Advise asked on basement wall insulation and fire blocking

[North90]

Hi!
I am in Saskatchewan, Canada (the cold part of Canada). I am in the process of preparing for finishing the basement of our 9 year old house. I would like you pro's to review my design for construction of my exterior basement wall finish. Also the fire blocking....

There are no signs of water problems and there is a working system of exterior drain tile connected to a sump pump.

I want to insulate the exterior walls with R20 and put some insulation on the concrete floor to keep the feet warm. There is no existing exterior insulation, the basement is about 9ft high between concrete floor and bottom of the floor joists. Grade is about 6ft above the concrete basement floor.

I want to use rigid XPS foam for insulation. 2" straight on the wall and extended into rim joists. Then a 2x4 wall on top of 1" foam of the floor, spaced about 1/2" from the foam on the concrete wall (walls are not flat).
Another 2" foam in between the studs of the 2x4 wall, no vapour barrier. A suspended ceiling and mineral whool in between floor joists will be added.

Any comments, problems or ideas for improvement?

thanks a lot!
Frank

 

[Dana]

From a moisture transport point of view there's an issue with putting fiber between the XPS and rim joist & foundation sill, since any air leakage into that area from the interior would end up delivering moisture into the joist. It's hard to make air-seals perfect and REMAIN perfect over time. It's better to put the foam against the band joist & foundation sill, and seal it in place with heavy beads of expanding foam, then put some fiber insulation on the interior side. To ensure a good air seal, cut the foam with about a centimeter gap on all sides so you can get the nozzle of the can-foam in there.

This diagram is almost right in terms of how to cut-in the foam:

If you have a ledge of concrete between the band joist & basement, filling in the ledge with some 1.5" foam, then putting a layer of foam on top of that that extends all the way to the band joist works better. You then can cut in the band-joist foam block and still get the can-foam nozzle in there to air seal it.

In Saskatchewan as long as the foam-R is at least 40% of the total R, you'll have sufficient dew point control at the foam/fiber boundary that it won't need more than latex painted wallboard for an interior side vapor retarder. This is consistent with the IRC 2012 chapter 7 prescriptives for above grade walls with class-III vapor retarders, and with much of the wall being below grade you'd have even more margin. (Southern Sask is equivalent to a US zone 7 climate, and way up north it's comparable to a zone 8 climate. Refer to the prescriptives for 2x4 framing in zones 7 & 8.) So, with tightly sealed R10 XPS, you could fill in with R15 rock wool and it would work just fine with the rock wool on the interior side. If the fiber is more than R15, you need more foam-R.

The rest of the assembly looks fine. Be sure to tape all the seams on all foam layers with housewrap tape (overpainted with duct mastic if you want it to still be air tight in 50 years)

In your climate polyiso has severe cold-temp performance derating, and wouldn't be the best choice on the cold side of the assembly. EPS & XPS gain performance as the average temp of the foam drops, and are the right materials to be thinking about here.

XPS delievers a fairly substantial greenhouse gas hit compare to EPS, since XPS is blown with HFC134a (about 1400x CO2), whereas EPS is blown with pentane (at about 7x CO2). When the foam is the primary insulation, it's nicer for the world to go with EPS instead of XPS.

Rock wool has an even lower environmental impact, and should be considered even for the lower portion of the wall. If you went with 3.0" of EPS (R12-ish), and insulated the studwall with 3.5" rock wool (R15), the "whole wall" R after factoring the thermal bridging of the studs will be about R21 (with 16" o.c. stud spacing) or R22 (24" o.c. spacing). At center cavity you'd have R27 for a total and the R12 foam would be fully44% of that total, which gives it adequate dew point margin for using latex painted wallboard as your interior vapor retarder.