Is a vapor barrier code-required or necessary in walls insulated with sprayed-in foam, applied by a professional installer?
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This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
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As a first time poster Welcome to Breaktime.
This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
Edited 7/13/2008 9:16 am ET by rez
"Is a vapor barrier code-required or necessary in walls insulated with sprayed-in foam, applied by a professional installer?"
Should not be required. That would be silly.
Why is it silly? A local contractor said many building inspectors require vapor barrier over sprayed foam because some foams do absorb vapor. Apparently there is a difference in the permeability of open vs closed foams, vegetable based foams and ico-based foams, etc. And some building codes are outdated when it comes to sprayed in foam. I'd like to learn more about this, rather than have my question called silly.
He didn't say your question was silly, he said the concept of putting a vapor barrier over foam was silly. And I agree.
High density foam (2lb foam - has an R value of about 7/in) is a vapor barrier at 2". No other vapor barrier would be required and would be a total waste of time. This foam is expensive, very expensive.
Lower density foam (like 1/2 lb foam - has an R value fo about 3.5/in) is not a vapor barrier. It happily absorbs moisture and --- here's the key --- gives that moisture back off without any degradation in foam. If fiberglass gets wet - it's dead and mold starts growing. Not so with foam. So I consider a moisture barrier on foam a total waste. I live in NC - we have a hybrid climate, so any moisture barrier you put in is on the wrong side 1/2 the year. So I sprayed open cell foam and no moisture barrier.
As an interesting example, I had left one bay of a gable end truss unsprayed so I could route my kitchen vent out later. I put a piece of pink foam in the bay to plug it up, but left a couple of inches air space behind the foam. A couple of weeks ago when it was cold here (about 20), I took the foam out to put in the vent. There was serious mold behind that foam and the OSB was saturated. No problem I understand what happened - i got indoor air working it's way behind that pink foam and it condensed on the cold sheathing. But here is the interesting part. Some foam had snuck into that bay under the sheathing. Just a little piece and only about 1/2" thick. I scraped it off and immediately under the foam there was absolutely no moisture on the sheathing. So I was sold on no vapor barrier.
MERC.
That is an interesting test there you had going on with the moisture behind that chunk of foam you put in. I was wondering if you could tell me how thick of open cell foam did you have sprayed and did you have it sprayed right to the roof deck? What do you think of the performance of the foam since you had your house done?
My roof was sprayed "nominal" 5". Therefore, some areas are 8" and some are 3". I actually probably average to more like 6", but I really don't care. The performance of the house is great. I sprayed 1/2 of the first floor walls and the entire 2nd floor. Not too many walls on the second floor since my roof is 12/12 framed to the floor w/ some dormers and cross gables. I had the guy spray all the way down the roof and right onto the floor (that joint is generally hidden behing kneewalls).
At any rate, performance....I've been really impressed. The house temperature is a lot more stable, so we keep the temp set 2 degrees lower than we used to. Presumably this is because the house isn't drafty. I haven't drywalled the 2nd floor yet, but it's open to the house via the stairwell. I have no HVAC up there. I am heating the house with the same furnace I had when my house was 1/2 the size it is now. So I'm heating 4800 sq ft with a 125k BTU furnace and it doesn't run that much. It might be on 25% when it was 15 here (that is about as cold as we get). I can't say enough good things about it. I wouldn't hesitate to do it again.
I actually think the foam paid for itself right away because I get to zone my existing hvac instead of installing a whole new system. I'm up to the task of zoning a system, but a new system I'd have to hire out. So that's a pretty big bill I just dropped because of the foam.
MERC.
If the building inspector really isn't willing to understand and demands the vapor barrier, just put it up, inspect, and tear it down.
"I'd like to learn more about this, rather than have my question called silly."
It's a good question. I meant any code that requires this is silly, and myopic. The principle advantage of foam is that it forms a continuous and monolithic air/vapor barrier. Sure, open cell foams are permeable to some extent, but this is not of great consequence. A lot of focus is placed on 'vapor' barriers, perm rates, and such, when the bulk of moisture problems are actually caused by air leaks into and convective currents within the wall cavity. Since the foam fully displaces the cavity, these problems are not present.
In many cases, vapor barriers can cause more problems that they solve. Hence the new thinking with 'airtight drywall approach', foam, and other methods. Codes are often behind the curve on the latest building science.
Doug- you are quite correct in noting the differences in the various foams. I am starting my own Polyurethane Spray Foam business this month and will use 2 lb density closed cell foam. I feel this is the best performance insulation on the market. And to answer the question, no vapor barrier should be required. I've seen US dept of energy documents quoted as saying just that, "PSF is an effective vapor barrier...". That evidence should be good enough for the inspector.
BTW, the bio based foams (soy, etc. ) are only 10-20% bio by volume. That is more marketing than anything else. The big difference is the density, 1/2 lb vs. 2 lb in general.
Not all local codes enforcement officers know anything about foam and still try to enforce outdated standards for old insulations on th foam work.
Not all foams are VBs themselves, but there is a good theory of practice that there is not really a need for a VB in the open celled foams if they are installed thick enough. It is not just the presence of moisture in the air that causes the condensation that a VB is intended to stop. It is when that warm moist air runs into a cold surface that condensation happens.
With a thick open celled foam, the oisture can move through it, but at such a slow rate that it never runs into a dew point situation, and the moisture will also dry out again when it is not trapped by a VB.
I would educate the inspector with studies from the foam people if it were a closed foam, not telling him hiow silly he was being, but I might go ahead and acrefully cover an open celled foam, depending...
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