Insulating the Space Between ICF Wall and Roof
This is such a weird space to consider in my design. I’ve read more than I care to admit and still can’t quite make sense of it.
My house is ICF, with one 66′ long, single-pitch slanted roof — from 10ft on the South to 21′ on the North. Think one big lean-to.
We built up the ICF walls, and then on top of those walls we set 2′ high, 66′ long trusses. On top of those trusses I installed an OSB roof deck, I&W underlayment, and a 2″ mechanical seam metal roof.
But between the soffit of that roof deck and the top of the ICF wall is 19″ of exposed truss, set 2 1/4″ to the interior of the ICF wall (resting right on the concrete).
My plan is to do an unvented roof assembly since the majority of the house does not have an attic. That means closed cell spray foam between the trusses. This is where the questions begin.
I had some leftover OSB so I went ahead and decked the exterior of the trusses. If you look at the wall assembly from the outside, it goes ICF, then OSB, then soffit/roof. Behind that OSB will be closed cell spray foam. Over the exterior of this entire wall assembly up to the soffit will be brick facade with a 1″ air gap behind it.
1) Should I put exterior rigid foam, like EPS, on the outside of the OSB? With closed cell spray foam on the interior, that means no path to drying and a greater chance of rot?
2) Should I just put a WRB like house wrap on the exterior of the OSB and not insulate the exterior of this space at all (since the interior will have 4-5″ of closed cell foam).
3) Should I *not* apply closed cell foam to the truss cavity behind this space and do a more traditional wall assembly? What would that look like?
It feels strange to just have sheathing + wrap on the exterior, especially when any moisture entering the OSB could get passed into the truss, since that is what the OSB is fastened to.
This 19″ space around the top of my house is about to drive me crazy. Send help.
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I'm sorry but I don't follow your assembly. Can you post a sketch?
Hey Michael,
Totally understand. Here's a picture from a few weeks ago that shows the area between the end of the ICF wall and the beginning of the soffit.
Ah, very helpful. I thought you were talking about the eave and/or ridge soffits. EPS is vapor permeable and will allow some drying to the exterior.
Yeah that makes total sense. Is there a proper name for the area I'm referencing?
So if I laid EPS board on the exterior of that space, taped the seam between it and the ICF wall, and then did 2" of closed cell spray foam on the interior of that OSB, you'd feel good about that?
I would call it the area under the rake soffit, or the top of the rake wall.
I avoid foam and concrete as much as I can for environmental reasons, so I can't say I feel great about it, but I think it would be safe from a building science point of view.
Totally understand that, Michael. What would be the best way to do this from a "green" perspective?
I almost always do vented roofs with R-60 cellulose insulation. On renovations, sometimes I'll do a flash-and-fill system of closed cell foam and cellulose. In your case, since your house is already made of EPS, which is one of the least-bad foams, I'd probably do the exterior EPS as described, and use a vented cellulose (or wood fiber) for a vented ceiling.