Replies

1. GBA Editor
   Martin Holladay | Jan 29, 2018 01:25pm | #1

   CB416,
   First of all, can you tell us your name?

   The risk of sill rot depends on several factors. The risk is higher if the basement is damp, and lower if the basement is generally dry. The risk is higher if the sill is close to the outdoor grade, and lower if the sill is well above grade. The risk is higher if your house is surrounded by bushes, and lower if the above-grade portion of your house foundation is exposed to sunlight and wind.

   In most cases, the advantages of insulating the rim joists on the interior outweigh the disadvantages.

   You want to aim for an airtight installation, not a "loose" installation. So yes, caulk before you insulate.

   Either EPS, polyiso, or XPS can work, although EPS or polyiso would be preferable from an environmental standpoint. (For more information on this issue, see Choosing Rigid Foam.)

   For more information on the insulating and caulking work, see Insulating rim joists.

2. 
   cb416 | Jan 31, 2018 10:16pm | #2

   Thanks Martin, it was actually the article for FineHomeBuilding that brought me here.

   My basement has two different elevations for the foundation. The finished side is above grade by more than a foot, the unfinished side is a crawl space, but with a fully enclosed foundation, the sill there is maybe 10" above grade. It's mostly exposed to sun and air on the exterior, with the exception of a deck that hides it on the south-facing wall. The floor above the crawl space has R22 in the joists but it doesn't do much for the floor. I'm thinking of putting XPS around the entire crawl space since the walls are only 5ft high.

   When we say caulk, exactly what type should I purchase? I've seen references to acoustical sealant and just silicone caulk. Is one preferred over the other?

   Would a tight XPS piece sealed with caulk be preferred over a looser fitted XPS piece with canned foam? My house is oldish, probably not much wood movement now, but caulk seems easier to work with.

3. 
   cb416 | Jan 31, 2018 10:16pm | #3

   Sorry, meant to sign off with my name!

   Thanks, Colin.

4. GBA Editor
   Martin Holladay | Feb 01, 2018 07:01am | #4

   Colin,
   You wrote, "The floor above the crawl space has R-22 in the joists but it doesn't do much for the floor. I'm thinking of putting XPS around the entire crawl space since the walls are only 5 ft high."

   It sounds as if you have realized that the crawl space insulation is in the wrong place. You need to insulate your crawl space walls, not your crawl space ceiling. For more information on this issue, see Building an Unvented Crawl Space.

   Although you could insulate your crawl space walls with XPS if you want, green builders generally avoid the use of XPS because it is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see Choosing Rigid Foam. Either EPS or polyiso would be a better choice.

   Q. "When we say caulk, exactly what type should I purchase? I've seen references to acoustical sealant and just silicone caulk. Is one preferred over the other?"

   A. Use either silicone or polyurethane caulk -- something a step up from latex painter's caulk. Acoustical sealant is notoriously messy.

   Q. "Would a tight XPS piece sealed with caulk be preferred over a looser fitted XPS piece with canned foam?"

   A. First of all, it's better to use EPS or polyiso, not XPS, for environmental reasons. Second, the best approach is to cut the rigid foam a little small, to make it easier to insert your nozzle when you seal the perimeter of the foam with canned spray foam. If you prefer to seal with caulk rather than canned spray foam, practice cutting the rigid foam so that the crack on the perimeter is neither too small nor too wide for caulking.