Hi all
I’ve been working on this new bath renovation.
I normally build my walls (exterior) from 2×6 insulated with fiberglass, then on the outside I sheath with 1/2″ ply, a layer of tar paper, then a layer of 1″ foam board and finally the siding.
On the inside, usually goes a vapor barrier then sheetrock.
I’ve also on inside walls used a layer of 1″ foam board then sheetrock and was thinking of doing it on this job.
I was talking with my tile person about the work in the bathroom yesterday and she said not to install 1″ foam board on the inside.
Her reason was that the moisture in the bathroom shower area would condense on the board and cause rotting near the floor where it would run to.
I guess I need to get opinions from the experts here about this.
In all the years I’ve installed the foam board on the outside I have never had a problem, but the inside is new to me. I’ve only done it once before.
Jeff
Replies
Sardog,
In Northern climates the foam should be on the outside or condensing wall. if the foam is sealed properly moisture cannot get to the "cold" wall to consense and fall out.. if you wanted to take a belt and suspenders approach to it you could put a little sheet of 5 mil poly to "catch" any condensation and funnel it back out of the wall cavity back into the bathroom.
First of all, where are you, climate-wise?
Second: "I was talking with my tile person about the work in the bathroom yesterday and she said not to install 1" foam board on the inside.
Her reason was that the moisture in the bathroom shower area would condense on the board and cause rotting near the floor where it would run to."
Ask her where she proposes that moisture otherwise would go and why it would condense on the foam surface and not on the painted surface of drywall. How does the foam under the sheetrock change the situation, other than to keep the sheetrock warmer in the winter (a good thing)?
Frenchy, what do you mean by "In Northern climates the foam should be on the outside or condensing wall." I don't think I'd want a "condensing wall" in my house.
Edited 9/15/2008 10:57 am ET by DickRussell
Dick Russell.
Nobody wants a condensing wall anyplace wood is, So you are correct.. However, even the best vapor barrier has the potential for leakage..
Dick
I'm in Vermont. A cold climate.
And this situation is confusing to me also.
I would think foam on both sides is ok, but I've always used it on the outside myself.
Jeff
You would not want foam on both sides of the wall assembly. That can trap moisture between. So if you already have it on the exterior, do not add to the inside. Other wise a good idea.
Frenchy is confused. A faom panel is best on the side that drives the warm wet air to keep the moisture out of the assembly, so in cooling climates like Florida, the foam belongs on the exterior, while in northern heating climates, the foam is best placed on the interior. Meanwhile, much of the country is in a moderate climate while the choice is optional.
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Hi Piffin
I don't quite get it.
Everything I've read and seen for years has put the foam on the outside.
In all my work, I've never had any problems, but then, I admit I don't often see my work taken apart for repairs for any reason.
I also admit I sure am no expert and can use all the help I can get, so I'm not afraid to ask for help.
Jeff
google up the buildingscience website and study away. Those guys do solid research and write white papers galore.You never ever in any climate want to have a VB on both sides of a wall assembly.Which side the foam is on depends on which way the warm moist air is moving. You want the first surface it hits to be the foam and VB. If you are moving bathroom air to th eexterior, and you have sSR over no VB and FG batts with foam on outside, the moisture will move thru the batts and stop at the exterior, condensing. Now you have moisture INside the wall assembly.But if you have the foam on the inside, It becomes the VB, The moisture never gets into the wall and if there is condensation, it is on the surface of the SR, where it can dry again.When the foam is on the wrong side and no problems ensue, one of three thing, or a combination of all three luckily have been in place...The VB has enough integrity that no moisture gets into the wall cavities.The insulation board is thick enough that now dew point exists and the moisture can move back out again in its seasons.CRS disease took the third one away from me....LOLGood night all.
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"Her reason was that the moisture in the bathroom shower area would condense on the board and cause rotting near the floor where it would run to."
For tile/wet areas, you should have an 8 mil poly moisture barrier behind the cement board anyway.
Jeff
Jeff
Would it hurt to have a double barrier?
Jeff
It could if they are separated.
That's why I mentioned the moisture barrier behind the cement board for wet tile areas - it's required.
Jeff