seems like a potentially positive legacy.

Also, I notice construction lumber typically (my shop) measures right at 19.9% MC, wet basis, just barely dry enough to burn in a modern wood stove. Hardwood for furniture is often seasoned (in house hold like climate control) for decades before being made into a rocking chair or a desk. I have some quarter sawn white oak in my office now, just seasoning to someday replace the flat packed computer desk I am working from right now. I bought the oak more than two years ago, it has been in my office ever since.

I am using the QSWO lumber as a foot rest. I have some cherry shipped up from the lower 48 (enough for maybe a humidor if I was a cigar smoker) that has been seasoning indoors since the 1990s.

Building with lumber that is not thoroughly seasoned - but trying to install perfect vapor control on it- seems like a group of folks are overlooking some basics like wood movement and frequently impaired laborers. Not all construction laborers are hooked on meth, but the sort of people that can actually labor all day getting stuff done are typically not 50 years old with multiple degrees.

 

[woodgeek]

You seem to be hung up on the idea that vapor migration into a wall cavity must be followed by condensation, mold and rot.

Not true. If vapor can migrate in, it can also migrate OUT. In other words, the goal is the allow the cavity to dry to the interior and exterior via a diffusive process.

No vapor barriers and diffusive transport results in a cavity with no condensation. There is a continuous gradient of vapor concentration. In a cold dry condition outside, the vapor concentration falls continuously from high in the warm interior to low on the exterior. This gradient tracks the temperature, so as to stay below the dew point everywhere. In a warm humid environment, the same gradients are reversed.

For example, imagine working outside in the winter in a rubber raincoat. It would trap the vapor leaving your body and you would be a sweaty mess. Even if the coat had a cloth liner, there would be condensation collecting on the inside of the exterior rubber membrane. Instead, your winter coat is designed to be be air impermeable (to prevent windchill) yet vapor permeable in the liner and the exterior shell... in this case the vapor travels through, and the exterior membrane dries by evaporation at a rate sufficient to prevent condensation, even if the outer layer is very cold compared to the to the surface of your skin (which has a dew point approaching 98.6 a fraction of an inch away). This is evidence of there being a gradient in vapor content across the coat insulation. The outer layer is made to be hydrophobic to shed water, while still being vapor permeable (the idea behind goretex).

Most people don't understand psychrometrics. https://en.wikipedia.org/wiki/Psychrometrics

I teach it.

You want to build your house like a good winter coat. All that is needed is vapor permeable insulation (like dense packed cellulose, or fiberglass or rockwool) and quality airsealing and no vapor barriers (no rubber raincoat to trap moisture). This is most easily achieved by careful wrapping and taping on the exterior with Tyvek. This is why this is such a common building practice now.

I get that you opened up your walls and saw mold everywhere. And though, 'Wow, even with this vapor barrier to prevent water going into the cavity, I had all this mold!' When in practice vapor barriers often result in MORE water in the cavity, by inhibiting drying.

You think that the science of this is poorly understood bc you do not yet understand it. In reality, a large number of different designs have been built in many climates, and the basic issues have been known for decades. Will every structure be perfect and 100% free of mold forever? No. If you want that, build your house out of concrete and metal. Will some designs become a 'mold farm' that grows mold all over the place...yes. Many such structure were built in the 70s and 80s, including your recent demo.

The goal is simple... build an airsealed, vapor permeable structure and it will not be a mold farm. This will involve vapor permeable insulation, sheathing and taped membranes (Tyvek), nothing exotic required. Adding poly and impermeable foam **in the wrong locations or thicknesses** can easily create a mold farm situation. The latter problem is even more likely in a mixed winter heat and hot humid summer situation.

One weakness of this approach: the interior paint you apply should be vapor permeable, as is common latex. But over years, many coats of particular paints can inhibit drying to the interior, leading to higher cavity moisture in hot humid conditions. Don't repaint every three years. Another weakness... the tape on the exterior can fail, or the membrane can degrade, esp in high solar heating locations. Structures that have had these failure then display higher cavity moisture. This was a problem I think with early versions of Tyvek and tape... many of which have now failed. If you see such a structure with mold after 30 years of service, you don't know if the mold started in year 1 or year 29.

In principle, the housewrap could be replaced every several decades. Easier than rebuilding the entire structure.

