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Should I put 2" foam board under, around, AND on top of concrete slab?

Insinuendo Insinuendo Aug. 5, 2022

Hey.  I have a question nobody seems to be able to answer: We're building an 1100 sqft ranch on a monolithic slab w/ 2" hard foam insulation under slab (+ crushed stone & eps vapor barrier), and 2" hard foam insulation vertically around sides/edges of slab and subfloor above.  On top of the slab will be a wood subfloor web-truss system where the electric & plumbing & hydronic floor heating panels w/ PEX tubing will be located - as these will not be buried in the concrete (subfloor space height around 18" from top of slab up to bottom of flooring). Here's my Question:  Do we also need another 2" hard foam board on top of the slab?  Or is the 2" foam board under the slab + the 2" foam board vertically around sides/edges of slab and subfloor enough to counter heat loss from above in conjunction w/ any heat from the PEX hydronic heating tubes located directly under the floor above?  Please let me know your opinion as I'm not sure if a second 2" foam board insulation on top of the slab is really necessary or not. Thank u VERY much in advance.  Look forward 2 hearing your answer!

 

Responses (3)

Eldon Krugman
Eldon Krugman 3 weeks ago

Prior to placing a concrete slab, one should consider if radon is an issue. If so, make preparations for subslab ventilation. When curing, a concrete slab should have the ability for the hydration to occur above and below, so there should be a layer of sand between the foam and slab. Please have a detailed heat loss calculation for the entire structure.  Depending on your heating degree day location, more than two inches of foam may be a cost effective solution. YOU really only have one time to get this correct. Finally, rigid foam should NOT be exposed in a crawlspace as it is a fire hazard, requiring some type of cover, perhaps drywall.