So, I've been pondering some thoughts on where walls meet roofs in the vein of Joe Lstiburek's "Perfect Wall" work and how to replicate that in a context that favors "house-shaped" forms i.e. those with gabled or hipped roofs and eaves instead of a flat roof surrounded by a parapet.

Based on the "connect the layers" principles, the use of continuous rigid insulation on the outside of the wall to supplement the native R-value of whatever wall system is used, and the use of rainscreen techniques for the fascia and wall claddings, I have come up with the following so far for an eave detail (disregard the lack of decking directly under the roof, I have some questions about how precisely standing seam roofing over rigid insulation works but they're separate from this):

eave detail drawing

However, what I have come up with raises a few questions in my own mind. First and foremost: what should I do with the airspace under the soffit board, given that this is very much a "hot roof" construction? Should I leave it vacant, fill it with more rigid insulation (mineral wool is my preference as far as rigid insulations go), or slope the soffit board to match the pitch of the roof?

Furthermore, is having a fascia that's as tall as the roof slab + twice the continuous insulation thickness going to be a problem in some way or another I'm not foreseeing? Finally, is there anything else majorly wrong with this detail that you see?

1 Answer 1


Your question is a good one. The company I worked for on the east cast followed Joes building science and worked with him too. The CEO of my company was also friends with him, so much for that.

My thoughts on you situation is to let the wall insulation cut around the rafters after the liquid applied water proofing membrane (LAWPM), is ran entirely over the roof, and of course the wall too, but add plywood between the rafters to allow the LAWPM to engage the bottom of the sheathing. The roof insulation would continue over the roof completely, the wall would continue to meet the under side of the roof sheathing, completing the thermal envelope and the soffit can be vented with a continuous screen vent to relieve heat build up.

This may help keeping down the "bulk" of the fascia, if the added layer of insulation makes it thicker than you want.

Then again, with the foam, it may not get that hot to begin with.

What the company I worked for did was apply the roofing in the usual way, sprayed foam on the attic side of the roof to make the attic a conditioned or at least a tempered space since many places had humidifiers as part of the HVAC system in the attic. The rest of the system was the same, seal up the outside completely, cover everything with LAWPM, and shoot 1/2 or 2lb foam over everything on the inside. It may have been 1/2 lb only, since I think 2 lb foam is considered non permeable.

Another note on how it was done, since the LAWPM did not have insulation over it, the soffits were skinned over with 1/4 or 1/2" plywood so the LAWPM followed a "rough" soffit. After that all finish material would fasten directly to that.

I know it is different how you are doing it, but it was the was it was done 7, maybe 10 years ago. I only mention it because you may be able to merge the 2 details together to get the system to work for you, since the illustration on the website shows a simplified commercial application, not residential with a soffit.


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