Insulating under deep, heated basement slab: rockwool vs EPS, & depth?

7 years ago

I wonder if anyone here has insulated a basement slab with Rockwool or similar. I believe the product we are looking at is the Comfortboard 80. (I prefer to limit our use of foam products.) Our basement slab will be about 10.5' to 11' below grade, in well-drained soil, zone 4/5(?) in Eastern Mass. Right now we have a gravel bed, and are planning to lay down a taped poly barrier, then insulation, then a 4" slab with hydronic radiant heating embedded. The walls will be insulated with R19 Rockwool batts, plus a smart vapor barrier taped to the poly. I am aware of the cost difference between the Rockwool and EPS foam board insulation, but am unsure about any differences in ease of installation, or how easily they can be damaged at the perimeter thermal break.

I am also uncertain as how many inches to put down under the slab. Our slab will be very deep in dry soil, and we plan to include a thermal break between the slab and the footing. The discussions and advice I have been finding online have been contradictory. 2" seems to be the standard, some people say perimeter only/none, and I am considering going to 3" or even 3.5".

If you have some personal experience with installing mineral wool under a slab, or educated opinion about the amount of insulation (inches/R) to instal at this basement depth, I would LOVE to hear from you! Thank you in advance.

Casey

Featured Answer

caseyb
Original Author

5 years ago

@Bongo - I ended up using 3" (2" + 1") of EPS. We used very little foam in the house, but it just made sense there.

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Comments (7)

just_janni
7 years ago
last modified: 7 years ago
How is rock wool going to meet the compressive strength requirements for a stable slab? And if you compress it that much - it's no longer insulates, right?

I have 6" of EPS under my slab. It's high compression EDITED - it's XPS that is 40PSI (i think) which is higher than the soil compaction PSI requirement. Over that a taped 15Mil vapor barrier - Stegowrap

Curious about the concern about EPS/XPS. We had a concern over the blowing chemicals (hexobromododecane? I think??? which is used in the manufacture / fire retardant process and is bioaccumulating etc - so we found a product that used CO2 in blowing process and imported from Finland......... FINNFOAM.

Briefly looked at a product called FOAMGLAS too - but that was stupid expensive.
caseyb
Original Author
7 years ago
The rockwool product in question is approved for use in this application. The concern about EPS foam in this case would be mainly over its 'green'-ness, more than an off-gassing concern, and rather than a fire safety concern.
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Bongo
5 years ago
@caseyb did you ever install the rockwool? we are looking to do exactly same thing in our build. thanks!
worthy
5 years ago
last modified: 5 years ago
When the Rockwool installation instructions caution you not to stand on the corners of the insulation, it makes you think twice how it will behave under tons of concrete. We're using EPS or XPS under our slab--depending on the relative costs.
Bongo
5 years ago
@worthy Rockwool states their board is approved for under slab installation. The rigid foam is not asvised for under ground applications when termites are a threat.
worthy
5 years ago
last modified: 5 years ago
∆∆∆∆

Definitely a consideration.

Rockwool installers are advised to use N95 or better masks to avoid inhalation risks. And, of course, high fossil burning heat is used in its creation.

XPS is now available without hexabromocyclododecane (HBCD) and legislation has halted the use of GWP HFCs. EPS (expanded polystyrene) is considered the most benign of foams.