WATCH MORE sustainable building VIDEOS
Once a decision has been made to choose between a crawlspace or a slab foundation , you need to know how to finish the edge off durably.
Typically the exterior edge of a slab on grade is made of EPS foam and can be parged. Having used rigid Rockwool panels meant we needed an additional solid exterior surface. Here we installed an exterior cement board, though that left the challenge of how to attach it without using metal fasteners that would lead to thermal bridging.
What we decided on was plastic straps that passed through the cement board and that were anchored in the concrete. It did hold as planned, though there is more play in it than we ideally would have liked and if we were to do it again we would increase the amount of fasteners. Like many things, this did work but we found it to be labour intensive compared to finishing an EPS foam slab exterior.
I understand your desire to limit steel fasteners into the slab and thereby reduce their thermal bridge. But what about use of a metal flashing above the vertical slab edge roxul tying into the wall's water barrier for drainage? Won't this come into contact with the cement and be a thermal bridge around the entire perimeter of the building? Or have you come up with a great solution to this as well? Really like this detail with the zip ties!
Thanks Mike, no flashing goes through enough to act as a thermal bridge. What we did with the zip ties was really a Hail Mary in a way to attach the cement board, and while it works, it's a little softer than I'd hoped it would be. That can be solved though with a thicker coat of more flexible spray on parging.
It would have also been better had we installed double the amount we tie straps, or if we had a more creative and easier solution from the get go. Finishing the edge is for sure is a trouble spot with thickly insulated slabs, and is one of the many reasons I've taken a shine to the pre-fab slab kits. The insulation is fastened to the concrete during the pour, and all you have to do is parge the outside. Best regards, Mike http://www.ecohome.net/guid...
If you were to do it again, would you recommend 2" of EPS on the outside for finishing and 6" of Rockwool inside? And how exactly woud you finish the EPS foam slab exterior? Just a large angled flashing? Thanks a bunch for the awesome content, very inspiring. I might be testing your technique in cold Whitehorse, Yukon when building next spring. Cheers!
No, we would not do it like that again, it worked but it was pretty labour intensive. We just built a new demo house and it is on a Legalett slab on grade with air heated floor, and the company takes care of that detail with a factory solution, They have an insulalated door threshold called ThermaSill that solves the problem entirely.