This week, I’m sharing a few observations, some technical, some not, from a deep energy retrofit of my own home in Burlington, VT. But, yes, I gotta get back to work, so just a couple more. Here’s Part 5 of the series
In yesterday’s post, I talked about spray foam. And I applied it against the sheathing on both roof and walls. What if moisture where to leak through?
Leak though? Yes, even pinhole leaks and a modicum of airflow could potentially create problems. IBACOS discussed this in a paper from last Fall, Moisture Risk in Unvented Attics Due to Air Leakage Paths. Yep, that moisture creates a potential for problem. (Conclusion–we need more field studies of this!) If moisture leaks through to the roof sheathing, in my cold Vermont climate, won’t it condense? And with typical vapor impermeable roofing material, isn’t this a problem?
Could be. So I geeked out a bit. Above the first roof, covered in a #15 felt, I left a 3/4″ air space, tied to the drainage plane at the walls, and then built a second roof over it. #15 felt has a perm rating of about 5 when dry and 60 when wet. It will allow the sheathing to dry. And if, either from condensation on the out layer, or an unlikely leak (I’m an optimist!1), it’s got a way to drain away.
OK, that’s the roof, but what about the walls? Closed cell foam inside, and that rigid foam, XPS, I talked about a few days ago. Could be trouble if a sandwich the OSB between two relatively impermeable layers. Well, I took a similar approach as with the roof. But here it was Tyvek over the sheeting followed by Cedar Breather (I went with materials stocked in the local lumber year.)If water were to hit the sheathing, from inside or out, it’s got some way to escape.
Makes sense. I hope! Check back in a few years.
1The outer most layer of the roof? Bottom to top: OSB sheathing, Ice and Water Shield, standing seam metal, double locked. The metal should provide good protection, from the water, and for the membrane. The Ice and Water shield should provide back-up protection. And below that? That drainage plane with 15# felt. So goes the theory anyway. Let’s take a look in a couple decades and see.