Renovating 2×4 walls and roof insulation options
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I’m in the 5a climate zone. Hot humid summers, cold for 3-4 months – with cool lake breezes and lots of lake-effect snow. Just bought a fixer-upper we wish to retire in. House is one story 2×4 walls that currently have paper-faced somewhat ratty looking rockwool insulation, no interior vapour barrier – with drywall that I plan on leaving intact as it’s in great shape.
The roof is 2x 4 rafters, with a few inches if fiberglass in the ceiling. I spray foamed (2 lb) the crawl space last year.
My wall renos will take place from the outside,with the old windows and wood siding , and (possibly) the 1×12 horizontal spruce being replaced.
My original thought is to rip off the outside, including 1×12, remove the rockwool, and having 3 – 3 1/2″ of (2 lb) closed-cell spray foam in the wall cavities – then an additional 3/4″ of rigid insul. board, along with housewrap before some kind of vinyl siding.
For the roof we plan on a shingle tear off, replacing with steel roofing (hopefully saving the old 1×12 decking) and spray foam underneath. Part of the roof we intend on tearing out drywall and having a cathedral ceiling. I would have an unconditioned roof. The steel is to eliminate snow build up as I just dont trust 2×4 rafters – even though they look fine after 40 years.
Its a little expensive to spray, but I’m guessing 2″ rigid would take a lot of build outs around windows/ doors- time is money and it is quick, also its our retirement home so we want it done right. It doesn’t have to be to code- just done right.
My questions include:
Does the combo of spray and rigid make sense? I’m hoping for the right combo of efficiency and ensuring vapour / mold does not become an issue.
What type if rigid foam is right for the job?
Can I tack a 2×2 (or any better idea?) to the underside of 2×4 rafters to make them thick enough to spray 4-6″ of spray foam under the roof deck?
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Replies
Stu,
First, I will address the question you didn't ask -- about your 2x4 rafters ("I just don't trust 2x4 rafters").
You need to hire an engineer to evaluate the rafters, and you need to follow the engineer's advice.
Q. "Does the combo of spray and rigid make sense?"
A. No. You don't want to encapsulate your sheathing (whether board sheathing, plywood, or OSB) with vapor-impermeable foam on both sides. If you like vapor-impermeable foam, choose one side of the sheathing to install it -- either interior or exterior -- but don't install it on both sides. (My advice: install the foam on the exterior, not the interior.) For more information on exterior foam, see How to Install Rigid Foam Sheathing.
If you really want some type of foam on both sides of your sheathing, choose open-cell foam between the studs. Even better: choose dense-packed cellulose.
Q. "What type if rigid foam is right for the job?"
A. Polyisocyanurate is considered the most environmentally friendly foam. It also has the highest R-value per inch.
Q. "Can I tack a 2x2 (or any better idea?) to the underside of 2x4 rafters to make them thick enough to spray 4-6" of spray foam under the roof deck?"
A. Yes (assuming that your engineer OKs the use of 2x4 rafters). For more information on how to insulate an unvented sloped roofing assembly, see How to Build an Insulated Cathedral Ceiling.
If you have the space using the existing 2x4 as the outer chord of a truss, using 1' sections of 1/2" OSB or ply screwed and glued to the side of the 2x4, and finger jointed 2x3 as the interior chord can add a large amount of structure with limited thermal bridging.
If you're going with more than 2" of 2lb foam it's advantageous to use semi-open cell water blown goods than standard HFC blown goods, for two reasons: It has a higher vapor permeance, which gives the roof deck a fighting chance to dry, and using water as the blowing agent doesn't have the fairly heavy global warming impact of HFC245fa (the most commonly used blowing agent for closed cell polyurethane.) HFC blown goods have a vapor permeance of about 0.8-1.2 perms at 1" thickness, which drops to about 0.3 perms at 3.5". At that permeance your drying times ends up being measured in months.
To date the only intermediate-permeance foams that fill the bill are both from Icynene, and both run about R5/inch. The higher-perm 2lb version is MD-R-200, which only drops below 1-perm at 4". The slightly denser 2.2.lbs stuff is MD-R-210, which drops below 1 perm at about 2.5". Either would be sufficiently low permeance at 3.5" to be protective of your roof deck ( even without exterior foam) while retaining reasonable drying capacity for a zone 5 climate. The addition exterior foam over the roof deck you would give you even more margin by running the roof deck at a warmer temp.
If you truss-out the rafters to provide more insulation depth 1-2" of HFC blown goods on the roof deck with the rest of the depth filled with high density blown fiber (3lbs cellulose, or 1.8lbs fiberglass) would still work without interior vapor retarders on the cathedralized ceiling section.
You could also do a full cavity fill with half-pound open cell foam, using Certainteed Membrain or Intello Plus "smart" vapor retarders on the interior. (Or if you promise to keep the interior RH under 35% during the winter, even spray-applied vapor-barrier latex is good enough, even though it's still 5 perms when applied directly to foam, as opposed the the 0.5 perm performance it hits when applied to paper facers on wallboard.)