Air-Seal an Attic Series: Improving Attic Knee-Wall Insulation

Fiberglass batts work best if they’re enclosed and air-sealed on all 6 sides. Adding a rigid-foam air barrier to the cold side of the batt is a great way to take care of this issue on a knee wall between finished and unfinished attic space.

Video Transcript:

Here we are inside an attic enclosed by a knee wall into a partially finished room and the outside of the house. Behind this insulation is nice conditioned air. In here it’s hot and humid. Most houses with this situation—namely, Cape-style houses and some Colonials—have the problem of air flowing through the insulation that’s in the ceiling and underneath the floor of the conditioned area. The air flows through the fiberglass insulation because nothing is blocking it. We want to provide a blocking panel that’ll stop the air from moving and improve the performance of the entire room.

The insulation in the knee walls of an attic is usually uncovered on the attic side. That has the potential to degrade the insulation as convective air loops through the insulation. To improve the insulating qualities of the insulation, provide a solid barrier so the insulation is encapsulated on all six sides: the drywall on the insides, the studs on either side, the top plate, the bottom plate, and then a panel we put over the face.

Choosing the best air-sealing materials

There are a couple ways of sealing this up. We could use a solid-wood block or a piece of drywall, but I find it’s easiest to use a piece of rigid-foam insulation. It’s easy to cut, easy to manipulate, and it fits into place. All we have to do is slip it in place and attach it. I’m going to use a screw. And then we seal around the edge of the whole perimeter with some caulking. I’m using acoustical sealant. I could just as well use expanding spray foam or average caulking for painting, as long as it’s a flexible kind of calking. You might be tempted not to wear a pair of rubber gloves when it’s hot in the attic, but you need to push the bead of sealant all the way around the edge, and the gloves keep your hands clean.

We could use a few materials: drywall, a piece of plywood, or even housewrap. It’s easy to work with, but housewrap is hard to seal around the edges. Having the seal improves insulation because it stops the air from moving through the cavity. I prefer to use rigid insulation. If you’re going to use drywall, which is an inexpensive material, you’re going to spend $7, $8, or $9 on a sheet. To get an inch of rigid foam insulation may cost you $15-$20, extra money, but for the extra insulation it’s worth the expense.

Installation is pretty simple

I’m using polyisocyanurate foam because we’ve got high R-value per inch, but you could also use some EPS or XPS insulation. I slide it along, and then screw it to the studs from one end of the room to the other. Now we have our rigid foam in place. The last step is to seal between the ceiling joist and the wall panel. I’m going to use some expanding spray foam to do that: just run a bead. It doesn’t matter if the gap is a 1/4 in. or a 1/2 in.; the foam will fill it right in. I’ll just work my way down the wall.

Videos in this series:

• Introduction
• Sealing Wire & Plumbing Penetrations
• Sealing a Ceiling Electrical Box
• Sealing Wall Intersections
• BONUS: Materials Overview
• Sealing Ductwork
• Sealing Soffits and Chases
• Insulation Ventilation in Rafter Bays
• Improving Attic Knee-Wall Insulation