Replies

1. GBA Editor
   Martin Holladay | Sep 19, 2016 01:37pm | #1

   Sacie,
   You guessed correctly -- the payback period for the investment cost of installing rigid foam on a crawl space floor is quite long.

   The main reason to install rigid foam under a basement slab has nothing to do with energy savings. It has to do with raising the temperature of the slab (in order to reduce the chance that moisture in the indoor air will condense on the slab, encouraging mold and funny odors).

   No one spends time in a crawl space, so the odor issue is a little less critical. Moreover, no one ever installs carpeting in a crawl space -- while they do install carpeting in basements. (Without rigid foam under the slab, installing carpeting on a basement slab is risky.) Finally, when homeowners use their basements during the summer, they are more likely to introduce outdoor air into the basement (and the introduction of outdoor air is associated with condensation). Since people don't hang out in their crawl space, this worry doesn't apply to crawl spaces.

2. 
   Sacie | Sep 19, 2016 02:09pm | #2

   Perfect Martin, thanks

   How about helping me with another question? In this very old house we are redoing, we plan to put a new standing seam type roof on it. We have gone through a number of proposals but now have come down to this:

   I like the gusset idea and think it will be the most cost effective and reduce thermal bridging to almost zero by using narrow plywood gussets and 2x4's at our desired depth. To that end, I propose we eliminate the XPS or Iso on the exterior all together to save move labor/money. Lets stick with 1.5-2" closed cell foam (R-10/13) on the underside of the sheathing to stop vapor transmission to the roof deck. lets make the gussets long enough to get 11" of cellulose on the inside and use the netting staples to the new 2x4's to achieve another R-40 for combined R-50+/-. We will use the typical air-tight high heat resistant peel and stick underlayment that is needed for the metal roof over the existing OSB on the roof.

   Will appreciate your comments on this scenario please. S

3. GBA Editor
   Martin Holladay | Sep 19, 2016 02:31pm | #3

   Sacie,
   Your plan to install only R-10 of closed-cell spray foam on the underside of your roof sheathing will only work if you live in Climate Zone Marine 4. If you live anywhere colder, you will need a thicker layer of closed-cell foam.

   Here are the minimum R-values for the closed-cell spray foam layer in this type of assembly:
   R-15 for Climate Zones 4A and 4B,
   R-20 for Climate Zone 5,
   R-25 for Climate Zone 6,
   R-30 for Climate Zone 7, and
   R-35 for Climate Zone 8.

   For more details on this approach, see How to Build an Insulated Cathedral Ceiling.

4. 
   Sacie | Sep 20, 2016 09:45am | #4

   Thanks Martin, I forgot to state our zone which is 4A/B. I had read that article you cite numerous time but along with all the rest forgot the details. Much appreciate the fast useful response.