Vapor Barriers, Foam Board, and Climate Differences

[rmcfall]

Bruce Harley's book "Insulate and Weatherize" is filled with lots of great information. On the issue of vapor barriers and climate differences, however, there are things that are confusing to me. I am hoping someone here can help me make sense of all this. Early in the book he talks about how kraft paper is the best interior vapor barrier to use in mixed humid climates and in cold climates in homes with central A/C. Mixed humid climates include Kentucky, which is where I am. He adds that polyethylene sheeting should only be used in the cold climates. This all makes sense to me, since in a mixed climate like Kentucky it is important for a wall to be able to dry to the interior OR the exterior.

Now, here is where I get confused. Later in the book he discusses the use of foam board in remodeling situations. He states that, "Extruded polystyrene and foil-faced polyisocyanurate board are both good choices for interior or exterior applications, regardless of the climate." Now wouldn't foil-faced polyisocyanurate (PIC) be equivalent to polyethylene sheeting, so why would it be acceptable to use in ANY climate??? This seems to contradict his remark about kraft paper being the preferred vapor barrier in mixed humid climates, and he does not appear to offer an explanation as to why foil-faced PIC would be an exception to this rule. I can see why extruded polystyrene (XPS) would be alright since it is somewhat permeable, but I can't figure out why something impermeable like PIC would be OK?? If polyethylene sheeting is not a good idea in a mixed humid climate, then why would something similarly impermeable like foil-faced PIC be OK?

Can someone shed some light on this?

 

[LLigetfa]

The issue with polyethylene is that it is not an insulator like PIC. The other side of the PIC is less likely to reach the dew point. Polyethylene needs to be on the warm side of the insulation to prevent moisture condensing on it. I don't think there would be enough air movement for moisture to migrate to the cold side of XPS on a normal above grade installation. Below grade however, PIC would be required. I have used PIC on the cold side with batt insulation and polyethylene on the warm side. I strongly advocate using an air barrier like Tyvek on the cold side to reduce air infiltration and at the same time allow moisture to escape.

 

[rmcfall]

The issue with polyethylene is that it is not an insulator like PIC. The other side of the PIC is less likely to reach the dew point. Polyethylene needs to be on the warm side of the insulation to prevent moisture condensing on it.

I see what you are saying about the PIC being an insulator, but in essence doesn't polyethylene become part of an insulated assembly when used with fiberglass? Thus, how is it that being an insulator prevents condensation from occurring? Or is condensation less of an issue with PIC or XPS simply because air cannot pass through them the same as with fiberglass? Still seems like the foil faced PIC might trap moisture, since it could only escape the direction opposite the PIC. ???

I don't think there would be enough air movement for moisture to migrate to the cold side of XPS on a normal above grade installation. Below grade however, PIC would be required.

My understanding from the building sciences website is that PIC should NOT be used below grade, but that XPS should be used due to its greater permeability.

I appreciate the reply.

 

[LLigetfa]

Sorry, I confused the PIC with XPS. The stuff that looks like beads should never be used below grade. The extruded stuff can.

It's all about the temperature differential, the relative humidity, and the dew point. Around here we go by the 1/3rd rule which says the vapour barrier must be within the warm side 1/3rd of total insulation. This of course is for heating, not cooling. When PIC is used on the warm side, the theory is that even the cold side of the PIC will be above the dew point and so condensation will not form. IMHO there will be a dew point somewhere within the batt insulation if all the insulation is batt.

As I mentioned, I will place XPS on the cold side of a wall to raise the temperature of the warm side of the XPS hopefully above the dew point. In my former home, the builder had placed 3 - 1/2" thick batts into a 2x6 crawlspace wall cavity and he did so by leaving an air gap on the cold side. The air gap allowed a large humidity to rise up from the ground and condense on the cold side of the batt and also on the PT plywood sheathing. Once the batt was saturated, the backside of the vapour barrier became soaking wet. I had to rip off all the poly and remove the batts to dry. I then cut XPS to fit between the studs and placed it on the cold side against the PT sheathing and then put back the batts. I also cut XPS baffles to block off the bottoms of each stud cavity and cleated the poly to form an air barrier. That solved the moisture problem for good.

