Under-slab insulation: XPS or EPS?
Claire Anderson
| Posted in Green Building Techniques on
We’re building a slab-on-grade home with radiant floor hydronics in the floor, and are wondering what type of rigid-foam insulation is best for under-slab insulation. (We’re planning on 3″ inches of insulation, no matter what product we use.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
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.
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
I assume you've seen the Oregon Residential 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.