The MC of the wood framing at construction doesn't matter. It dries out in service, unless you somehow perfectly sealed it on purpose (which would be very difficult). Also, my understanding is that newly framed houses are often left open for some time once some roof goes on to dry out the framing timbers prior to sheathing and drywall. I thought that was a common practice to avoid warping of the framing after the sheet goods have been applied.

And yes, the siding over the Tyvek should have a small air gap behind (blocked to insects) it to facilitate drying to the exterior and prevent bulk water from wind driven rain getting driven into the cavity.

 

[Ashful]

For the purposes of argument, lets say that crazy advances in medical technology allows both of us to live for another 77 years, to 2100. Current projections for climate change indicate an increase in global average temps of 2.5°C above the 20th century mean... If you are worried about sea level rise, be above 50' elevation and you won't have to worry about that either.

Damnit! Are you saying I should stop building that boat dock and bulkhead in my back yard? I was banking on my future Zestimate reflecting a water view.

Admitting that my experience is limited to the relatively narrow climate region that makes up southeastern PA, I will say I've torn open many dozens (hundreds?) of wall cavities in more than a dozen homes, built everywhere from 1700 to 1970, and have failed to find any evidence of mold in a single one of them not involving masonry. Framed houses around here, whether insulated with fiberglass bats, blown-in cellulose, or polystyrene beads, just don't seem to favor the flourishing of mold that so many fear, despite many variations of exterior sheathing and interior finishing methods.

I have seen mold inside of walls, but it has ALWAYS involved insulation on masonry or masonry-faced cavities, such as foam board nailed to originally-uninsulated plaster on brick or plaster on stone, or basement foundation wall cavities using several different insulation and vapor barrier types.

Poindexter, your climate is much more extreme than ours, so I have no doubt in your observations. But I cannot believe some of the extensions your making from these observations, particularly when it comes to the implied issues with home cooling, mostly due to the very small delta that will typically exist between your interior and average exterior temperatures in any cooling period north of the Mason Dixon. Our traditional vapor barrier configuration is all wrong for cooling, in theory, but when average temperature is only 10F above indoor temperature, there's not much opportunity for troublesome levels of condensation.

 

[sloeffle]

The foam guys then say to avoid this, you need to make the foam THICKER than the cavity insulation, and you need to make this thickness higher the colder your climate. A 'minimum safe foam thickness' to avoid mold. In cold climates, you get numbers like 6 or 8+ inches of foam, and still a big risk of condensation if the owner is a little aggressive with the indoor humidification (due to a sick family member).

This has me cringing a little bit. When I did my house addition I was going to do flash and batt but the foam guys got a little carried away at the beginning so we decided to just fill every 2x6 cavity full with open cell insulation. My *hope* is that since it's open cell insulation that it will let the moisture permeate through it. Outside of the house does have Tyvek on it. No whole house humidifier though.

I've walked through dozens ( it was somewhat of a hobby years ago when I lived closer to the burbs ) of houses under construction and I've never seen a house in Ohio with vapor barrier behind the drywall. As woodgeek pointed out, a house needs to breath and wrapping the interior in visqueen doesn't allow it to do that. I assimilate it to foaming the roof of your attic but then you don't treat that space as conditioned.

 

[woodgeek]

This has me cringing a little bit. When I did my house addition I was going to do flash and batt but the foam guys got a little carried away at the beginning so we decided to just fill every 2x6 cavity full with open cell insulation. My *hope* is that since it's open cell insulation that it will let the moisture permeate through it. Outside of the house does have Tyvek on it. No whole house humidifier though.

I've walked through dozens ( it was somewhat of a hobby years ago when I lived closer to the burbs ) of houses under construction and I've never seen a house in Ohio with vapor barrier behind the drywall. As woodgeek pointed out, a house needs to breath and wrapping the interior in visqueen doesn't allow it to do that. I assimilate it to foaming the roof of your attic but then you don't treat that space as conditioned.

I think open cell foam would score as vapor permeable, and thus aok if reasonably airsealed.

 

[woodgeek]

Admitting that my experience is limited to the relatively narrow climate region that makes up southeastern PA, I will say I've torn open many dozens (hundreds?) of wall cavities in more than a dozen homes, built everywhere from 1700 to 1970, and have failed to find any evidence of mold in a single one of them not involving masonry. Framed houses around here, whether insulated with fiberglass bats, blown-in cellulose, or polystyrene beads, just don't seem to favor the flourishing of mold that so many fear, despite many variations of exterior sheathing and interior finishing methods.

