Solving Comfort Problems Caused by Attic Kneewalls
Insulation and air-sealing are key
In Texas, they call them “hot walls.” My friend Mike Barcik likes to say they’re what separate you from the blast furnace. Down here in the warmer climate zones, where attics get up to 8,000°F (well, that may be a slight exaggeration), many people call them a liability. (Sadly, architects haven't gotten the message.)
In the world of building science, we call them attic kneewalls — walls that separate conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. from an unconditioned attic. They’re also the cause of many comfort problems, especially in bonus rooms.
What’s wrong with attic kneewalls?
People who have a bonus room find that it’s often the least comfortable room in the house. Why? The main reason is that they’re the most flawed part of the building enclosure, especially the attic kneewalls. In the photo above, the kneewall is the short wall with the door in it.
Now, an attic kneewall has to perform better than an exterior wall in summer because there's a bigger temperature difference across it than there is across an exterior wall. If it's 95°F outside, it could be up to 130°F in the attic. Since heat flow increases when you have a higher temperature difference, you can see why attic kneewalls must perform better to keep the house comfortable and efficient.
Unfortunately, if you have a bonus room or FROG (finished room over garage), you may be aware that rooms with attic kneewalls are often a liability. Many of bonus rooms have surfaces that are nearly all part of the building enclosure, allowing for more heat loss or gain than a typical room in the house. (See Figure 2, below, for a diagram of the heat flows in summer.) If you're adventurous, you may have even made a little visit to the other side of those walls and seen horrors like I’ve shown in Figures 3 and 4 below.
Bonus rooms have a number of problems, and kneewalls are often one of the biggest. Kneewalls also occur in other parts of homes, especially in larger homes. We did some quality assurance on a home recently that had over 1,000 square feet of kneewalls. So, let’s look at the main problems with kneewalls now:
- The fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. has fallen out or is displaced from the cavity.
- The insulation doesn't fit properly or completely fill the cavity, allowing attic air to be in contact with the drywall.
- The insulation doesn't have any attic-side sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
- Although not strictly a kneewall problem, floor joists running below the kneewall often lack any kind of block or air-sealing.
Where the first problem occurs, the insulation is doing nothing. It may be rated R-13 or R-19, but it performs as R-0 because the unconditioned air goes right around it. The second problem is similar, but if the insulation does make good contact with the kneewall, you get a little benefit. The third problem, lack of sheathing, degrades the performance so that an R-13 batt may perform like an R-7. Fiberglass doesn't stop air movement, which is why the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. comes from testing done on fully encapsulated batts. The fourth problem, lack of blocking between the joists, means that there’s nothing to stop hot or cold attic air from getting under the floor of the conditioned space. (See Figure 5.)
If you think I had to look hard to find photos like those two shown with problems, think again. I've got photos like this from a majority of the houses with attic kneewalls that I've been in. Unless the builder certified the house in a program that requires attic-side sheathing or someone fixed the problem later, this is the standard practice for insulating attic kneewalls, at least here in the Southeast.
How to do kneewalls the right way
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), 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. (See Figure 6.) It's possible to use housewrap on kneewalls and to 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. (See Figure 7.) It looks like the installers just shoved it in the direction of the kneewall and hoped the inspector wouldn’t look, notice, or care. 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 room in the house.
I'd prefer to see a rigid material on attic kneewalls (Figures 8 and 9). 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, though, 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 for new construction.
Another word of caution here: A lot of times, framers don’t use top plates for kneewalls. They just nail the studs to the rafters instead because it’s easier and faster. For new construction, you’ve got to make sure the build a proper kneewall by including a top plate. If you’re fixing an existing home, you’ll have to put some kind of sheathing at the top of each cavity as well so the insulation will be completely encapsulated on all six sides.
Do it right
Doing things in the right order is key, no matter what material you use. Sheathing the kneewalls should happen during framing, before the electricians, HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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 (Figures 7 and 8), which I saw while doing quality assurance in Nashville for our friends, E3 Innovate.
There you have it. Attic kneewalls often cause big comfort problems in homes. Getting them right is fairly straightforward but takes time and attention to detail. The results are worth it, though, in happy clients, lower energy bills, and personal satisfaction that you’re doing great work. Now, if we can just get designers to cut back on the amount of kneewalls to begin with!
- Energy Vanguard
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