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Video Series: Exterior Insulation Retrofit — How to Install Foam On a Roof

This video is only available to GBA prime members

Learn how to cover an existing roof with rigid foam,
and prepare for installation of a new roof

with David Joyce


Narrator: Before installing the roof foam, cut back the wall panels to match the roof plane. Like when cutting back the roof overhangs, cutting the foam exactly flush with the wall isn’t that big of a deal. In fact, it’s better to cut off a little too much than too little. If the wall plane is a little proud of the roof plane, it will be difficult to weave the corner.


Insulation Retrofit Complete

1. Introduction (free)

2. Strip Siding and Roofing (free)

3. Install Housewrap and Roof Underlayment

4. Install Rigid-Foam Over the Walls

5. Install Rigid-Foam on the Roof

6. Flash Windows Over Rigid Foam

7. Choosing Details and Materials (free)


Remodel Project: Deep Energy Retrofit

Podcast: What Is a Deep Energy Retrofit?

The History of the Chainsaw Retrofit

Roofing and Siding Jobs Are Energy-Retrofit Opportunities

David Joyce: Here we’ve installed two layers onto the wall. We’ve taped all the seams, and we’ve cut it flush with the existing roof. Now we can bring the membrane from the roof down over all of this and either tape it to the Tyvek or tape it to the face of the foil. Capping this will help with the air seal. When we’re done with that, we’ll start with the roof, putting foam on top of the roof. We’ll cut that flush again, maintaining a straight edge because later on we’ll be putting up fascia and soffit. It’s easier to work on a flat surface. We’ll wrap Ice & Water Shield, helping with the air shield in the future. Narrator: Dave’s crew used a regular handsaw to cut the foam in place. The first layer is…


  1. azad_lassiter | | #1

    A quick Question for this
    A quick Question for this retro-fit Outsulation Projects.
    When adding the Rigid Foam you are now creating a vapor barrier on the outside of the house.
    You can bet that b/c of the popularity of interior Vapor barrier on the warm side of the walls that there is now two vapor barriers You could be trapping Vapor inside of the wall.

    Is there a work around for this type of work in the retrofit? Or is this system shown in the videos missing a huge component?

  2. GBA Editor
    Martin Holladay | | #2

    While it's true

    While it's true that a layer of interior polyethylene is undesirable when the walls and roof have a thick layer of exterior rigid foam, it's also true that reports of moisture problems due to the presence of undesired polyethylene are virtually nonexistent. There are several factors explaining why this is so; most of them are discussed in this article: The Exterior Rigid Foam is Too Thin!

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