I installed floor trusses over a slab. I had applied 6 mil poly over the slab then filled the depth of the trusses with weather shield blown insulation. I then sealed the space ,with 6 mil poly across the top before putting down the subfloor. The cavity has approx. 3″ air space between the top of the insulation and the underside of the poly covered subfloor. This space between the two layers of poly is vented into an adjascent heated space. The room above the floor trusses is heated as well. Have I made a mistake by placing the poly over the slab under the blown insulation?
Grant
Replies
If the material under the slab is dry and drains well you should not get too much moisture but I think you will get some between the concrete and the poly. Poly on concrete tends to draw moisture out. It sounds like the joist are exposed on one end and air can circulate into the 3" space, this is good. But it is never good to make a poly sandwich, I take it you laid a layer of poly on top of the joists then nailed down the sub floor.
I would have raised the joists off the concrete with pwf material and insulated so the material did not touch the concrete. Keeping the 3" space between the concrete and the insulation, and then nailed and glued the sub floor without poly. I am not sure if it is ever a good practice to poly over concrete. The chances are, if your house is fairly new, there is a layer of poly under the concrete. So you would have a triple decker sandwich.
Time will tell, the 3" air space vented at one end is good, this will keep things dry. If you sealed the concrete poly well this will also help.
Thanx for the response Dave. I built a 2000 sq ft workshop. One end approx. 36x14 is intended as the finishing room where I will assemble and apply finishes to the furnature. I wish to maintain the temperature in there at 72. The rest at 55-60.
The slab is about 6" thick, with 16" compacted crushed stone over sand base. The portion under the finishing room does have rigid insulation under it. The floor trusses are sitting on top of sill plates c/w gaskets so they are not in contact with the concrete at all. I wrapped the poly then installed the headers. My intent was to create a sealed dead air cavity in the winter and open the vents and provide mechanical air movement under the floor for three seasons.
However the day I blew in the insulation was very hot and humid and there was a lot of condensation trapped in the plastic envelope. It took a month or so before I was able to get the building roof and walls closed in and get around to cutting in the vents for the raised floor area. I was concerned at the dampness I found in the blown insulation. I knew it would have some but I was surprised at the amount. My real concern was that there was a dew point with-in the poly envelope and it was effectively trapping and retaining the moisture.
After posting my question I contacted the insulation people and explained my concern. They have assured me that the insulation will dry out, but it may take a couple years unless I pull some of the floor off and let it breath for a few months to extract the moisture. Once it's gone though I can close in the floor and should have no problems in the future.
You live and learn. I would never do it that way again.