Replies

1. 
   Mucho | Mar 04, 2017 02:15am | #1

   Sorry, I forgot a sub-question: Will I be losing out on moisture barrier if I used 1" instead of 2" XPS in this situation?

2. GBA Editor
   Martin Holladay | Mar 04, 2017 06:25am | #2

   Dow Styrospan is ordinary extruded polystyrene (XPS). It will perform well in the applications you list. That said, most green builders avoid the use of XPS, because it is manufactured with a blowing agent that has a high global warming potential.

   Most green builders would choose either EPS or polyisocyanurate for these applications. For more information, see Choosing Rigid Foam.

   -- Martin Holladay

3. GBA Editor
   Martin Holladay | Mar 04, 2017 06:33am | #3

   Q. "Will I be losing out on moisture barrier if I used 1 inch instead of 2 inch XPS in this situation?"

   A. I'm not sure what you mean by a "moisture barrier." Are you interested in knowing the vapor permeance of the XPS?

   1 inch of XPS has a permeance of 1.1 perms
   2 inches of XPS has a permeance of 0.55 perms.

   These figures won't tell you much about whether these materials are suitable for use as wall insulation or rim joist insulation.

   In the U.S., building codes require a minimum of R-15 insulation for basement walls in Climate Zones 5 and 6. If you are using XPS, that means a minimum of 3 inches of insulation.

   For more information on these topics, see these two articles:

   How to Insulate a Basement Wall

   Insulating rim joists

4. 
   user-2310254 | Mar 04, 2017 12:09pm | #4

   Reclaimed rigid insulation will be much less expensive and, if using XPS, have less environmental impact.

5. 
   Mucho | Mar 04, 2017 12:40pm | #5

   Right, I forgot about that aspect of XPS - thanks for reminding me Martin. I'm less excited about the sale now..

   Also, yes, vapor permeance is what I am wondering about. Since I am retrofitting, it would be easier for me to use 1" rigid foam, but all specific information and examples that I find online talk about using 2".

   So, the question is: Since I will up the R value to of these walls with Roxul comfortbatt to meet/exceed code, am I losing anything by using 1" rigid foam compared to 2"?

6. GBA Editor
   Martin Holladay | Mar 05, 2017 05:07am | #6

   The reason that the vapor permeance of the rigid foam is irrelevant is that you shouldn't expect any diffusion drying through the rigid foam. The idea is to install a thick enough layer of rigid foam to ensure that the first condensing surface (the innermost surface of the rigid foam) is above the dew point during the winter.

   If you will be installing a combination of rigid foam and mineral wool batts on the interior side of a basement wall, you need to make sure that the rigid foam is thick enough to avoid condensation. You can use the guidelines in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

   (Even though that article discusses above-grade walls, the ratios of rigid foam R-value to fluffy foam R-value that are discussed in the article represent a conservative approach for below-grade walls.)

   So, if you plan to install a 2x4 wall insulated with mineral wool on the interior side of your basement wall, you would first need to install a continuous layer of rigid foam with a minimum R-value of R-5 in Climate Zone 5, or R-7.5 in Climate Zone 6.

   -- Martin Holladay

7. 
   Mucho | Mar 05, 2017 11:05am | #7

   Thank you for clarifying that for me Martin, I wasn't sure that article was relevant for insulating interior walls. Think I'm going to go 2 inch to play it safe.