I'm making plans to finish my basement, adding three rooms and insulating it. The plan is to use the foam board insulation attached directly to the concrete, then framing over it. My question has two parts: do I have to leave a space at the bottom of the Sheetrock (I've seen posts that say 1/2" to 3/4"; the second part is, I've seen posts that say to leave a dove of 1/2"-1" between the frame and the insulation. Is this necessary to do, and why? It would seem to make sense to have it right against it. Thanks

 

If you mean a space between the concrete floor and the gyproc, yes you do want that 1/2" or so. This is so dampness from the concrete cannot wick up into the drywall and lead to mold.

Do not leave any space between framework and foam. You want to avoid any air circulation behind the wall.

 

Basements can be a pain and one of the primary problems is moisture, botf liquid water and moisture vapor. Even a basement that appears to be dry will have some vapor passing through. That is where the rigid insulation can help as it can allow a small amount of vapor to dry to the inside, avoids moisture accumulating behind.

So, be sure you have addressed all water/moisture issues before the walls go up.

Additionally, continue that rigid foam over the top edge of the foundation, sealing the sil plate to the concrete. Then detail some rigid into each rim joist cavity. Link below, Fig 10.
Understanding Basements | Building Science Corporation

Bud

 

Thanks guys! The link was good to read, and I was thinking it made more sense to keep things tight rather than leaving a space.

 

The only good argument I've seen for leaving a space is when the basement walls are not straight or plumb. Then spacing the wall out just enough to make the pall good is not a major issue, especially if you are filling the cavity with fiber insulation. BTW, I like Roxul for cavity insulation as it does well in basements.

Bud