Retrofit exterior insulation can’t be continuous
I’m re siding my 1953 house (south dakota, zone 6a) I’m having them add 2″ of eps and a rainscreen along with the LP smartside. I just realised I’ve been reading about how important it is for the insulation to be continous but mine wont be. The basement is mostly below grade but the tops of the basement walls go above grade for a foot or two. The front and one side of these foundation walls have decorative rock, the other sides are just normal. I wasnt intending on covering the two bare sides of foundation or the decorative rock. in addition I also have concrete steps that dont have enough wiggle room for insulation to fit between them and the house.
my house has no insulation in the walls of the main floor except for in the bathroom a previous owner remodeled. I’m adding in r13 kraft faced fiberglass to all stud bays then covering with osb, 2″ eps, tyvek drain wrap, rainscreen then the siding.
-Will the R value of that wall be lowered because of the exposed foundation below it?
-How much lower R value would it be?
-Should I add more insulation to help cover any r value losses I have?
-Or even if it is less R value is it really going to be just fine as is and I will be patting myself on the back all next winter?
-Off subject question- exterior insulation & rain screens are both very rare in my area so my crew is totally inexperienced. I’m thinking I should add an additional layer of housewrap over the osb sheathing as insurance incase of air barrier failures happen in the taped foamboard or tyvek drain wrap? -What kind would be a safe and hopefully inexpensive choice?
-Any glaring errors in my plan you would point out and/or extra advice/opinions are extremely welcome.
Thank you GBA, you are amazing!
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Replies
"Will the R value of that wall be lowered because of the exposed foundation below it?"
The R-value of the walls themselves won't really be lowered, but total heat loss may be affected. Think in terms of heat paths. It won't be all that effective to just up the R-value in one location (your wood walls) because it is lacking in another (foundation walls). In other words, you should be considering insulating your basement if it is not. You don't have to do it from the outside:
https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
There may be a thermal bridge in the area of the sill plate where the insulation transitions from being interior to exterior, but it's better than not insulating the basement at all. Be sure to get the rim joist insulated and well sealed.
You may want to consider making the OSB your air-barrier by taping that.
As for 2 housewraps: I'm not sure its a big advantage. Consider having them put the WRB over the taped OSB like they normally would (high chance of success) then don't worry about having anything over the EPS. (Depending on how they're flashing the windows) Or alternatively, keep it exterior to the foam, and consider the foam your secondary WRB. But the most robust air-barrier, in either case, will be created by taping the OSB seams.
https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go