Replies

1. GBA Editor
   Martin Holladay | Jun 21, 2013 05:08pm | #1

   Doug,
   The most important part of the wall to insulate is the top of the foundation wall -- the above-grade portion. You say that you want to leave this portion of the wall exposed on both the interior and the exterior. But if you want to insulate it -- and I think you should -- you'll only be able to leave one side of the wall exposed. The other has to be covered up. It's your choice which side to cover.

   Choose one side to insulate, and insulate the wall from the footing to the top of the stone wall. Rigid foam won't work for this application, so you'll have to use closed-cell spray polyurethane foam. It's perfectly OK to use closed-cell spray foam below grade, and in contact with the soil.

2. Expert Member
   Dana Dorsett | Jun 21, 2013 05:16pm | #2

   Some benefit, sure but leaving 2.5' of exposed stone is a HUGE thermal bridge, and usually leaks air to boot.

   How bad is it?

   Pretty bad!

   You don't say how thick it is or the type of stone, but say it's about a foot to 18" thick which will have a U-factor of about 1 BTU per hour per degree-F per square foot. When it's +20F outside and 70F indoors, it's losing on the order of 50BTU/hr per square foot. In a house with a ~1500 square foot footprint you'd have about 150' of perimeter, and with 2.5' of exposure, that's 375 square feet, and (375 x 50=) ~19,000 BTU/hr.

   That's more than half the heat load of my whole house at +5F (my 99% outside design temp.) And that's just the above-grade section. The average heat load of the below grade section might average might add another 5000-8000BTU/hr, but insulating the exterior only up to grade would only cut that part by half, since the stone also thermally bridges downward. At -10F your basement will have a truly huge heat load.

   So, by insulating the exterior up to grade you'll make a tiny dent in the basement's heat load, maybe even as much as 15%, , but it also leaves the thermal bridge to the soil at the footing, which in cold climates represents a real heat load 24/365 (and don't even get started about "free air conditioning"). But if you insulate the interior from your sub-slab foam all the way up and over the foundation sill and band joist you'd be reducing the heat loss by a major fraction, more than 90% if you do a decent job of it.

   Insulating on the exterior-only and below-grade-only is a bit like building a so-so barn, but leaving an open archway rather than a door that closes.

3. 
   poky1234 | Jun 21, 2013 06:00pm | #3

   Thanks for the quick and detailed replies. I'll take the advice you both gave and I will insulate all the way up the wall. The rest of the house has been spray foamed with closed cell foam. The stone foundation wall is 18 inches thick with mortared joints and granite stone.
   One question for Martin though. Why cant rigid foam boards be used? Would the spray foam be applied on the outside of the dimple waterproofing membrane?

   Thanks,
   Doug

4. GBA Editor
   Martin Holladay | Jun 22, 2013 02:32pm | #4

   Doug,
   If you are insulating the exterior of the wall, I would spray the foam insulation directly against the stone. Then the dimpled drainage membrane could be installed on the exterior side of the cured foam.

   Rigid foam won't work well because of the roughness and unevenness of the stone wall. The air gaps between the stone and the insulation allow convective currents to form and undermine the thermal effectiveness of the insulation.

5. 
   poky1234 | Jun 22, 2013 10:10pm | #5

   Thanks for the reply Martin. I assume from your answer that rigid foam would not be effective on a stone wall due to the fact that the stone is irregular and that the rigid foam would not be able to sit tight and flat against the wall. My question then is why do the 2 major manufacturers of dimple moisture membranes , Delta MS and Platon, recommend putting the dimple moisture membrane against the foundation wall first then rigid insulation behind, The dimples hold the membrane and foam away from the wall just as the irregular profile of the stone would.

   Thanks,

   Doug

6. GBA Editor
   Martin Holladay | Jun 23, 2013 05:18am | #6

   Doug,
   Q. "Why do the 2 major manufacturers of dimple moisture membranes, Delta MS and Platon, recommend putting the dimple moisture membrane against the foundation wall first then rigid insulation behind?"

   A. Good question. I don't know.

   If you want to use rigid foam instead of spray foam, I would advise sealing the perimeter of the rigid foam with canned spray foam when installing the rigid foam. On my own basement, I insulated with rigid foam on the exterior of my stone walls, and was never satisfied with the installation. Spray foam makes more sense to me.