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Raised-frame floor above slab

I understand the importance of super insulating the underside of a slab with large amounts of XPS foam, especially in my very cold climate (northern Maine). I have run across a unique foundation design of a passive house in Falmouth, MA, by architect Steven Baczek (http://www.deapgroup.com/Falmouth_PH.html). It involves insulating ABOVE the slab, essentially leaving the slab out of the conditioned space. His design involves 4" of XPS rigid foam resting on top of the slab, followed by 2x8 16" OC boards spanning the slab, resting on a 2x4 strut. 3/4" T+G subfloor covers the 2x8s. Blown-in cellulose fills the cavity underneath raising the R-value to tremendous amounts, which I imagine is significantly less expensive than underslab foam insulating. He weighs the loss of a thermal mass but says the loss is negligible based upon the heating load of the house and amount of insulation that otherwise would be financially unrealistic with foam underneath the slab. He doesn't take into account the labor and extra materials required to essentially build another floor. I am guessing that this is not the best system in most climates (warmer), but in mine (Zone 7-8), I am wondering if it's worth it? Any other testimonials or lessons learned out there? Thanks.

Asked by Matthew Michaud
Posted Dec 30, 2012 8:55 PM ET
Edited Dec 30, 2012 9:14 PM ET


4 Answers

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The Fine Homebuilding article on the house can be found here: A Practical Approach to Passive House.

I'm attaching an illustration from the article.

Floor insulated with cellulose above slab.jpg
Answered by Martin Holladay, GBA Advisor
Posted Dec 31, 2012 7:48 AM ET
Edited Dec 31, 2012 7:50 AM ET.


I'd hate to have a pipe burst and flood in that home.

Answered by stephen edge
Posted Dec 31, 2012 12:32 PM ET


This is where I learned about the house. Any suggestions about this design. Yes, I would not like to risk plumbing issues.

Answered by Matthew Michaud
Posted Dec 31, 2012 12:35 PM ET


Matthew, It i an interesting idea although for the life of me I can'f figure out how they built it unless the joists are separated from the foam by enough of a gap to provide access. It is essentially a crawlspace, but one which is then completely filled with cellulose. Given that (here anyway) a crawlspace must have a minimum clear height of 18", or 2 ft where services are present, you are looking at filling the floor with at least 32" of blown in insulation! If that ever did get wet, surrounded by foam on three sides, it would never dry. To my mind that isn't a very resilient assembly over the long term.

Answered by Malcolm Taylor
Posted Dec 31, 2012 12:54 PM ET

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