Home>Discussions>INSULATION & HVAC>Polyfilm Needed? Mold Growth?
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David Hopper
Polyfilm Needed? Mold Growth?


I have a question about the placement of a poly vapor barrier.. I hope that someone can help.

I'm in the process of framing in some offices inside of a concrete block building. The building itself is over 40 years old, and has been repainted several times with block seal, Also the interior surfaces of the block have been painted several times as well.

The building is built on a slab( entirely avove grade), and has 8" X 24" Pilasters every 10 feet on the inside.

My plan it to frame in the new walls 1" in front of the pilasters, so that there will be some room for air movement behind the wall. this air space will be open to the area above the drop ceiling.

I'm framing the offices with 2X4's, and R13 ( 3 1/2") faced insulation to the inside. The building owner keeps insisting that I place a poly film on the back side of the framing.. as a Vapor barrier.. ignoring my arguement that this will result in a double barrier ( the insulation facing & the poly) with the insulation in the middle, and will just be providing a breeding ground for mold. Am I correct in this?

Secondly, He did say that If "I wanted to" I could apply the poly directly to the concrete block.. althought I tried to explain that this would be essentially the same thing, that the poly film would hold the moisture against the painted ( read "SEALED")block walls, and also would provide a perfect enviroment for mold growth. Any sweat or condensation on the block wall would not be able to evaporate.. and the poly film would always be wet.

I'm recommending that the area just be left open to the air.. and that the ventilation will prevent the growth of mold.. which is what he's trying to accomplish with the vapor barrier.

I Argue that the painted block will pass little , if any, mosture thru, and that the faced insulation and Semi Gloss paint in the offices will effectively prevent moisture from migrating from within the building. leaving the "void area" dry.

I'm no expert by any means.. and will gladly admit it if I'm wrong.. so someone please chime in.. Barrier or not?

Thanks in advance


Re: Polyfilm Needed? Mold Growth?

Dave ---
Having the air space between the exterior wall and the insulation is a bad idea also considering it would also be open to the ceiling above.
The last thing you want is this air space since you will end up with warm moist air from the interior reaching and contacting the cold block walls and condensing.
Also you will be creating a stack effect of moving air which will carry cooler air ---- fiberglass is terrible within moving air and it's performance is greatly reduced.

I have to agree with your assesment regarding the vapour barrier placement suggested by the owner.
Besides faced batts aren't really all that effective in controlling vapour transmission.

To offer a suggestion ------

Using an adhesive formulated for foam ----- glue 1-2 inch rigid foam insulation to the exterior block walls making sure to seal all joints with house wrap tape and seal along edges and along the tops and bottoms with spray foam.

The foam will provide a continious layer of insulation thaat is far superior than the interuppted method. The foam will also be the vapour barrier.

It's required here that the insulation be continious from floor to the underside of the roof for commerical spaces --- it's also a good idea to do this anyway.
You will have to cover the foam with drywall as a fire rating.

This way you there isn't a need to move the walls so far out if you don't want to.

2 cents.:)

David Hopper
Re: Polyfilm Needed? Mold Growth?

Thanks for the reply Canuk, I'll look into the foam insulation.. sounds like it might be the answer to my problems.

The only issue i may have is the poor quality if the walls. they have been repaired before. windows and doors have been moved over the years, I've got lintels protruding 1/2" in places and low spots in others.. so getting the foam to sit well, may be a trick.. as the wall is not a flat plane.

to make things worse, the longest wall in the room, has 3 pilasters in it.. they are not plumb not even flat.. a straight edge placd floor to ceiling.. shows a 3/8" - 1/2" bow to the pilasters ( actually there is 1 straight one).

Given the condition of the walls,I'd prefer to build my framing in front of the pilasters, rather than between them ( with drywall bridging them. I'd need to place my framing partially in front of 2 of them, just to establish a straight line.

what are your thoughts on building the walls as described in the original post, except closing the voids off at the top, just creating air pockets within the walls? this would eliminate the stack effect of moving air.

I guess i should have mentioned in the first post.. I'm in cincinnati, so we get a wide range of weather.. and high humidity in the summer.

Any thoughts?



Re: Polyfilm Needed? Mold Growth?

That's the beauty of the rigid foam sheats they can used to conform to irregular wall surfaces.
If need be you can apply different layers of different thickness to build out to a truer surface.
Then with strapping secured with Tapcons you could apply the drywall right over top.

If you go with your proposed system just make sure the rigid foam is applied to the exterior wall if you are leaving a gap with the framing.
You will have to ensure the gap is completely sealed more for fire code reasons in this case.
If your commercial codes are anything like ours --- the inspectors can be bears to deal with.

Re: Polyfilm Needed? Mold Growth?

Frame the wall as planned. Spray it with closed cell foam. Closed cell to keep interior moisture from condensing on the wall as it could with open cell foam. I would want to be sure the wall isn't leaking from the outside.

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