07:02AM | 08/08/06
Member Since: 08/07/06
1 lifetime posts
Hi there, I'm building a 1400 sq ft two-story home in Southeast Alaska (cool, rainy climate) and need some advice. The house is framed with 2x6 (24 in. OC) walls sheathed with 1/2-in. CDX. I'll have R-21 fiberglass batts in the wall cavities. I also plan on having rain-screen walls, i.e., horizontal wood siding (5/8" local hemlock) nailed on to 1/2" furring strips to create an air space behind the siding. In addition, I plan on adding 1-in. rigid foam insulation (unfaced extruded polystyrene, i.e., pink board) to the entire exterior for added insulation and a thermal break. This seems simple enough -- there is even an article in the current issue of Fine Homebuilding (Aug/Sept 2006) on how to install rigid foam insulation -- however, there are a couple of important details left out in the article that I can't figure out. Mainly, how to integrate the foam panels with a rain-screen wall and also with window/door flashing. The article also doesn't explain how to integrate/install housewrap/felt when using unfaced rigid foam panels. I'm using 15# felt instead of Tyvek. Where does the felt go in the layering? Do I install the felt directly over the CDX as would be traditional, and then nail the foam over the felt? (then nail on the furring strips and finally the siding) If so, then the back of my airspace in the rain-screen is going to foam, not felt, which seems wrong. But if I put the felt over the foam panels, that doesn’t seem right either.

The FH article also skips over how to flash the windows and doors when using rigid foam panels. I have Certain-Teed vinyl windows. The article suggests furring out each rough opening (to match the thickness of the foam) to create a solid surface for nailing on the siding. They suggest you build a simple frame around each window/door opening and butt the foam panels against the frame to create a flush surface. However, they don’t explain how to flash this configuration. I plan on using one of the flexible flashing products like Fortiflash/E-Z Seal around the windows and doors. These strips would traditionally be installed directly against the rough opening (i.e., against the CDX) but if I furr out the openings as suggested, then do I wrap the flashing around the openings and then OVER the furring strips, treating the whole unit as one? How do I integrate this drainage plane with my felt drainage plane?

I guess my question comes down to -- if I want to integrate rigid foam panels with a rain-screen wall, how do I keep my drainage plane intact?

Any advice on how I should assemble all of these layers would be much appreciated: CDX sheathing, 15# felt, 1-in. foam panels, ½-in. furring strips for rainscreen, window/door flashing, windows, siding? Thanks in advance from a novice builder who is trying to do it right…


07:21AM | 08/08/06
Member Since: 04/25/05
1915 lifetime posts
You might want to look at the Building Science website.

I am not sure if they so any designs that incorporate both features. So you will need to go through the different ones to see.

Also go through the different papers and articels that they have.

FHB also had an article on rain screens a couple of years ago.

Another option is to put the foam on the inside wall.

Also you might want to discuss this in the FHB Breaktime forum.

They have a lot more volumne of messages and more pro builders.
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