Combined permabilty of 1" unfaced (expanded) EPS and 3/4" extruded?
I am residing a 30 year old house in North Jersey. In order to match the exisiting house I need to add about 1 and 3/4 inch filler over the 1/2 plywood sheathing to some exterior walls before reinstalling vinyl siding.
Over the 1/2 plywood, I have been using using 1 inch unfaced expanded poly (i remove the plastic backer as I dont know if that is permiable) then use 3/4 extruded poly (I punch about 100 nail holes first to ensure it breaths)
THere is no vapor barrier inside other than standard r 11 fraft faced insulation
So my questions are,
1-do I have enought permabilty to prevent any condensation / mold issues?
2-Do I need to perforate the Extruded Poly
3-is it necessary to remove the clear film on the expanded poly?
Asked by Anonymous
Posted Wed, 03/10/2010 - 11:09
Edited Wed, 03/10/2010 - 11:23
Posted Wed, 03/10/2010 - 11:09
Edited Wed, 03/10/2010 - 11:23
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Ken,
The combined R-value for 1 inch of EPS and 3/4 inch of XPS is about R-7.7. In climate zone 5, where you live, all you need is R-5 or greater to take away any worries about condensation in your wall cavities. Obviously, your wall cavities can dry to the interior.
You're making a mistake to punch holes in your foam sheathing. Put away the 16-penny nails and stop abusing a perfectly good building material. If you want to remove the plastic film on your EPS, go ahead.
Posted Wed, 03/10/2010 - 11:22
To prevent mold and other moisture problems, it's more important to control indoor relative humidity. Kraft facing is a vapor barrier (class 2, less than 1 perm), and plywood and XPS are also about 1 perm. So your wall system will restrict vapor diffusion and allow some minimal drying in each direction. Preventing exfiltration of warm winter air or infiltration of humid summer air are more important than the vapor permeance of the assembly.
I agree with Martin - there's no need to punch holes in the XPS. It really won't change the vapor permeance unless more than 50% of the area is holes.
Posted Wed, 03/10/2010 - 16:32
Thank you gentlemen. This helps
Posted Wed, 03/10/2010 - 17:14