I have a home that was built in the mid 90s with builder grade vinyl windows in dire need of replacement. I have air sealed and insulated the attic and basement well I believe, but I have a number of battle to fight still before the home is where I want it to be.
If I have to replace the windows, I am looking to bite the bullet and replace the siding as well, which will allow me to add 2″ of rigid foam to the vertical exterior. The existing interior walls were unfortunately built in the following inside -> outside order…
drywall -> poly -> batting -> sheathing -> WRB -> vinyl
The question I have is, given the poly on the interior will block the majority of moisture from entering the wall cavity, is it safe to use 2″ of XPS rigid foam on the exterior?
Replies
Unless that poly is a smart membrane that allows water vapor to permeate you should get rid of the poly. Having poly and a WRB creates a moisture sandwich- if any water or moisture gets into your wall it has no place to dry out and will result in mildew, mold and rot.
The 2inches of exterior XPS may be enough or may not be enough. You need to calculate where the dew point is in your wall assembly and its dependent on where you live. I wont be doing that for you. You need someone well versed in this to help you or do more research yourself.
The poly is definitely not a new smart product. Afaik, houses were just built this way for like 30 years. We're in Rochester, NY so the poly goes on the warm side in accordance with practices back then.
From my recollection, when I performed the due point calculations for my area a number of years ago, I needed 1" XPS below grade and 1.5" above grade (I think), though I went with 2" when I insulated my basement walls and band joists.
I guess the question I really had was, if I put an adequate amount of XPS on the exterior to raise the sheathing to an adequate temperature, is the existing poly still an issue? I might as well build a new house if the only option is to remove the poly.
The existing poly becomes an issue if you get moisture inside of your wall assembly. On a humid day any air getting into your wall from outside will condense on the inside face of the plastic and those drips have no where to dry to... So rot and mold. Not definitely but maybe.
Consider a vapour open exterior insulation, this removes the "vapour sandwich" potential.
Mineral wool, wood fiber, or any other vapor open rigid insulation would be a much better choice.
I was hoping to air seal with the use of the foam board as well, but I may just need to attack it on two fronts and use mineral wool. That would bump things out quite a bit though, so I need to figure out if that screws up my soffit vent. I just replaced my roof last fall...