Codes run afoul of advice when it comes to insulating my slab
A few months back, I asked about the best way to layer materials for my garage slab on grade with hydronic heating in MN (zone 6):
Consensus was to place 6 mil polyethylene vapor barrier ABOVE XPS insulation, directly beneath concrete slab. PEX tubing would then be stapled to XPS foam boards, and rebar on top of PEX.
However, building inspector failed my installation, stating code requires 4″ of rock for radon mitigation, followed by polyethylene sheeting, THEN foam boards, and finally the PEX and rebar for the pour.
So, my question is this: should I care? I know what Joseph Lstiburek says… no sand between sheeting and concrete, and no poly beneath foam board. But, what is the risk, really? (I know what the risk is if I don’t do it like the building inspector asks!!) Most builders here in MN routinely put the poly on top of the rock, then put down the foam board insulation, and not simply because of code, I’m told.
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Replies
Kent, print out the BSC details and buy your inspector lunch while you show him the PDFs.
The plastic below foam is not horrendous. If you have to do it with just that one switch, your will be fine as you are not trapping water in the gravel. The foam will not have room for much water to bleed into. Not the best but... maybe also you could find an inspector to convince your inspector... or blackmail or...
And lastly, I hope your plans were not accepted verses your actual installation in,but denied. Around here our plans are approved way before we build.
Kent,
If you're lucky and you have dry weather during the hours between when the polyethylene is installed and the concrete is placed, you'll probably be OK. The worst problems occur when it rains after the rigid foam is installed but before the concrete is placed. At that point you have wet foam, and you have water under the foam and between the foam sheets. That water takes a long time to dry out, delaying the installation of the finish flooring.
It's too bad that building inspectors don't know more about building science. One tactic is to politely ask your local building inspector to cite the code that requires the poly to be under the foam. I don't think such code language exists.
Thanks, gentlemen. The inspector cites the section of code that specifies "vapor barrier, such as polyethylene sheeting, must be placed on earth or compacted fill material, beneath the concrete slab..." So, he is hung up upon the sheeting being in contact with "earth" or "dirt".
Only a 30% chance of rain before the concrete pour next Tuesday, so I guess I'll just play the game by his rules, and not buck the system. So long as there is no definitive risk of compromising the foam insulation, radon mitigation, or integrity of the overall structure, I think I'll pick a more critical design element to butt heads with him over! Thanks!
Read and look the last detail, Figure #21 -Slab Insulation, on the last page of a study in Minneapolis done by Building Science Corporation: http://www.buildingscience.com/documents/profiles/etw-minneapolis-profile
You can print it and take it to your building inspector.