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Exterior Foam Insulation Retrofit: How To Install Polyiso Panels, Part 2

The second layer of foam is the air barrier and drainage plane
October 01, 2015

This is part two of a two-part series on installing exterior foam. Part one is here.


With the first layer of a two-layer exterior foam retrofit installed, Calvin and Damien turn to the money layer. They begin with a starter piece, again attaching it with two screws until the furring strips go on.

You may notice that the seams are already taped. That is because they wanted to pre-fit the pieces before we shot a video, and then they got carried away and taped the seams. So we had to cut the tape and shoot the installation.

The foam is tucked tightly into the bug shield, and they install as close to full sheets as they can. Certainly, they cover seams in the first layer with solid panels in the second.

Again, the pieces should be snug.

They work from the corner in. This is the opposite corner on the same front of the house because the center of the wall is broken up with a large entry stair.

Once the corner is plumb and level, they fasten it off.

Succeeding pieces are fit tightly. Cantilever sections, like outside corners, have their ends staggered so that there is no straight line for air or energy leaks.

The bottom of the foam and any exposed edges, such as at a basement knee wall, are wrapped in a site-bent metal channel that keeps bugs out. The metal is sealed to the foundation with silicone caulk.

Next, the seams are edges are taped with contractor tape. The metal bug shield was just taped to the foam also, with contractor tape …

… as are the inside corners and cantilevers.

Outside corners are sealed with a wide strip of peel-and-stick membrane to cover the staggered corner sheets.

Next, foam is sliced at a downward angle to extend the sill pan sloping to the outside …
… and the peel and stick are peeled and stuck to the foam, similarly to how it is done in the rough framing.
Calvin explains the process:

Calvin quote

Notice that he cuts the corner shy and bends the membrane to keep the bottom point water-tight.

With the seams, edges, and corners taped, they turn to install strapping for a rainscreen siding assembly. 1x3 utility grade strapping is fine; as long as the strips are fastened into studs, they will do their job of giving the siding something to fasten into.

The second layer screws are longer than the first layer screws need to be— about six inches compared to four inches.

Screws are countersunk by cutting partial holes in the furring strips with a spade bit.

As the furring strips are secured to the framing, the foam is compressed inward. This can make for a wavy wall, so when it is time to install siding, the crew will pull a string line across the wall and either drive in or back out the screws.

That string line process is not worth doing now because the wood framing may still expand or contract before it is time for siding.

This is part two of a two-part series on installing exterior foam. Part one is here.

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