I’m not sure I understand the use of foam sheathing on exterior walls and why you need to have enough R-value to keep the plywood above the dewpoint. Don’t you have the same problem, only more pronounced, with no foam sheathing? Or is the problem that the foam sheathing prevents the plywood condensation from drying to the outside?
What about just preventing moisture from entering the wall at all? Could this be achieved with a layer of foam board on the interior, under the sheetrock?
Thanks in advance.
Replies
Yes. the issue is that foam board is too restrictive of moisture flow to allow materials to move moisture to the outside.
Yet it does not perfectly block moisture.
So in a heating situation, it is not enough of a vapor barrier to let you discount the water which would condense inside the wall in some cases.
OK, thanks UncleMike.
I'm in zone 5 and I see that I need R7.5 to prevent condensation. If I used 1" on the outside of Zip Wall could I then put strips of 1/2" or 1" foam in the stud bays against the Zip/OSB on the inside? Will that foam sandwich keep the interior facing of the foam board above the dew point and prevent condensation?
Structure is all framed and trimmed and going to 1 1/2 foam on exterior and then furring stips before siding messes it all up. Gable rakes were done allowing for 1" foam. I was planning to nail cedar clapboards right onto the foam. I'm thinking I'll use a vapor retarder paint on the sheetrock and monitor relative humidity in the living space during cold season.
Will this work? Is there a better way? Would a membrain layer somewhere help?
Thanks for any info. Much appreciated. The structure was raised before I became aware of these issues.
I think you are on the right track with the thicker layer of foam on the outside. Pay attention to the R values of various foam options as you choose (if it is not already up)
On other thing you can do is make sure your moisture sources in the house have adequate ventilation to avoid over-taxing your vapor control measures on the inside surface of the walls. make sure any penetrations into that wall are sealed. (switches, outlets....)
The use of foam sheathing on exterior walls serves to prevent issues related to condensation and to meet necessary R-value requirements. While it might seem counterintuitive, the purpose is to avoid moisture-related problems, including preventing plywood condensation. Using foam board on the interior beneath the sheetrock could be an alternative to address moisture, but it's essential to consider the overall building science and climate conditions for effective insulation and moisture control.
" I was planning to nail cedar clapboards right onto the foam". This is not good practice as foam board is not a WRB and will eventually allow water into the sheathing and framing. Best to seal all foam bd joists and then install a product such as "Hydrogap". The clapboard nails would need to be long enough to penetrate into the studs.