Foam and Siding over Existing T1-11 Siding
Location Zone 3A OKC, OK. House constructed 1979.
Wall details
-5/8 Sheetrock
-2/4 Wood Stud Fiberglass batt (Unsure if faced or not. I know areas I have demoed in interior walls separating the garage space have been but have not looked at an exterior wall)
-Denny board sheathing foil faced on both sides. ( They do not make this any more and can not find specs on it. Closest thing I can find is Thermoply. It seems similar in every way. Thickness, material, and in the way it overlaps.)
-Siding is a mixture of brick, T1-11 vertical panels, and tongue and groove installed diagonally. I have attached pictures for reference.
My concern and questions are over moisture.
1) I have to do something with the exterior siding and brick. I brought in a siding contractor and he suggested placing rigid foam over the existing siding then adding a drain matt and residing with Diamond Kote, LP, or Hardie. My concern is that because of the foil faced sheathing I might be creating two vapor barriers on the exterior side and this will create moisture issues with the T1-11/tongue and groove siding.
2) My second concern deals with the interior of the wall. Because the exterior sheathing is foil faced on both sides should I be concerned that the house can not dry to the exterior? If so what can I do to mitigate the risk? Install a dehumidifier perhaps?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
To my eye, that is a great looking house.
I understand you want to make it better but it seems to me you have a fairly good wall. If we guess you have R4 continuous exterior insulation in place and R 10 in the wall or about R14 or so. To see any measurable improvement, you would need to end up over R28. Starting a R14 there is just not that much energy loss to be stopped and replacing the siding and adding that much insulation to a wall is very expensive in fact my guess is the return on investment is never.
Consider spending your efforts at making the existing walls as air tight as possible. Caulking is cheep and you will make the house more comfortable while lowering the fuel bills.
Have you had a blower door test?
Walta
Deleted
Walta thanks for the response. Sorry I guess I wasn't very clear. This isn't a question over energy efficiency. They are not adding the foam for R value. Its 3/4 so the R Value in minimal. They only do it to flatten out the surface to apply the Diamond Kote 5D building wrap. The siding needs replaced because t1-11 is a terrible product and there is no good way to keep water out of siding when its run diagonally like it is. I wish I could just paint and caulk but it not really an option. Trust me I wish it could be saved but it just wasn't properly maintained over the years. It already has rot in multiple areas. So those areas will be replaced with a plywood.
My energy bills are actually significantly lower than the majority of my neighbors. My 45-year-old single pane windows will be replaced when I do my siding. That will offer me the biggest improvement.
So, I am guessing the plan is to bury the rotten T11 and use the foam to give you a flat surface for vinyl siding. If you want to avoid adding another vapor barrier, use something like plywood or rockwool.
Walta