bathroominsulation
I'm renovating my bathroom and I need to insulate the floor. It's a concrete slab with 2×4 joists set on one brick piers.
The home was built 50 yrs ago.
There was no insulation at all in the floor. And the bathroom is freezing in the winter and hot in the summer.
If I slide in pieces of rigid foam board will I have problems with condensation rotting out the 2x4s?
Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.
That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.
The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.
Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.
Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.
Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.
There is a rigid polyisocyanurate foam core insulation, often sold under the brand name Super Tuff from Dow that has a higher R value than XPS. It is R-13 in 2 inch panels and is higher if you trap a small air barrier behind it (around R-15).
The foam has a foil vapor barrier on the back.
You could
HOWEVER: The paneling could hold itself up when mounted this way, and you could mount lighweight things, such as pictures. The walls would NOT be suitable for mounting shelves, fixtures, heavy mirrors, etc. You would need to take care and inform any subsequent owner that the walls were not intended for bearing any signifciant weight!
Best Answer
Your plan is what most people & contractors do. But, latest basement testing has revealed that even with foam board or spray-in foam you need 6-mil poly against the concrete & then also on top of it. Under-it stops moisture & air from wanting to come in & On-it stops moisture & air from wanting to penetrate it. Here's a really good video breakdown I recently ran across of what's been determined.