Replies

1. Expert Member
   Dana Dorsett | Apr 18, 2013 04:28pm | #1

   Cost

   insulation type

   exterior finish type on the foundation

   thermal bridging at the foundation sill & band joist

   siding type

   intended total R-value

   Performance wise it's often useful to keep the thermal mass of the concrete fully inside the thermal boundary of the house, and exterior foam on the foundation aligned with a foam-clad upper story wall can make a seamless thermal break at the band joist & foundation walls.

   But with brick-veneer houses you can't get the continuous foam, suffering a huge thermal bridge at the foundation top, and performance can be better with an interior foam aproach.

   Also, if you're insulating the slab it's easier to get a continuous thermal break at the slab/wall interface with an interior side foam approach, with the slab-foam meeting the wall-foam, "floating" the slab.

   Polyiso can be used on the interior side only for below grad apps due to it's slightly hygroscopic nature. Wedged between dirt & concrete it'll almost always saturate over time, losing a large fraction of it's thermal performance. EPS does much better, and is preferable to XPS on a number of basis'.

   Insulated concrete forms (ICF) give you something of the best of both worlds- for a price. From a "time is money" perspective it speeds up the whole process of an insulated foundation, but from a raw assembled cost it's usually a bit more expensive than an insulate-from-the-inside-only approach. A bare-bones ICF starts at R16 (R8 inside, R8 outside), and plenty for foundation insulation in a US zone 5 climate, but R20s are pretty common and may not be a big up-charge (if any).

   Only if you're looking for VERY high performance (Net Zero Energy, PassiveHouse certification, etc.) would going much over R20 make much sense in that climate, and even R20 might be a stretch.

2. 
   user-1112730186 | Apr 18, 2013 04:54pm | #2

   Thanks. How much do you consider interior condensation on a concrete wall if you go with an exterior foam approach?

3. Expert Member
   Dana Dorsett | Apr 18, 2013 05:33pm | #3

   With an exterior foam approach the concrete is inside the thermal envelope where it's warm. If the concrete is cool enough to condense moisture out of the air, so is everything else in the room. At normal healthy interior humidity of say 35% RH @ 68F the dew point of that air is ~39F, a temperature the concrete will never reach when it's inside the insulation.

   With healthy 50% RH @ 75Fconditioned air the dew point of that air is ~55F, again a temp the concrete won't be experiencing in a 75F room when it's inside the insulation.

4. 
   user-1112730186 | Apr 18, 2013 07:46pm | #4

   Interesting to note the following building science recommendation with much suggestion for the interior application:

   http://www.buildingscience.com/documents/information-sheets/basement-insulation/files/bscinfo_511_basement_insulation.pdf

5. GBA Editor
   Martin Holladay | Apr 19, 2013 07:02am | #5

   Heather,
   In the following article, I provide a list of the pluses and minuses of both insulation approaches (on the interior of a basement wall or on the exterior of a basement wall): How to Insulate a Basement Wall.