What to do about the vapor barrier?
I am currently using a formerly unheated basement room as home office. The room is not properly insulated and rather hard to keep warm. I am thinking about installing a nicer floor and insulating it to make the climate more pleasant. The floor is currently bare concrete and I assume there is no vapor barrier under the slab. About 50% of the room is below the ground. We live in the Arctic, so the temperature ranges from about -35C to 25C, and it is mostly considerably colder on the outside than inside.
I don't have much experience and am on a very tight budget.
The initial idea was to lay out PE foil as vapor barrier, build a wooden grid (60cm wide) to fill with glass or rock wool and to install wooden planks on top to walk on.
I am very unsure about the materials and vapor barrier thoug.
According to previous replies, I would end up with trapped water under my foil without channels to escape if the foil was placed directly on the concrete. As I can not build a pit, that makes me consider to ditch the vapor barrier altogether, to allow some ventilation, but that seems to eventually soak my materials.
I am not primarily concerned about the materials as in bent wooden floor planks after a few years. I am mainly concerned about mold, I don’t want to make the room a few degrees warmer but trade it for smell or an unhealthy environment.
I have attached an artistic masterpiece (sorry!) of what I mean and hope that someone might have a pointer what you would do.
I am still lost and would appreciate any help, I am also open to different materials!
Best Answer
The best place is for the vapor barrier to be is UNDER the concrete slab. This prevents moisture from the ground from seeping through the slab into the living space.
Assuming that's not the case, then your next best choice is to apply a moisture channeling product on top of the slab. You don't want to use poly film here as that will cause moisture to condense between the slab and the poly with nowhere to go.
There are any number of commercial moisture channeling products but these provide both a vapor barrier and a system of channels for the trapped moisture to drain, most often to your sump pit.
Even though your floor is "dry" now, that doesn't mean there is no moisture coming through it. Concrete is porous and allows water to penetrate. The reason it seems dry is that the moisture is evaporating into the air. Try taping a sheet of poly to a small area of the floor for a few days and you should see moisture trapped under it.