Fill stud cavity with fiberglass insulation?
seabornman
| Posted in Green Building Techniques on
I am finalizing a house design in zone 5. Exterior walls to be insulated with R10 XPS exterior of the sheathing and R13 fiberglass in the wood stud cavity. Peel and stick will be directly applied to sheathing per REMOTE style walls.
My concern: wood studs will be both 2×4 and 2×6 construction, with 2x6s where walls are tall (10 ft.). Is R13 insulation pushed towards exterior (I’ll have to figure out the best way to do that) OK or should I fill the cavity with insulation in the tall walls? Will ratio of R19 to R10 work re: moisture control?
Thanks in advance.
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Replies
Go ahead and fill the cavity in the 2x6 walls. The extra cost will be minimal, you'll get a higher R-value and you'll avoid problems with unimpeded convection in the empty space. And in zone 5 you are fine with 2" of foam on the exterior of a 2x6 wall as far as moisture control. (See https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing for more detail on that.)
But if you haven't bought the foam yet, consider EPS or polyiso instead of XPS. XPS in North America is made with a blowing agent that has a massive climate impact, more than 1000X worse than CO2. EPS and polyiso completely avoid that. XPS has slightly higher R-value per inch than EPS, but that advantage decays over time, whereas EPS is more stable. If you want higher R-value than EPS offers, you can use "Neopor" graphite infused EPS. Or you can use polyiso, which is more expensive, but you can often buy reclaimed polyiso for cheaper than XPS.
Joel,
Charlie gave you good advice. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.
Use something denser than R19s for the cavity fill though- an R19 only performs at R18 when compressed to 5.5" in a 2x6 cavity. It has the same amount of fiberglass as an R13, but is manufactured at a higher loft, which gives it very low resistance to convection or infiltration air movement. Damp sprayed cellulose (or dry blown) would deliver about R20 @ 5.5", and both R20 or R21 fiberglass would perform at spec, as would R23 rock wool.
With R10 on the exterior you're good up to at least ~R27 cavity fill from a dew point control point of view. Even with R23 rock wool you have sufficient margin to use standard latex paint as the only interior side vapor retarder.