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Can a Slab Foundation be Insulated Without Rigid Foam?
comments (3) January 18th, 2012 in Blogs
Scott Gibson, contributing writer
4 users recommend
To avoid rot, straw bales must be kept dry. Lucas Durand calls this a "concept sketch," and appends a few caveats: "This idea is less than half-baked. There are many unresolved issues that I haven't thought about - like air sealing, attachment of rainscreen cladding over parging, the drying potential of the under-floor insulation, etc."
That's essentially the question posted by Roy Harmon in a post in the GreenBuildingAdvisor Q&A forum.
Specifically, Harmon is wondering about something called the Archibio Sandwich Slab, which is described in a 1994 book entitled Straw Bale House. It consists of two layers of concrete with straw bales between then. Although the technique is said to require more concrete than a conventional slab foundation, the cost of the insulation itself is low, $1 or less per square foot for R-50 performance.
Low cost is good, and so is the fact that no petrochemicals are involved. But the organic nature of straw raises concerns, too, chief among them the potential for decay.
Read the whole article at GreenBuildingAdvisor.com.
posted in: Blogs, insulation, rigid foam, straw bales
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Comments (3)
About the only thing that I might about the proposal depicted in the sketch is that the idea might be serviceable IF the floor joists were extended outwards so that they carry the wall bales.
This would enable the straw bales used as floor insulation to be pulled out and replaced as necessary -- and make no mistake, it will be necessary.
The T&G floor is extremely air-leaky and that will surely result in moisture from the interior getting into the floor bales. That is a fact that can be verified by the experience of other SB builders who did the same.
The 'impermeable membrane" placed on the crushed stone drainage layer under the bales will be the plane where the moisture will condense. Microbial activity will be supported followed by Fetid Goo Brew .
WRT to the query re: Why downward sloped perimeter (foam) insulation ?
It creates an umbrella of insulation which retards the outwards/upwards flow of warmth from deep earth, extending the non-frost-susceptible zone underneath the floor, outwards from the "foundation" a bit, which is a Good Thing if frost heave of the foundation elements is considered a Bad Thing. ie Frost expansion is three-dimensional. Lateral pressure can wreak significant havoc too (as well as popping things upwards out of the ground).
Posted: 11:51 am on January 24th
Posted: 7:35 am on January 23rd
This was built 6 years ago and the slab has not developed any cracks at all. I live in north western Maine were winters are cold
Posted: 7:21 am on January 23rd
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