I have seen mold inside of walls, but it has ALWAYS involved insulation on masonry or masonry-faced cavities, such as foam board nailed to originally-uninsulated plaster on brick or plaster on stone, or basement foundation wall cavities using several different insulation and vapor barrier types.

This makes sense. Wood is naturally mold resistant, things like chipboard mold bc of the glues added. Also, a lot of people think brick walls (or stucco covering) are impermeable to water, when in fact wind driven rain goes right through them, so you need to have a gap behind to drain the liquid water away.

A colleague had a new construction house in a development with chipboard wall sheathing with stucco directly applied to it without a gap. Every unit in the development was a mold farm on the sheathing a few years later, and the builder was long gone.

---------------------

You also have to worry about some residents... they can be stupid like me. When I first moved into my house (which was super leaky) with a small child with asthma, I wanted to humidify her bedroom (a basic parental instinct). I discovered that I couldn't humidify the whole house (several gallons a day didn't move the needle), so instead I humidified two upper story bedrooms. In the winter, all that vapor was blown out the leaky window+storms and into the leaky wall cavities. The sills in the affected (50 yo) window all rotted out and had to be replaced. I assume that there must be huge deposits of dust /dirt /pollen in those cavities (filtered by the 50 yo fiberglass) and have seen black fiberglass in may other places in the house. I assume that my water wet/moldered that black fiberglass but good.

I haven't opened up the walls, but I assume I grew a mold farm in them prior to doing a lot of airsealing. After the airsealing a small humidifier keeps the whole house reasonable in January, I don't bother running it the rest of the year.

I can now tell that I am 'reactive' to the air in my master bedroom, and probably have been for a decade, perhaps contributing to apnea and some side effects of apnea (like hypertension). This is esp bad in the summer and fall (reverse stack effect with high outdoor humidity). The effect appears localized to my bedroom.

I recently realized that the drywall on the bedroom wall does not go to the floor where there are hydronic baseboards, and has partly pulled away from the studs due to years of thermal cycling. The builder left a gap, perhaps for thermal expansion. In my preTyvek house the retrofit airsealing is done at the drywall plane. And I now think that gap is where all the water went 15 years ago, and where the mold spores have been blowing back in ever since 8 months a year.

Solution: rip out the hydronic baseboard, seal the drywall gap, and put on a trim baseboard. Oddly, I did that to the other bedrooms years ago.

Moral to the story: I grew a mold farm in my house by turning off my hydronic baseboards (that probably kept that cavity too dry/hot for mold for 50 years), switching to ASHP heat and central air, and humidifying the chit out of us due to a sick kiddo for a few years. And a pro round of airsealing on the house missed the a large key gap in the drywall because it was obscured on the FLIR by the radiator. And the single speed AHSP leads me to overcool to get enough dehumidification, leading to indoor temps often being below outdoor dewpoint.

 

[EbS-P]

So I’m going summarize thoughts here for myself. Air sealing, like really good air sealing say 3 ach50 or less is ideal. Exterior vapor barrier now moves dew point to the inside of this vapor barrier and humid house air can condense and cause mold.

So the solutions as I see them are more exterior insulation. Like all of it. Leave the wall cavities open. Put all needed insulation outside of the vapor barrier. And dehumidify the house with a ventilating dehumidifier. Don’t rely on AC for dehumidification. Ventilated attics are a nono.

So what Constrction methods would work well in this situation?

 

[woodgeek]

So I’m going summarize thoughts here for myself. Air sealing, like really good air sealing say 3 ach50 or less is ideal. Exterior vapor barrier now moves dew point to the inside of this vapor barrier and humid house air can condense and cause mold.

So the solutions as I see them are more exterior insulation. Like all of it. Leave the wall cavities open. Put all needed insulation outside of the vapor barrier. And dehumidify the house with a ventilating dehumidifier. Don’t rely on AC for dehumidification. Ventilated attics are a nono.

So what Constrction methods would work well in this situation?

Or much more simply, NO vapor barrier. Just a vapor-permeable air barrier.

And why are ventilated attics a no-no... that is a great way to have a dry attic.

What is wrong with AC dehumidifcation (if properly sized or variable speed to dehumidify at a good setpoint)? That allows drying the cavity to the inside during the summer.

 

[Poindexter]

To summarize, I see a consensus that vapor impermeable layers just don't work. This confirms the suspicion I had when I opened the thread.

I see a consensus that vapor permeable layers like house wrap do work or can work.

