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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.

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 easy to cut flush, using the roof plane as a guide. Contractor’s tape on the teeth of the saw protects the roof membrane from being cut. For the second layer, use a block of foam taped to the saw blade as a depth gauge. Valleys are a lot more difficult than simple roof sections. To get a good air seal, you need to bevel the valley cuts and weave the layers. It’s good to use a handsaw, because circular saws can’t cut a bevel through a 2-inch piece of foam. To keep the cuts consistent, make a pattern, just like you would for cutting roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . At the lower edge of the roof, set up a staging area so that you have a safe place to work from.

After two layers of foam, apply an ice shield, furring strips, and plywood. Then, install roof jacks. It’s difficult to get the overhangs exactly right from a scaffold below. So, Dave runs the new overhangs a bit long and then cuts them off cleanly after snapping a line. Installing long screws through plywood, furring, and foam is something you get the hang of over time. Don’t try to get them perfectly square in both directions. Instead, concentrate on hitting the framing.

Aug 10, 2017 8:54 AM ET

User-6884681, While it's true
by Martin Holladay

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!

Jul 26, 2017 10:11 AM ET

Edited Aug 10, 2017 8:35 AM ET.

A quick Question for this
by user-6884681

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?

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