绝缘阁楼/ kneewalls
I have 3+ car garage with a second floor that I am finishing off as living space. The space has kneewalls ~5 feet high, and I am trying to determine the best way of insulating the attic space/kneewalls. From research I’ve done, most seem to agree that the best solution is to insulate the roof rafters (bringing the kneewalls into the thermal boundary), as opposed to insulating the kneewalls themselves. In my scenario, since the living space is above an unheated garage (which has R-19 in the walls), I have already insulated the garage ceiling/2nd-level floor with R-30. One other factor that I’m not sure of it’s impact – there will be a small hvac system (gas) in the attic space to heat/cool the living space.
虽然我能够在我的场景中绝缘阁楼椽子，但我不会通过未绝缘的膝盖陷入阁楼空间来减少大量的热量吗？所以除了在阁楼中绝缘屋顶椽子外，我还可以隔离膝盖墙吗？我读到这一切的所有文章都给出了任何一个/或场景，但没有提到两者都作为可能性。我住的地方（南部纽约），我最关心对加热的影响，而不是冷却。
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Replies
The usual idea is to insulate the rafters all the way to the exterior wall, which then ties the entire attic -- and your living space -- into the thermal envelope of the rest of the home. This assumes that the "floor" of the attic/living space is also the "ceiling" of the upper floor of the home, which isn't the case if you're over a garage.
In your case, since the garage is unconditioned space, you'll want to insulate all the way around your living space which includes those kneewalls and the floor under the living space too. What I would do is put a layer of polyiso over the back (attic) side of the kneewalls, then fill the stud bays with mineral wool. If you have the ability to put polyiso under the joists, you can use loose fill in the cavities between joists. Polyiso is easy to air seal, which is extra important with living spaces connected to a garage.
Try to allow for vent channels under the roof sheathing for the portion over your living space if you plan to build a cathedral ceiling in there. Vented assemblies are much less prone to moisture issues.
Bill
Bill - thanks for helping to clarify this for me!
Follow-up question - is using Polyiso on the backside of the kneewall a better solution than XPS or would either work? I've read that the R-value of polyiso drops significantly in cold weather.
Polyiso is usually the best choice. The R value issue has been overblown, and even when derated, polyiso is usually at R5 per inch -- the same as XPS. You loose nothing relative to XPS in cold weather, but in shoulder seasons (spring/fall), and in summer, you win with higher R value per unit thickness compared to the other rigid foams.
Bill