Wall Construction With Exterior Rigid Foam Questions
Long time reader, first time poster. I thank all those involved with GBA for the resources they provide.
-SW Ontario (Zone 5/6 border so we’ll go with 6)
-2 storey house (approx. 2500sqft total, even split per floor)
-standard 2″X6″, 16″ OC frame construction
-Zip sheathing planned
-10″ thick basement walls
-2″X4″ sill plate
-4″ of surplus polyurethane rigid foam insulation against the Zip sheathing (I understand polyu to have a similar chemical formula & insulating properties to polyiso)
-2″ of surplus XPS rigid foam insulation on the exterior of the polyu
-1″X4″ vertical furring strips (12″ GRK screws)
-insulated vinyl siding
-perforated metal J-channel approx. 7″ deep to help keep out bugs, support foam, support furring strips, etc. at the bottom
-flashing between exterior of Zip sheathing and top of basement concrete wall
-Membrain or equivalent interior wrap( ie. not “regular” poly)
1 – Are there any foreseen issues with that construction methodology? I’m using surplus polyu simply because it’s quite cheap in my area ($0.33CDN sqft for a 4″ or 6″ thick piece 6′ to 8′ tall).
2 – Would the 2″ XPS be necessary? At 6″ thick, I calculate the absolute worst case scenario at an R-value of 12 (6″X2R per inch) based on the GBA article on sheathing, rigid exterior foam, and the dew point. Since 11.25 is required, I’d just squeak by (notwithstanding the most recent article on less being passable). I’m more concerned with the polyu becoming waterlogged/severely underperforming in the cold.
3 – Is the 2″ XPS recommended? I have a large amount of surplus XPS. Placing the 2″ outside the polyu is do-able from my perspective as I’m doing the installation myself. I prefer, unless told otherwise, having the XPS as it will help to serve as a WRB (*only to the polyu*) when tapped . Ie. the XPS will help keep the polyu from taking on water. Are there any technical reasons NOT to put up the XPS on top of the polyu?
4 – Would the J-channel need to be elevated slightly above the top of the foundation walls? Ie. 1/8″ or so? The window boxes will be flashed to the exterior XPS and the XPS itself will have its seams sealed with tape. I doubt there will be much water able to get into the polyu. I also feel that rainsoaked polyu would be able to drain even if the J-channel was resting on the concrete.
5 – Are longer GRK screws required or recommended? At a 10degree inclination, I’m calc’ing over 1.5″ of penetration into the 2″x6″ (plus, the 1/2″ of the Zip) and most of the documentation I’ve read recommends at least 1.5″ penetration for vinyl siding, 16″ OC. Any “real world” experience to back that up out there? Since I’m over 4″ of foam I’m guessing that I’ll need to consult an engineer.
6 – Any preference between GRK and Headlok screws using 1″x4″ furring strips?
7 – Any problems using polyu as the rim joist insulation? I’ll be using the “peanut brittle” or “tight fit & caulk” method and consulting my building official over what he wants in regards to fire code. I’ll likely have enough XPS scraps left over that I can use that if it’s a far superior choice.
Insulated vinyl siding was only chosen for aesthetics (my wife hates the way some vinyl goes “wavy” when installed; the budget cannot afford brick or stone veneer and I, surprisingly, have found quotes for Hardy board higher than expected in my area). I’m not counting it to actually insulate anything.
Thanks in advance for any discussion.