Yesterday I was doing a home energy rating for quality assurance in Augusta, Georgia, and the house I was in had a bonus room. Bonus rooms are often the least comfortable room in a house, and the main reason is that they are the most flawed part of the building envelope, especially the attic kneewalls. In the photo at left, the kneewall is the short wall with the door in it.
The problem with many kneewalls is that they have fiberglass batt insulation with nothing covering them on the attic side, as shown in the photo below. What happens then is that the batts don't make contact with the air barrier (the drywall), air moves through and around them, and they fall out of the attic kneewall. In other words, they're pretty much worthless because of poor installation. It doesn't matter who made the batts.
Georgia recognized this problem about a decade ago and started requiring all attic kneewalls to have sheathing on the attic side and to be insulated to at least R-18. The best way to provide that sheathing is to use a rigid material. I've seen OSB (oriented strand board, the flaky plywood), foamboard, and structural or non-structural cardboard sheathing materials like Thermo-ply.
I've also seen attic kneewalls sheathed with non-rigid materials, mainly housewrap. The house I was in yesterday had housewrap on the kneewalls, and you can see it above. It's possible to use housewrap here and do it well. On the left side of that photo, however, notice the opening around the light switch.
On the other side of that bonus room, the kneewall was in much worse shape. It looks like the installers just shoved it in the direction of the kneewall. Notice the batt that has fallen away from the wall in there, too. It's right above the yellow part of the housewrap. This clearly will lead to extra energy use for heating and cooling and possibly callbacks from the new homeowners who find this room to be the least comfortable place in the house.
I'd prefer to see a rigid material on attic kneewalls. Both can be done poorly, of course, and I've seen both done poorly. I think the rigid material will do a better job with air sealing and will stay intact longer. Here are the three keys to sheathing an attic kneewall properly:
- Cover the whole wall.
- Seal all the edges, penetrations, and seams.
- Sheathe the kneewall during framing.
Doing things in the right order is key, no matter what material you use, though. Sheathing the kneewalls should happen during framing, before the electricians, HVAC contractors, and other trades get in there. Then, anyone who cuts a hole in the sheathing is responsible for sealing it. Do that and you avoid the mess you see with the housewrap above and instead end up with something like the kneewall below, which I saw during another HERS rating for quality assurance, this one in Nashville.