I don't see a consensus for a small range of vapor permeable products that are effective in a wide range of climates and circumstances.

And it is true I do not understand psychrometrics. I just read the wiki page again until my eyes rolled. I know if I want to buff the fresh wax on my car as the dew is condensing on the fresh wax I need to be out in the driveway with a bunch of dry towels around 3 or 4 AM.

If I have even a clue about what @woodgeek has been trying to explain, what happened at my house was

1. interior vapor impermeable layer installed under the drywall imperfectly.
2. c 1980 oil burning furnace in the garage, with outside air intake next to boiler on garage wall, boiler chimney through roof.
3. Warm interior living space air (carrying vapor) leaked out through the holes in the vapor barrier and left condensate in the insulation over 34 years
4. Done.

So my idea, from even a couple weeks ago, was wrong.
1. Wood stove installed in 2014, drawing combustion air from living spaces exposes air leaks in "vapor impermeable" plastic sheeting under drywall.
2. Incoming leaked cold air carries smell of mold.
3. Air leaks in vapor impermeable barrier layer are drawing cold water vapor into the insulation blanket and leaving condensate secondary to wood stove drawing combustion air from living space.

So if I maybe learned anything in this thread, I might have avoided mold smell inside my living space all along if I had _started_ with an outside air intake feeding combustion air directly to the woodstove so I could have, net, continued to push warm vapor laden air into my insulation blanket through the existing leaks in the "impermeable" vapor barrier layer.

I still think trying to upholster chairs built from green wood that isn't done moving is a bad idea.

Appreciate the discussion so far, my original question is answered.

 

[woodgeek]

Happy to help.

I would add the following.... for a long time, builders have been making leaky framed structures, and pretending that they were not leaky.

In my house, my vented attic would _sometimes_ collect significant condensation on the bottom of the roof sheathing, in the vented attic. This would then drip off the nail points and onto the insulation and stored items... like magical rain. Since a vented attic should only live at the outdoor dewpoint, its hard to see how such a large amount of condensation would occur (dew formation should mostly be on the outside and this did not occur during dewy weather).

Ofc, one of the many 'easter eggs' in my house was the that the clothes dryer, rather than being vented out a block wall to the outside, was vented through a different wall into an attached unheated 2 car garage. We discovered quickly after moving in that if there is no dehumidifier in the garage, anything stored in the garage will rapidly grow mold. In fact, we discovered that two weeks after moving in in July, when half of our possessions were still in the garage temporarily, and we had been doing a bunch of laundry bc of our two small children.

Well, it turns out the framing of the interior walls was not airsealed anywhere, either to the garage below or the attic above. So even with a little dehumdifier keeping the mold in check in the garage long term, that vapor from the dryer vent was being drawn by the stack/chimney effect through the interior framed walls and ejected into the attic, where it condensed on the nails and fell as rain on the insulation. While the attic was technically vented, and such might have diluted the vapor, the venting was ofc well below code levels, and far smaller in total area than the air sealing gaps to the garage! So in practice the attic was vented by the dryer two stories below.

I rerouted the dryer vent to the outside, and the rain stopped. QED.

IOW, I think of older framed construction as being somewhere between a chitshow and a farce when it comes to airflow and condensation.

I like your explanation OK @Poindexter. The true explanation may be more nuanced, involving air flow through the walls behind the vapor barrier (e.g. through wire and plumbing penetrations). For example, if a previous owner vented the dryer into an unfinished basement for 'free heat' that vapor could follow wiring penetrations into the 'air and vapor sealed' wall cavities and condense there, being pulled by the chimney effect.

One would also have to ask if the outer sheathing was of low vapor permeability, and was the siding not allowing bulk water intrusion.

 

[maple1]

When we built 25 years ago, vapour barrier under the drywall was code. AFAIK, it still is, here. (I must remember to ask a contractor friend about that). So that is what we have. We are about due for re-siding. At which time I would like to improve the air sealing on the outside and maybe add some exterior insulation before new siding goes on. A lot around here are just throwing on a layer of foam board when they do that. Which I have always wondered about. Are there certain foam products (I.e. open cell) where it would be safe to do that without making a moisture trap out of your walls? I am behind on my building materials knowledge.

 

[Ashful]

When we built 25 years ago, vapour barrier under the drywall was code. AFAIK, it still is, here.

