Foam Insulating the roof on an old home
truckaxle
| Posted in Energy Efficiency and Durability on
Remodeling an old house in Climate zone 5 (Boise Idaho). To this point, the house never had any insulation. Recently opened up the eves to make more space and I am now wondering the best way to insulate.
I have a bid to place open cell foam but from reading here and elsewhere it looks like closed cell foam is recommended and less risky. Is that the current consensus?
I only have about 4 inches of room… should I build out the 2x4s with say 2x8s running alongside the existing rafters and get more insulation?
Any recommendations are greatly appreciated.
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Replies
Truckaxle,
If you haven't done the roof yet, your best option is to install rigid foam above the sheathing. See here for more: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing
You can also install a combination of insulation materials above and below the sheathing. In addition, it is important to air seal the structure as much as possible.
If doing it all from the interior side, 3" of HFO blown closed cell foam delivers about R20, leaving you an inch of additional depth on the full dimension 2x4s. Installing 2x6s perpendicular to the 2x4s would add another 5.5" of depth, for 6.5" total. Installing 6.5" of cellulose would add another R24, bringing the center-cavity R up to R44-ish.
That puts 45% of the total R outside the foam/fiber boundary, which is fine from a dew point control point of view, and it'll be close to the code-min performance on a U-factor basis due to the lower thermal bridging, since the rafters don't fully penetrate the insulation except at the intersections of the 2x4s and 2x6s.
Alternatively, installing 6.5" of 1.8lbs density blown fiberglass (Optima, Spider, L77 etc) adds R27, bringing the center-cavity R up to R47, and guaranteed to meet code on a U-factor basis, and still a high enough R-ratio to have a bit of dew point margin.
If taking an exterior rigid foam approach, install 4" of roofing polyiso (R23) and filling the 2x4s with 1.8lb fiberglass (R17) comes to R40, but with the full continuous R23 thermal break over the framing it'll be pretty close to code on a U-factor basis. If you made that 5" of roofing iso (R29) , for R46 at center cavity it would beat code min performance with margin.
OK thank guys. At this point I am working from the interior only. Some day I will replace the roof and add rigid on the exterior.
Dana if I understand you correctly you thinking close cell against the roof deck and then cellulose to the interior of that via a built out rafters?
That's pretty much it. But rather than rafter-extenders (which create a continuous thermal bridge of R1.2 /inch wood through the much higher R/inch insulation) make them perpedicular to the existing rafters, "Mooney Wall" style:
https://www.greenbuildingadvisor.com/sites/default/files/Mooney%20wall%20detail.jpg