In my current home, I placed 2" thick XPS between the PT sheathing and the soil and then filled the 2x6 cavity with R22 batts.

 

[rmcfall]

Thanks for the explanation. Being in a mixed humid climate, I wonder if I'd be safer going with the XPS on the interior? Actually, what I have done is filled the wall cavities with XPS foam. I have cut, fit, and sealed the XPS foam between studs with spray foam. A lot of work, but I believe it will be well worth it in the long run. I can already tell a difference now that the temp is up and the humidity is high. With the air conditioning running, this room feels very cool relative to other areas of the house that I have not finished. Anyway, I would like to cover all the wall studs with foil-faced PIC to insulate the wood studs, but I am a little concerned about having a wall that cannot dry to the interior. Based on Harley's book, foil-faced foam on the interior is acceptable, regardless the climate. However, his other comments about using a permeable vapor barrier in mixed climates has recently made me lean toward using XPS across the studs instead. That said, as you pointed out, the foil faced PIC is an insulated air barrier, so it sounds like it would be fine. Plus, the PIC provides a slightly higher R-value and doesn't cost much more than the XPS.

I suppose I could always punch some nail holes in the foil-faced PIC to get the advantage of a higher R-value, plus the benefit of permeability????

 

[rmcfall]

When PIC is used on the warm side, the theory is that even the cold side of the PIC will be above the dew point and so condensation will not form.

Is it safe to assume that in a mixed humid climate the opposite is also true? That is, when PIC is used on the cool side (i.e., summer months), even the warm side of the PIC will be above the dew point so condensation won't form???

So where does all that vapor go? Does it just stay inside the PIC? I suppose it can't, since the foil-faced PIC has an impermeable layer on both sides. Is the vapor then forced into the surrounding wood framing?

 

[LLigetfa]

The moisture will find its own equilibrium. You cannot make the vapour barrier 100% sealed anyway so moisture will not be trapped forever. I think an air barrier is more important to control the movement of humid air through the wall cavity. Even if the styrofoam is not impermeable, it will still be an air barrier so air will not carry moisture to the cold side.

Here in our brutal Winters, it is said tha just one leaky electrical box letting moist indoor air leak out can condense as much as 5 gallons of water into the wall cavity.

Again, because of the R value between the interior and exterior of your styrofoam sheet, and with the smaller temperature differential, warm moist air hitting one side and cold hitting the other, the warm side won't be cooled enough for condensation. If the styrofoam is impermeable on both sides, there should not be moisture migrating through the foam.

One thing that is seldom discussed is that styrofoam insulation can degrade over time but supposedly the double foil faced foam doesn't deteriorate as much as unfaced.

 

[rmcfall]

Great response. Thanks. So because of the small temperature differential created when foam board is used, it really can be used on the interior or exterior no matter the climate, and regardless whether it is foil-faced PIC or XPS. To use an extreme example, foil-faced PIC would be fine on the INTERIOR even in the deep south, even though in that area a vapor barrier would normally be placed along the exterior since that is the warm side of the wall. Just so I am understanding this, the foam reduces the temperature differential thereby minimizing or eliminating condensation, which is why the foil-faced PIC can be used on either side of the wall, regardless of the climate. I sure wish Harley had explained this in his book. Perhaps I overlooked it, but I looked all over for an explanation.

Given condensation is not an issue, PIC seems like the way to go. Compared to XPS, the PIC has a higher R value and offers some reflective qualities. With the exception of below grade walls, what benefit is there to using XPS instead of PIC? Since the cost is about the same, I can't think of any.

 

[benjamin]

Use the PIC on the inside, just be sure the outside of the wall can shed water, drain water and breathe. Wall systems get into big trouble when there are two vapor barriers, because no wall is waterproof and vaporproof. Synthetic stucco houses and many finished attics rot out the framing because of moisture buildup that can't escape because of double vapor barriers.