You know, it's funny that in my sweeping statements above about never finding mold, at least 90% of the work I've done is on houses built with NO vapor barrier. Indeed, even my current kitchen and master bath have plaster on lathe with no vapor barrier, having been added to the house in the 1890's, presumably before such a thing was even considered. That said, plaster (esp. pre-electrification) with pre-latex paints would have made an awful good vapor-permeable air barrier, all by itself.

One thing I'm still not getting from the Poindexter / woodgeek banter is how an OAK (or lack thereof) on your stove or furnace might impact migration of moisture into a wall. Poindexter's last summary might have some thinking that running your furnace or stove with no OAK might be a good way to keep outside air moving through the wall (versus inside air moving out), and thus might help avoid issues for those running indoor humidifiers in winter. However, woodgeek's second sentence at the very top of this thread states that moisture migration into a wall is by diffusion, not convection. So, other than increasing the requirement for water to achieve a given RH%, would the use or avoidance of an OAK have any impact on how moisture migrates into a wall from a closed space?

In a leaky old house like mine, I suspect the amount of air being drawn by a few stoves and furnace is so small relative to the "leak potential" of the house, that no matter what the theory, the actual CFM's in question make it irrelevant. It's more likely that pressure differentials created on the walls by a mild breeze outside completely swamp the net effect of what a stove is pulling through the walls.

 

[ABMax24]

Interior vapour barrier is code here. It must be installed under the drywall, taped at every seam, and all outlet boxes on outside walls must have hats to seal themselves to the vapour barrier.

I couldn't image a house in this climate without that, the humidity in the house would be in the single digits and every outside wall would be solid frost on the inside.

Impermeable vapour barrier on the inside, permeable on the outside.

I think the flash and fill technique is backwards, spray foam should be against the drywall and loose fill or fiberglass batt on the outside. Problem is no one wants to drywall a house that isn't enclosed, or sheet a house worth of walls while standing.

 

[Poindexter]

One thing I'm still not getting from the Poindexter / woodgeek banter is how an OAK (or lack thereof) on your stove or furnace might impact migration of moisture into a wall. Poindexter's last summary might have some thinking that running your furnace or stove with no OAK might be a good way to keep outside air moving through the wall (versus inside air moving out), and thus might help avoid issues for those running indoor humidifiers in winter. However, woodgeek's second sentence at the very top of this thread states that moisture migration into a wall is by diffusion, not convection. So, other than increasing the requirement for water to achieve a given RH%, would the use or avoidance of an OAK have any impact on how moisture migrates into a wall from a closed space?

@woodgeek 's second sentence for reference: {The basic idea you are missing is that if the air is stagnant in the cavity, vapor motion is by diffusion, not convection.}

When I first started running a wood stove (~2012) I found cold air leaks all over the house, mostly the window frames, but many of the jiffy boxes for receptacles and switches on exterior walls as well. It took 2 summer upgrade seasons to just fix the big ones, and probably three more years to get the little ones.

I have been running the same wood stove since 2014, maintaining roughly the same indoor temperature for coming up on 9 years. I am still pulling combustion air from my living space. My remaining "one big leak" to feed the woodstove is under the entry door on the lower level.

I like having that airleak there. It is handy to walk by it barefoot on my way to my office in the morning for a first impression of how brutally cold it is out there today. If I cave to Mrs. Poindexter and spray foam insulation under the entry door, I am putting in an OAK first. My second biggest air leak is the sliding glass door out onto the deck (on the wood heated upper level) that will pull all my combustion air through the dining room/ kitchen area. In that scenario Mrs. Poindexter's agitation level might require medical intervention.

I can feel that stream of cold air, sometimes, in a layer against the floor under an inch thick, moving from the top of the stairs directly x the stove room to the hearth. When my woodstove is really cranking I can push the thermocline between the wood heated upstairs and the oil heated down stairs partly down the enclosed stairwell- in which case I am feeding the wood stove preheated air.

In all of these cases my indoor RH is often 8-10% upstairs. Running 2-3 gallons daily through humidifiers I can get to, but rarely exceed 20% RH in ~10k cubic feet of volume upstairs.

What I am getting at, with air leaks supplying combustion air to the wood stove, air movement through my wall was demonstrably present and vapor movement was riding on the air.

Outside water sealing is adequate to subpar. The house was built with T-111 panels nailed directly to the 2x6 studs and then painted.

