How to Insulate and Air Seal a Knee Wall?

by Erik North on February 7, 2012

How to Insulate and Air Seal a Knee Wall?

 

Knee walls are among the biggest air leakage and heat loss sources in home in part because they are such a pain in the rear to insulate and air seal properly. Anytime I conduct an energy audit of a Cape Cod or 3-story Colonial or a Salt Box, I know that a good chunk of the homeowner conversation and audit report will deal with the knee walls.

These are important conversations to have since a properly air sealed and insulated knee wall can make all the difference in energy savings and comfort for many homes. So what is the problem and how do we air seal and insulate the knee wall?

What is a Knee Wall?

Knee walls are the bane of energy and comfort in homes … by which I mean they are vertical half height walls built to convert normally unconditioned attic into liveable space.

The bottom plate of the knee wall rests on the open floor joists and the top plate is secured against the roof deck rafters. The knee wall runs vertically, making a 4-foot half wall which helps more clearly define the attic’s space. So what’s the problem?

The Problems with Knee Walls

The building envelope should ideally have the structure, air barrier, thermal controls and vapor controls working in tandem as one assembly. The knee wall space could be insulated along the horizontal joist flat and vertical wall or the roof deck.

Might as well re-use this chestnut so we have an idea what we’re referring to:

Air Leakage in Attic Knee Wall

Image: DOE's Office of Energy Efficiency and Renewable Energy

What problems do these approaches face? The eaves usually have vented soffits (and occasionall vented gable ends) allowing unconditioned outdoor air into the knee wall space.

Any recessed lights in the flooring (or vent fans or pipes or duct work) unless properly sealed will leak conditioned air into the knee wall space.

The bottom plate of the knee wall rests on top of the floor joists, leaving a giant hole underneath. This is one of the largest structural holes (we call them thermal bypasses) in any building. Hooray.

The vertical wall has built in bookshelves, drawers, electrical outlets and usually a door access to the crawlspace, all leaking conditioned air.

All this leaked warm (or cool if you’re south of the Mason Dixon line) air stays in the knee wall crawlspace, right? Ha!

The conditioned air escapes out the unsealed top of the knee wall, where the top plate is attached to the roof rafter. The air moves right up the roof deck and out the ridge vent. Knee walls may create open, useful space from former attics but they are heat loss and homeowner comfort disasters. So what to do?

How to Insulate and Air Seal

Important note: In discussing this insulation and air sealing work, we’re assuming the slopes above the top of the knee wall and the attic cap are already vented, air sealed and insulated like crazy no matter how much it’s not true.

OK, first thing. The number one most important step in insulating and air sealing a knee wall is having a continuous air barrier. The biggest heat loss problems with knee walls are those gigantic holes in the air barrier.

So, before we get into the details, there are a few choices to make. Will the attic be vented or unvented and will you insulate along the roof deck or along the horizontal and vertical walls. I won’t get into vented vs. unvented much (see this article for way more detail). Suffice to say, vented roof designs work when properly installed and unvented designs work when the thermal, air and vapor controls are properly installed to prevent condensation. The one significant advantage to insulating the attic slope is that the crawlspace becomes conditioned space. One can store liquids, perishables and other delicates without worrying about freezing damage.

Insulating and Air Sealing Knee Walls – Vented Version

If you choose to insulate along the horizontal and vertical walls, you have four key air sealing points. The ceiling and knee wall drywall will act as your air barrier. The first point is sealing the soffit venting. Assuming styrofoam vent chutes are installed, rough cut a foam block to fit between the rafters. Fit it flush against the vent and first floor top plate and foam seal around it. The goal is no unconditioned air in this space.

The second point is between the floor joists below the knee wall bottom plate. This one is huge. Just like above, rough cut a foam block to fit and seal with foam sealant.

The third point is the knee wall and crawlspace floor drywall. Any penetration between the conditioned interior air and the knee wall space needs to be sealed tight. This includes built-ins, recessed lights, vent fans, duct work and electrical outlets.

The fourth point is between the roof rafters over the knee wall top plate. Install a rough cut foam block against the venting and air seal.

The advantage here is that your knee wall is probably already insulated this way so it’s just a matter of buying some foam … so cheap.

The disadvantage is that it would be very very hard to reach the desired R-value (from R-49 to R-60 depending on climate) for attic insulation.

Insulating and Air Sealing Knee Walls – Unvented Version

Another approach is insulating along the roof deck without venting. This approach has several advantages with one giant caveat.

First the crawlspace would now be conditioned space. No matter the climate, you could store delicates in the crawlspace without fear of environmental damage.

Second, remember all that air sealing? Forget about it. You’re moving the air and thermal layers to the roof; all those air leaks are inside the house now and not heat loss.

Third, remember how I casually tossed in ‘built ins’ for air sealing? Built in shelves and drawers can be next to impossible to properly seal. Far easier to seal at the attic slope than the built ins themselves.

The big caveat? Moisture control and your roof deck. The air and thermal barriers have to be perfectly intact. If warm-moist air contacts the old roof deck, the moisture will condense and over time possibly damage the roof. When the air barrier is intact, that air can’t contact the roof. When the thermal insulation is wholly intact, the surface temperature will stay at room temperature. Tada, no condensation or dew point problems.

There are two common approaches. The first is securing XPS or foil faced polyisocyanurate to the roof rafters and dense packing cellulose behind it. The other is spraying at least 2″ of closed cell spray foam directly onto the roof deck, encapsulating the rafters. Then either fiberglass batts secured in place or netted cellulose is used to fill the rafter cavities.

Air sealing and insulating the knee wall can make the attic space warmer, more efficient and more comfortable.

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