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Details on a rigid foam chainsaw retrofit

bobww | Posted in General Questions on
I am retrofitting a century old, uninsulated house with exterior rigid insulation. It is in zone 6a in northern MT.  The plan is to put 9 inches of polyiso foam on the roof deck and 3.5 inches on the exterior walls with some fluffy insulation blown in as well. Then furring and cladding on the walls, sheathing snd asphalt roof on top. A chainsaw retrofit.  I am also replacing the (vinyl replacement) windows. The new windows will be “outies”. 

No one out here seems to have much experience/expertise with this approach, so I have been relying heavily on GBA and Joe Lstiburek‘s excellent site. I’ve read everything I can find, and I think I have a good handle on things. But I have a few questions. 

(1) air barrier: if I use two layers of foam, seams offset, gaps foamed and seams taped, will the foam serve as the air barrier? Does the WRB also need to be detailed as an air barrier? I will use peal and stick on the roof. 

(2) WRB: my plan is to install the rigid foam over the sheathing, then a WRB over the foam. That makes the window detailing simpler. However, ice and water membrane will go in the roof deck and will extend over the 3.5 inches of foam and fold down over the top of the WRB on the walls. Then 9 in of roof foam on top. a) is this okay? (b) is there a reason to use anything other than a basic cheapie housewrap like Tyvek? (c) how should the housewrap be detailed? Does it need careful taping etc, given that the foam will be taped? i.e. is the WRB in this assembly just for water, or also for air sealing?

(3) flashing: how should the bottom edge of the 3.5 inches of foam “overhang” on the wall be supported/flashed? An aluminum shelf with a turndown? (not sure the terminology, I’m out of my linguistic depth here.) Or some other material? To let any errant moisture drain, keep the bugs out, and protect the bottom edge of the foam. 

(4) Moisture: This is an old house, with an old full basement and some crawlspaces.  The basement gets moisture.  It doesn’t usually flood (though has once or twice in the past when the sump went out and the water rose up from below.  Am I looking for trouble with moisture issues if I insulate this way? The house has had no moisture or rot issues for a century, but it is uninsulated and leaky as hell.  This is MT, so winters are snowy, springs are wet, and summers are hot and dry. 

Thanks in advance for your advice. 


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