Vapor Barrier Insulation Question

[Timnus]

Hey everyone,

I am looking to install my wood stove in my living room, and want to get rid of the old drop ceiling and put up drywall instead. The attic has kraft-faced insulation stapled to the ceiling joists (paper facing down) and then there is blow-in insulation on top of everything. The trouble is that the faced insulation is sagging and coming down in places. So, I would like to staple up plastic to hold it all in place before I put the drywall up. Will this be an issue if I add another vapor barrier on the underside of the joists (where the kraft-faced paper is also at)? My house is in Central PA. The attic has some venting at the peak, but they didn't cut out the OSB everywhere underneath the ridge vent.

 

[semipro]

Water vapor that makes its way through the drywall ceiling (and it will) would condense on the vapor barrier and create a mold potential on top of the drywall. If you just want something to hold up the insulation so you can install drywall how about landscape cloth? I've used it in similar situations.
A house wrap like Tyvek would also hold up the insulation and allow the water vapor to pass through while providing an air barrier. I'd prefer the former though.

 

[Lumber-Jack]

I don't know much about the building codes in the USA, but here in Canadian homes the vapor barrier is generally installed on the inside of the walls directly before the drywall board goes on, so what you are contemplating would be the proper way to do it.
What Semipro is suggesting wouldn't pass code here.

 

[woodgeek]

Poly vapor barriers are not necessary or recommended in Pennsylvania, too humid in the summer. Kraft paper is pretty harmless, because it is always leaky enough (untaped) that it basically doesn't work as a VB. The drywall will make a (slightly vapor permeable) air barrier, which will be perfect.

I would just staple the paper back up (as best I could) to hold it temporarily until I got the drywall up.

 

[woodgeek]

Water vapor that makes its way through the drywall ceiling (and it will) would condense on the vapor barrier and create a mold potential on top of the drywall.

This is not quite right SP. Any water coming form the interior side can go right back into the space (cuz the drywall is permeable), so condensation would only happen if the poly were very cold, which it won't be (in PA) with insulation above it. In the summer, however, the poly would be cold (from AC inside) and could get condensation on the top side, that couldn't dry to the interior.

That is why poly is ok up north, but not in solid AC country in the (humid) east.

 

[semipro]

What Lumber-Jack says may be true in his area as it is in many colder regions but very likely not in PA where you have a warmer climate. Installing a poly vapor barrier behind the drywall in PA could result in some very bad moisture/mold problems.
Check out these articles:
http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?searchterm=vapor barrier
http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/reviving-old-debate-vapor-barriers
http://www.energyvanguard.com/blog-...54110/You-Don-t-Need-a-Vapor-Barrier-Probably

Edit: I originally wrote this in the morning. I posted it later without reading the other responses on the thread.

 

[semipro]

This is not quite right SP. Any water coming form the interior side can go right back into the space (cuz the drywall is permeable), so condensation would only happen if the poly were very cold, which it won't be (in PA) with insulation above it. In the summer, however, the poly would be cold (from AC inside) and could get condensation on the top side, that couldn't dry to the interior.

That is why poly is ok up north, but not in solid AC country in the (humid) east.

I don't disagree with you WG and may be wrong but this is a controversial subject. Even Joe L. of construction science recently changed his position on moisture movement in walls...although I think it was in reference to basement walls.

I, and I believe others, have actually found mold between poly and drywall. I don't recall whether it was in walls or ceilings. I'm assuming the problem might even be worse in ceilings due to stack effect and the lack of air integrity of the loose fill insulation above letting cold air reach the poly. I can't believe that its ever a good idea to put organic material, in this case the paper backing on drywall, in contact with a waterproof barrier especially when humid air is being physically driven through the organic towards the poly. In a ceiling the pressure gradient driving this transfer would occur year round.

In the scenario you describe the condensation would could readily evaporate through the insulation back into the attic space.

I'm actually starting to find myself more in the "walls need to breathe" camp. I think the damage potential with vapor barriers is higher than the potential benefits and that all barriers should be sufficiently impermeable to stop air infiltration while being permeable enough to allow drying... that is until we find a better way.

What we really need are barriers that are engineered to change permeability depending upon temp and dewpoint. Allowing vapor movement in only one direction depending upon conditions would also be useful.

 

[woodgeek]

SP, definitely an emerging topic with different practices in different areas. We agree, poly==bad.

My current position is that wall/ceiling assemblies should be airtight and at least slightly vapor permeable in one or both directions. The cases of mold in walls IMO are 90% bulk water getting in, and 9.9+% air transported water, with 0-0.1% diffusion water (that would be prevented by a VB). That is, there is little to no evidence that diffusional water has ever caused damage in an insulated and airsealed assembly (at least in a lower-48 climate).

If the OP needs a membrane to get his insulation to stay, he should use something vapor permeable, as you said. If not, he should skip it.

I also think it is good mold 'insurance' in older homes to......
1. never humidify >30%RH in the winter when it is freezing out.
2. never 'over AC' in the summer. If your AC is not getting enough dehumidification, a lot of folks will just turn the temp further and further down, and get mold popping up in their basements, exterior door and window frames, etc. Better to run a dehumidfier, and keep the AC setpoint a few degrees higher. More comfortable and prob cheaper too.

 

[semipro]

2. never 'over AC' in the summer. If your AC is not getting enough dehumidification, a lot of folks will just turn the temp further and further down, and get mold popping up in their basements, exterior door and window frames, etc. Better to run a dehumidfier, and keep the AC setpoint a few degrees higher. More comfortable and prob cheaper too.

Good point and one that I've not thought of. All the more reason for proper design of HVAC Manual J or better. Its funny how many folks are convinced that their AC is undersized if if runs a lot on the hottest days.

 

[Timnus]

Thanks everyone for your feedback. I will still need to put up something temporary to hold the insulation from falling. So, one idea was landscaping cloth. What about Red Rosin paper? House wrap is a bit expensive, but if it is a good option, I can do that. Any other inexpensive ideas?

 

[semipro]

What about Red Rosin paper?

As long as its water vapor permeable enough it may work, providing its strong enough to hold up the insulation.
According to this reference it is.
http://www.beronio.com/pdfs/products/hardware/BldgPaperHouseWrapFlashing.pdf

As woodgeek mentioned, the kraft paper backing on the insulation should be enough though. The permeability is right.