1 Helpful?

Two vapor barriers in one wall

I have a new home under construction in very cold climate ( more than 8,000 heating degree days). Walls are insulated with 1and1/2 to 2 inches of closed cell foam and r13 batts in 6 inch walls. The insulation contractor has also placed plastic sheeting over the walls, stapled in place.

I told the buildert that the walls now can not breathe, but he says all of his recent houses have been done this way, and have been done this way by several different insulation contractors.

The home has a HRV and electric heat with wood stove backup.

Should I have them tear off the plastic? Any advice greatly appreciated.

Asked by Bill Birck
Posted Nov 29, 2010 2:44 PM ET
Edited Nov 30, 2010 1:11 PM ET


21 Answers

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The plastic seems redundant and possibly dangerous to me, but I have another question: is the spray foam flat enough to lay batts against it? I haven't seen a lot of spray foam, but it has always looked fairly lumpy, and doesn't seem like it would work as well with batts as it would with blown-in.

Answered by David Meiland
Posted Nov 29, 2010 4:16 PM ET


yes the spray in is very bumpy, and quite inconsistent in thickness as well. But the batts have enough give to conform to the irregular surface.

Answered by Bill Birck
Posted Nov 29, 2010 4:29 PM ET


For a flash-and-batt job like yours to be effective, you need to be sure that your closed-cell foam is thick enough. In your cold climate, 2 inches is the bare minimum -- so if you have substantial areas of 1 1/2 inch insulation, that's not great.

You don't want the poly. You probably want to call your spray foam contractor back and ask him to install at least 2 inches of foam.

Answered by Martin Holladay
Posted Nov 29, 2010 4:47 PM ET


Bill, I don't have direct experience with this system, but it sounds like you will have a lot of air pockets between the foam and the batts. Perhaps someone with more experience can comment on the wisdom of doing it this way.

Answered by David Meiland
Posted Nov 29, 2010 5:08 PM ET


The Flash-N-Batt system, as I understand it, typically uses a minimal layer of SPF just as an air barrier and then fills the cavities with batt insulation. This sounds like an attempt at more than that or a poorly controlled flash layer.

The air barrier and thermal layers should be contiguous with each other to avoid air pockets that can become saturated with moisture and consequent condensation zones. A lumpy flash layer will prevent contiguity and could lead to problems.

It appears that the contractor tried to compensate for his poor foaming job by adding a vapor barrier, which is likely to create additional problems by trapping moisture in the wall cavities.

Answered by Riversong
Posted Nov 29, 2010 5:30 PM ET


When flash 'n batt was first shown to me, the SPF was a thin layer (~1/2") and I believe it was open cell... strictly an air seal The insulation contractor I have used was considering adding it and gave me a DVD to look at. The SPF went in fairly cleanly in the video (under perfect TV studio conditions) and the batts looked OK going in against it... BUT... the 1/2" thickness quickly got thicker once the dewpoint issue was brought into the discussion, and I believe the SPF was changed to CC as well. Product/marketing people trying to remember to pay attention to the lawyers who were pointing at the scientists...

Honestly, if I were going to use foam I would probably consider this system--but I would want blown-in fiber rather than batts. I don't see any way that you'd avoid having air gaps and areas of over-compression.

Answered by David Meiland
Posted Nov 29, 2010 5:48 PM ET


Bill, you definitely do NOT want plastic on the inside of the wall. We use flash and batt occasionally. There is usually a better way to insulate but it does have its place. Cost is usually less than full closed cell foam but more than dense-pack.

There is some amount of air gapping, but because the inside face of the foam is fairly warm it shouldn't affect the efficiency too much. As far as I know there's nothing wrong with compressing fiberglass insulation; it actually raises the R-value per inch of the batts. Maybe if it was completely crushed it would no longer insulate but that would take some effort.

Answered by Michael Maines
Posted Nov 29, 2010 6:35 PM ET


is the batt insulation faced or unfaced? I have read that the the use of faced batts in a flash and batt application increases the chance of mold growth.

Answered by Anonymous
Posted Dec 7, 2010 9:37 AM ET


Too many anony mice skittering around here!

I don't think anyone's interested in what some unnamed person has read somewhere. I read somewhere that Obama is a Muslim terrorist who wasn't born in the US of A and is probably a socialist fascist nazi liberal.

Answered by Riversong
Posted Dec 7, 2010 2:22 PM ET


I don't think anyone's interested in what some unnamed person has read somewhere.

I don't think anyone's interested in what your opinion is on how many people are interested in what some unnamed person writes. But then, of course, that's only my opinion...
The relevant comment on the sole anonymous posting in this thread would be whether what they have read is correct or not and in what circumstances. Besides, you're not trying to help "Anonymous" but Bill Birck, the original poster.

Answered by Timmy O'Daniels
Posted Dec 7, 2010 5:39 PM ET



Actually, I am quite sure that at least some participants here ARE interested in my opinion on the liabilities and worthlessness of anonymous posting, since several share my concerns. But, regardless of whether you and any others are, or are not, interested in my opinion, I will continue to express it until the policy here is changed.

