Replies

1. 
   STEPHEN SHEEHY | Aug 07, 2016 08:39am | #1

   Both are fine. Buy recycled.

2. Expert Member
   Dana Dorsett | Aug 08, 2016 03:06pm | #2

   The R5/inch eventually drops to R4.2/inch for 1.5lb density XPS (Foamular 150), as it's HFC blowing agents leak out to do their climate damage. At 20 years it can't be counted on for even R4.5/inch despite the "lifetime warranty" of always retaining at least 90% of the labeled R. (It's a warranty that would rarely be taken, since the cost of sampling and re-testing, and the cost of re-installing would make collecting on it prohibitive.)

   At 1.5lbs density "Type-II" EPS, is also R4.2, and does not rely on blowing agents for it's performance- it 's performance on day 20,000 will be pretty much what is was on day 1.

   Cost-wise Type-II EPS is typically 20-25% cheaper per labeled R than XPS (and it's a stable R over time.)

   XPS has a somewhat higher compression rating than EPS, but they are both several times higher than necessary for supporting a slab. (But neither are good enough for going under a footing that is supporting the whole house unless the footing has been engineered for it.)

   Type-I EPS (1lb per cubic foot nominal density) is fine for below grade walls, but can take on enough moisture under slabs to take a performance hit under super-wet conditions. It's usually labeled ~R3.9/inch.

   Cutting EPS with a heated 4" putty knife with (with the side sharpened) makes quick work of up to 1.5" material (R6-ish). Some people rig up heated wire for doing artsy-craftsy complex shape cutting of polystyrene, but for straight cuts heating a putty knife blade works fine.

3. GBA Editor
   Martin Holladay | Aug 09, 2016 05:12am | #3

   Peter,
   Stephen and Dana gave you good advice. For more information, see this article: Choosing Rigid Foam.