Hi, I have retrofit basement slab insulation question. My basement walls and perimeter joists are insulated with a slightly more than R15 factor. The basement is overall well insulated and very air tight. But still, the basement feel cold in the winters. I live in Canada (near Quebec city) and I realized that it was probably because of the uninsulated basement slab. When the set room temperature is 22 degrees Celcius, the floor temperature is at more or less 19 degrees. The soil beneath the slab is a fine one that drains water very well. I guess the soil beneath the slab acts like a big thermal mass because when the set temperature is 18 degrees, the floor is at 17 degrees and it takes several days for it to reach 19 degrees when the set temperature is at 22 degrees. The floor covering is actually a very good quality porcelain tile (well, that's an issue because I guess I would have to either destroy it or put some insulation over it and then put another floor covering). What would you recommend to have a confortable basement? I guess the only option is to have a basement slab retrofit project? And/or radiant heating? If so, what would you suggest? Thank you very much!

How about a rug, with a cm of XPS foam underneath it? Bevel the edges so you don't get a bump.

The deep subsoil temperatures around Quebec City are about 5-7C, so the soil beneath your slab must be pretty dry if the tile stays at 19C in a 22C room! But it's still a significant heat loss.

As long as the basement doesn't have any water issues, 1-2cm of XPS foam with OSB or plywood on top, with a wooden finish floor and area-rugs would increase comfort levels. If you were to put radiant floor there you would need to go at least 5cm on the foam, but that would be even MORE comfortable.