Insulation on basement slab question: Polyethylene vs. Delta FL under rigid XPS?
We have a 6 year old cottage in Ontario with walkout basement with bedroom and rec room on the window end and utility area at other “subterranean” end. It is not heated in the winter except weekends when we light up the wood stove. Walls are insulated to R22. Floor is not. Basement floor temperature has been observed to drop to near 0 degrees C at exposed daylight wall in middle of winter.I don’t know if there is poly or clear stone under slab but am certain there is no insulation. I use dehumidifier to keep moisture down to 50% in summer. The goal is to get a thermal break so that the floor is not so cold in winter.
The concrete floor has no leaks and is white/ dry everywhere where exposed. In bedroom where I had laid a 12×12 carpet over 6 mil plastic for past few years. Last weekend I found the concrete is very dark and the underside of plastic was wet to touch. I am taking this to be vapour coming up through the floor and being stopped by the plastic. When the plastic was removed and floor left to air dry a few hours it started to lighten up. So after an initial moisture panic I think it is evidence that the plastic was doing its job and the white floor elsewhere is evidence of the slab drying into the basement.
My goal is to insulate the floor in bedroom and rec room. Reading through numerous web sites and information here there is a lot of information-some conflicting- as to best way to proceed. Under slab options are not possible. From information on this site I have settled in on 3/4” T&G OSB laid over 1.5″ rigid EXP insulation on vapour retarder on slab. What is challenging me is the vapour retarder. Poly sheeting is referenced numerous times on this site, as is the dimpled sheeting such as Delta FL. My questions are:
For this situation is there benefit of using the dimple sheet instead of the poly?
Does the air gap from the dimple make a difference?
Should I be sealing the perimeter of whichever one I use?
I ask the latter questions because some dimple sheet web sites suggest leaving a 1/4” gap to let vapour escape to dry the slab; whereas others suggest sealing it tight.
I see one Building Science web page promoted the air gap as good thing stating it enables a vapour stasis to be achieved; however, the page didn’t show details at perimeter. I’m assuming it must have been a sealed air space.
Any insights/advice would be appreciated.
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