Fastening Vinyl over Foam

Q:

After going through my remodeling budget, it looks like vinyl siding will be in my near future. I still want to put 2-in. XPS rigid-foam insulation outboard of my structural sheathing, so my question is: Is it OK to fasten the vinyl siding through the foam, or do I have to apply furring strips?

—Josh Braun, via email

A:

Mike Guertin: As with a lot of construction, on the one hand you have the installation instructions from the manufacturer and/or association, and on the other you have what is being done in the field. I’ll try to give you the full picture and let you make the call.
The by-the-book answer to your question is that vinyl siding manufacturers require a flat, solid supportive surface, and limit foam installed under their siding to a maximum of 1 in. thick. So, technically speaking, you can’t nail through or even use 2 in. of foam, and you can’t use furring strips (which are not a flat, solid surface) without voiding the warranty.

That said, I’ve nailed vinyl to furring strips set 24 in. on center that were fastened to the underlying studs. In these cases, you must follow a screw manufacturer fastening schedule for attaching the furring strips. Both FastenMaster and Simpson Strong-Tie have these details on their websites.

If you want the flat, solid-surface option, you could consider a second layer of sheathing atop the foam, but there isn’t a prescriptive fastening schedule for this approach, so an engineer-designed fastening schedule will likely be required. You might be tempted to use a product like Huber Zip System R-Sheathing for this purpose, because it carries code approval, but be aware that R-Sheathing has strict installation guidelines that you should first review to ensure the product is suitable for your wall framing and seismic and wind zone.

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