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Learn how to fasten and air-seal rigid-foam sheathing
to the outside of an existing house

with David Joyce


David Joyce: We’ve stripped the house of all the siding, we’ve put on the housewrap, and now we’re applying the two 2-inch layers of polyisocyanurate.

Narrator: Using multiple layers of foam rather than a single layer of thick foam helps you get a better air seal, because you can offset the seams on both edges of the sheets. Start with a 2-foot rip at the bottom of the wall and stack a full sheet on top of it. The next layer will begin with a full sheet. Take a little time to square up the first sheet so that the corners are good and plumb. This way, you won’t have to cut all the factory edges from the subsequent sheets.

There are a lot of obstructions on the outside of a house—spigots, electrical conduit, meters. Some can be worked around; some need to be moved. If a spigot is too short to absorb an extra 4 inches of wall thickness, cut it off and replace it later.

Dave likes to use roofing buttons to hold the foam to the walls. In this video, his crew uses spikes and screws interchangeably. Now they use screws exclusively. Screws are easier to remove, if needed. Screws are also easier to adjust. One of the tricky parts about installing rigid foam is that it compresses. Being able to adjust its tightness will be important later on when prepping for siding. When attaching the roofing buttons, be extra careful not to overdrive the screws. Just drive them until they’re snug. When installing the furring strips later, compress the foam using a stringline to get a nice flat surface for siding.

Dave’s crew tapes all the horizontal seams before moving on to the second layer. But they don’t spend too much time taping the inside corners of the first layer, because the corners will pull apart as the foam is compressed. On the outside corners, the first layer of foam is sealed with peel-and-stick right to the housewrap before the next sheet of foam is woven in. Start the second layer with a 2x4 ledger along the bottom of the wall. The 2x4 will provide backing for trim and the bug screen assembly. Tight joints between foam panels are important. But don’t sweat it too much. Small gaps are no big deal because the gaps will be filled with canned foam later after the rigid foam is compressed with the furring strips. Do not use canned foam before compressing, or it will expand and become a lumpy nightmare. With multiple layers, this compression is magnified. This house has only two layers of foam. On homes with four or six layers of foam, the compression is magnified even more.

Jan 6, 2016 12:02 PM ET

Edited Jan 6, 2016 12:03 PM ET.

Thanks for this - Very
by Gayle Borst

Thanks for this - Very helpful.

Apr 10, 2015 5:03 PM ET

Response to Paul J. Boniface
by Martin Holladay

Response to Paul J. Boniface

Here are some links to help you:

Window Installation Tips for a Deep Energy Retrofit

Extending Window Openings for a Deep Energy Retrofit

Nailing Window Flanges Through Foam

‘Innie’ Windows or ‘Outie’ Windows?

Apr 10, 2015 2:10 PM ET

Informative, but how do you
by Paul J. Boniface

Informative, but how do you treat the windows?

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