What Architects Need to Know About Attic Kneewalls
If you want your building enclosure to perform well, you need to start with good design details – especially for kneewalls
We were working on a project, so we got a set of plans to get started. It includes the attic kneewall and vaulted ceiling section you see at right. This is typical of plans that architects draw, and builders build houses this way all the time. Unfortunately, it contains several errors. Can you spot them?
I've written about attic kneewalls and vaulted ceilings a few times before because they often create problems. This may be a regional thing because my friends in the Northeast are often surprised that builders don't just move the building enclosure to the roofline with spray foam insulation, structural insulated panels, or foamboard on top of the roof deck.
What’s wrong with this section?
Nevertheless, I've been in a lot of homes that have been built to the architectural specifications above, so let me point out the problems with the section you see in that diagram.
- Vaulted ceiling. Although there is a line, the designer has not called out that there should be blocking at the base of the insulation in the vaulted ceiling. Fiberglass, the insulation usually installed here, is air permeable, so ventilation air is as likely to flow through the insulation as above the baffle.
- Attic kneewall 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. . Without sheathing on the attic-side of the kneewall, hot (or cold) attic air can get into the air-permeable fiberglass insulation most often used here. Even worse, the fiberglass batts often fall out, as shown in the photo below.
- Attic kneewall insulation. The section above calls for R-19 insulation. Fiberglass batts that meet that specification are typically 6 inches thick. Home builders usually build 2x4 kneewalls that are 3.5" thick. The diagram shows the insulation sticking out beyond the kneewall framing. That extra bit of insulation hanging out in space will have little to no effect because attic air can easily move right through it.
- Floor joist blocking. The floor joists that go under the kneewall are often a source of comfort and energy performance problems. If the attic air can move through unimpeded — and fiberglass insulation offers little resistance — that floor can be a heat source in summer and can rob heat from the room in winter.
How to design an attic kneewall correctly
The diagram below comes from the Georgia state energy code supplements and amendments (pdf), which has a lot of nice building enclosure details created by Southface. Notice that the diagram below calls for blocking at the bottom of the vaulted ceiling insulation, attic-side sheathing for the kneewall, and blocking between the floor joists under the kneewall. This should be standard fare for any architect working on homes with these details.
The advice in this article doesn't apply to all architects, of course. I know many who know how to do this correctly. Some even specify the building enclosure at the roofline to avoid having the problems cited here. Martin Holladay wrote a nice little piece for Fine Homebuilding a while back showing the two ways to insulate an attic kneewall.
For you architects and building designers still drawing sections as shown at the top of this article, however, it's time to change. If you're going to specify a building with attic kneewalls and vaulted ceilings, these details matter. If you don't get them right, the builders probably won't build them right, and evidence of the problems will be plain for all to see. Not to mention that the rooms surrounded by these defects will be the most uncomfortable rooms in the house. One man I know even abandoned his man-cave because of these flaws.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.
- Energy Vanguard
- Southface Energy Institute
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