And, in case you hadn't noticed, I already posted a constructive response to the OP, and that has been representative of the overwhelming majority of my contributions. I don't, however, see a constructive response from you to the original question - merely a non-constructive diversion of the thread.

Answered by Riversong
Posted Dec 7, 2010 5:50 PM ET


Tommy, what's your sport? If Robert gets his disciples together we should challenge at next years First Annual GBA Open.

We'll bring the mice and a keg of Sam Adams, they can bring their bibles and all natural Riversong brand H2O.

Let's get this event off the ground.

Brooks, you game?

As to the wall design, I haven't heard of huge failures like the stucco homes horror stories. I would opt for no plastic and try to get the drywall installed as an air barrier.

Answered by aj builder
Posted Dec 7, 2010 6:21 PM ET


Of course I had noticed your, typically, constructive response to the original posting. The absence of any such response from me simply reflects my unfamiliarity with the niceties of flash-and-batt insulation. The thread was not diverted by me; I was merely commenting on your diversion of it to make, yet again, a point which is is totally irrelevant to the value of GBA as a place for the exchange of information on green building.

Answered by Timmy O'Daniels
Posted Dec 7, 2010 6:50 PM ET


Foregive me for foregetting to leave my name. I was not trying to cause such an uproar. I have read in fine home building or possibly this old house. That you may want to install a nonfaced batt with spray-glue in a flash and batt application, because the paper on the kraft faced can promote mold growth if there is a moisture problem. I have never installed flash and batt. It was just what I read as I stated before. Thought someone with more experience with flash and batt could weigh-in on what I had read. Sounded like Bill was dealing with a contractor that didnt know for sure what he was talking about. Thought one more giving him advice couldnt hurt. Thanks for having my back Robert

Answered by Mike
Posted Dec 7, 2010 7:55 PM ET


a point which is is totally irrelevant to the value of GBA as a place for the exchange of information


If you believe that anonymous posting is irrelevant, then don't comment on it or on others raising the point who KNOW that it's highly relevant to this forum (as other thoughtful regular participants agree).

Perhaps you haven' t been around here long enough, but a very significant number of (though certainly not all) anonymous posts are either ignorant or malicious, or are industry hacks promoting a product. I'm not alone in feeling that allowing anonymous posting detracts from the quality of discussion on this forum and there is simply no legitimate reason to allow it (Martin's protestations to the contrary).

Now let's get back to the subject at hand.

Answered by Riversong
Posted Dec 7, 2010 9:06 PM ET



You're partly correct about the danger of kraft facing on fiberglass batts, though the same is true of conventional installations without the foam flash because paper is vulnerable to mold. But the kraft facing, if properly installed, can also reduce vapor migration into a fiberglass insulated cavity and hence reduce the likelihood of condensation and mold growth where it's most likely - on the sheathing.

But, at least, the kraft paper allows some drying to the interior, whereas poly does not. And a sandwich of two impermeable layers is always at least potentially problematic - with or without kraft facing.

Answered by Riversong
Posted Dec 7, 2010 9:10 PM ET


Would the best solution for Bill be A consistant 2" flash, followed by kraft faced batt. Then a water resistant primer to not trap, but impede moisture infiltration into the wall cavity?

Answered by Mike
Posted Dec 7, 2010 9:31 PM ET



I'm not a fan of spray foams in general and flash and batt is no improvement. At best, it air seals the stud cavities but not the plates and other junctions where most framing leaks occur. And, while it might protect the sheathing from condensation, it does nothing to protect the studs which are still thermal bridges. Fiberglass is the most worthless insulation on the market, and batts are nearly impossible to install without voids and compressions. Kraft or any other facing only makes them more difficult to install well.

If, however, you do use kraft-faced batts, then that creates the vapor retarder and no additional vapor retarder is necessary, but the drywall still needs to be airtight to avoid channel flow through the framing cavities (between electrical boxes, for example).

Blown fiberglass is a significant improvement over batts and dense-pack cellulose is an order of magnitude better. There are other, better ways to increase the walls R-value and also interrupt thermal bridging, such as interior rigid foam board or cross-hatched framing. And NEVER use a poly vapor barrier, with the possible exception of 10,000+ HDD climates.

Answered by Riversong
Posted Dec 7, 2010 10:43 PM ET


there is simply no legitimate reason to allow it

And no legitimate means to prevent it.

Now let's get back to the subject at hand.

Quite - should never have left it in the first place.

Answered by Timmy O'Daniels
Posted Dec 8, 2010 4:20 AM ET


Is it possible to contact administration?
Hope for answer

Answered by WikiFunna
Posted Dec 16, 2010 12:08 PM ET


What administrators do you hope to contact? People who administer what?

Answered by Martin Holladay
Posted Dec 16, 2010 12:10 PM ET

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