Re: slab insulation
ed21 and jkirk ----- if the slab was below grade ( basement ) then i agree with you guys.
However - if it's a slab on grade this changes things. The temperature differential will be greater and with no thermal break between the slab and ground , the floor will be colder and could result in higher heat loss ---- though 50% seems high.
Using rigid foam laid on the slab and covered with plywood is a fairly common system used around these parts in finished basements and on grade slabs.
Originally Posted by paulnederland
Ideally with slab on grade it also goes a long way to insulate along the exterior edges of the slab to provide a thermal break from the exterior.
While I'm uncertain as to the origin of the method I believe it was adopted from the commercial roofing systems where rigid foam is laid on the decking first as a thermal insulator and covered over with a wood ply material and roofing layer.
This system for floors is an effective way to provide a comfortable continuous thermal insulated floor as well as providing some softness as opposed to the hard concrete.
Depending on the type and brand of rigid foam they will have different compressive strengths. The common types found at the home centers will usually be in the 25 - 30 psi range regardless of thickness. Generally --- for recreation areas --- such as a finished basement --- they will work for the floor. For high traffic and for the main living areas it might be advisable to go with the type having 40 psi or more compressive strength.
While the foam itself is soft and easily punctured having a solid surface like the concrete slab on one side and a plywood surface on the other creates a solid base. Having this type of sandwich with solid material displaces any point loads which doesn't affect the foam. Personally I would recommend 5/8 inch tongue and groove plywood over the 1/2 inch since the thicker plywood provides a better base for any finish floor covering.
As for putting wood flooring down --- this can be tricky depending on the final temperature of the floor surface regardless of putting the foam down. If the floor surface temperature is cool enough and compared to the temperature and relative humidity of the air in the home could cause condensation under the wood flooring causing cupping or other moisture related issues ----- depends.
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