Replies

1. 
   Robert Riversong|Sep 28, 2009 09:57pm|#1

   Both XPS and EPS have been used successfully below grade, but XPS is still the standard because it typically has more compressive strength and higher R-value per inch.

   Your thermal design program should specify a certain required R-value for sub-slab insulation rather than a certain thickness. 3" of EPS would offer no more than R-12, while the same thickness of XPS would give you R-15 (a 25% increase).

   Where are you building and what is the heating Degree-Day total for your climate zone? In a cold climate (>7200 HDD) and for a highly-insulated house, R-20 to R-30 subslab insulation is commonly recommended for radiant slabs. Be sure to insulate the slab edges at least to the level of the sub-slab insulation, as most heat is lost from a slab at the edges to the ambient air temperature.

2. 
   Anonymous|Sep 28, 2009 11:38pm|#2

   Hi Robert ~
   We're building in southernmost Oregon (Medford area), with a HDD total of 4,566. Code calls for R-10 under-slab insulation, but we'd like to boost it to R-15, since we're relying on radiant floor and passive solar heating.

   I did find a good PDF on slab insulation details at buildingscience.com. Do you have any other sites to recommend?

   Thanks,
   Claire

3. 
   Robert Riversong|Sep 29, 2009 06:14pm|#3

   我想你已经看到了俄勒冈州的住宅Energy Code brochure on slab insulation:http://www.oregon.gov/ENERGY/CONS/Codes/docs/ResPub_9.pdf.

   They require R-15 slab edge insulation for all heated spaces. The slab edge insulation value can be the combined R-value of perimeter insulation and slab-edge, if it's not a monolithic pour. But since the majority of heat loss is from the edge, if you're increasing subslab insulation to R-15, you might consider increasing slab-edge insulation to R-20.