One thing I notice is water vapor takes up a lot more space or volume than liquid water. So when the dewpoint is reached at a particular point, more vapor carrying air is going to move to that place as each molecule of water condenses. I think this would be an example of the diffusion process woodgeek is getting at.

 

[EbS-P]

Or much more simply, NO vapor barrier. Just a vapor-permeable air barrier.

And why are ventilated attics a no-no... that is a great way to have a dry attic.

What is wrong with AC dehumidifcation (if properly sized or variable speed to dehumidify at a good setpoint)? That allows drying the cavity to the inside during the summer.

If you are installing AC and live in a green grass climatic, generally there is enough days a year where you need dehumidification but not cooling. there is a decent amount of the season and with new construction you are very tight with really good insulation so you need to actively manage humidity without cooling.
Now that you are really tight you need fresh air ventilation and you want to be able to control the humidity of that fresh air. Doing a lot of cooking for a gathering get extra fresh air and take all the moisture out you added. For the cost of installation I think it with it for any new tightly constructed well insulated house.

As for the attics. Again you could build a basement and run ductwork there but if you are going two stories ductwork and equipment is easier access in the attic. If you put and hvac stuff up there it should be encapsulated.

How much of this has direct applications to central AK I’m not sure but if you are adding AC I think there is enough crossover that these ideas/concepts need to at least be thought about.

 

[peakbagger]

This vapor barrier issue and historically cheap electric rates is reportedly why southern homes were frequently not insulated very well if at all. Someone I worked with 20 years who did construction in southern California commented that most of the construction at the time was 2 by 4 with minimal insulation.

 

[woodgeek]

When I first started running a wood stove (~2012) I found cold air leaks all over the house, mostly the window frames, but many of the jiffy boxes for receptacles and switches on exterior walls as well. It took 2 summer upgrade seasons to just fix the big ones, and probably three more years to get the little ones.

I have been running the same wood stove since 2014, maintaining roughly the same indoor temperature for coming up on 9 years. I am still pulling combustion air from my living space. My remaining "one big leak" to feed the woodstove is under the entry door on the lower level.

I like having that airleak there. It is handy to walk by it barefoot on my way to my office in the morning for a first impression of how brutally cold it is out there today. If I cave to Mrs. Poindexter and spray foam insulation under the entry door, I am putting in an OAK first. My second biggest air leak is the sliding glass door out onto the deck (on the wood heated upper level) that will pull all my combustion air through the dining room/ kitchen area. In that scenario Mrs. Poindexter's agitation level might require medical intervention.

I can feel that stream of cold air, sometimes, in a layer against the floor under an inch thick, moving from the top of the stairs directly x the stove room to the hearth. When my woodstove is really cranking I can push the thermocline between the wood heated upstairs and the oil heated down stairs partly down the enclosed stairwell- in which case I am feeding the wood stove preheated air.

In all of these cases my indoor RH is often 8-10% upstairs. Running 2-3 gallons daily through humidifiers I can get to, but rarely exceed 20% RH in ~10k cubic feet of volume upstairs.

What I am getting at, with air leaks supplying combustion air to the wood stove, air movement through my wall was demonstrably present and vapor movement was riding on the air.

Outside water sealing is adequate to subpar. The house was built with T-111 panels nailed directly to the 2x6 studs and then painted.

One thing I notice is water vapor takes up a lot more space or volume than liquid water. So when the dewpoint is reached at a particular point, more vapor carrying air is going to move to that place as each molecule of water condenses. I think this would be an example of the diffusion process woodgeek is getting at.

Another factor is stack effect. Even when your woodstove is not burning, your house is still a chimney. Usually the air flows out in the upper story and in on the lower story (like under the front door). In a hot climate, this reverses in summer.

Not that this means that vapor leaking out in the winter is mostly upstairs, and vapor leaking in in the summer (also a problem if humid outside and AC inside) is ALSO upstairs. My rotted window sills and suspected mold farm are all on my upper story. Winter window condensation is an upper story thing... never downstairs.

Were your moldy cavities upstairs or down?

Somewhere there is a balance plane where the pressure with the outside is basically zero. The air draft from a woodstove or a bathroom fan basically shifts this balance plane up. When it is windy outside that messes up this simple picture, with infiltration upwind and exfiltration down wind.

Vapor diffusion is a molecular process that moves vapor even when there is no air/fluid flow. It is the same as heat flow when there is no matter flow (i.e. conduction). If you see a small drop evaporating on your counter in a room with no airflow... that is diffusion.