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Learn how to enclose the back side of a kneewall with an air barrier and make batt insulation perform better

Video Transcript:

Here we are inside an attic enclosed by a kneewall 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.

There are a couple ways of sealing this up. We could use a solid wood block or a piece of dry wall, but I find it’s easiest is 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 caulking. You might be tempted not 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.

The insulation in the kneewalls 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 plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate., the bottom plate and then a panel we put over the face.

We could use a few materials: dry wall, a piece of ply wood, or even house wrap. 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 to $20, extra money, but for the extra insulation it’s worth the expense. I’m using polyisocyanurate foam because we‘ve got high R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch, but you could also use some EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. or XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. 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.

Dec 11, 2016 3:25 PM ET

Great video. You mention
by Andrew Ross

Great video. You mention sealing the bottom of the polyiso you added to the walls with foam, but you don't say anything about the top of the polyiso or the seams in between each piece. Should these gaps be sealed as well? I have a '50s house and the kneewall has no top plate and no air space between the kneewall attic and the small attic space at the roof peak. Would it be okay to add polyiso up to bottom of the rafters without sealing the top gap?

Feb 20, 2015 10:03 AM ET

Edited Feb 22, 2015 9:44 AM ET.

Awesome, thanks Martin. The
by Peter Rogers

Awesome, thanks Martin. The fact of attics being typically warmer had occurred to me as well... would also help get the full benefit out of the polyiso. If you've never heard of this causing an issue, that's good enough for me! I'll check with local code authorities on the EPS as you suggest. I've always recommended EPS in this situation, but was recently questioned on this recommendation in terms of the ignition barrier issue.

Feb 20, 2015 8:43 AM ET

Peter, 1. If you want to be
by Martin Holladay

1. If you want to be on the safe side, you should follow the recommendations in this article when choosing the thickness of rigid foam that you intend to install on the attic side of your kneewall: Calculating the Minimum Thickness of Rigid Foam Sheathing.

However, thinner foam than would be required by the table in that article will probably work just fine -- because attics aren't quite as cold as the outdoors. I've never heard of a moisture problem that can be traced to the use of too-thin rigid foam on a kneewall.

2. Local code authorities vary widely in their interpretation of code requirements for thermal barriers and ignition barriers over rigid foam. In most jurisdictions, these barriers aren't required when the rigid foam faces an inaccessible area like the tiny attic behind a kneewall.

When in doubt, talk to your local code official. If a thermal barrier or ignition barrier is required, the easiest way to comply with the code is to protect the rigid foam with 1/2-inch drywall.

Feb 20, 2015 8:27 AM ET

Two questions: 1) isn't
by Peter Rogers

Two questions:
1) isn't putting an inch of foil faced polyiso on the exterior a risky practice in many climate zones, as this amounts to a vapour barrier without moving the dew point far enough inside the wall assembly?
2) if one uses EPS instead, does it not need to be covered with an ignition barrier on the outside? If it does, any suggestions for how to do this, short of drywalling?

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