Posted By robert.thompson on 10 Feb 2013 12:51 PM
Hello:
The point of the following question is to determine the most cost-effective way, both in materials and heating costs, to keep the floor of my house warm.
My 1200 sf single story house foundation will be built on ARXX Prime ICF's (R-22) directly on bedrock and will vary from 2' to 6' in height - average about 4'.
The floor of the crawl-space is cleared of all earth and is 100% exposed bedrock, roughly 1200 sf of solid stone. It is very uneven.
I am wondering if, after sealing out moisture by covering the bedrock floor with heavy plastic sealed to the ICF walls and supplying 'some' heat to the space, filling the floor joists above with Roxul insulation batts (to insulate the floor above) would be a good idea?
The walls of the house will be double-wall with an R-value of either 42 or 50.
The approximate cost to insulate the floor joists to R-24 Roxul 5.5” batts would be $2000.
The approximate cost to spray foam the bedrock floor to R-12 would be $2800, due to the it's uneveness.
Any suggestions would be appreciated.
Rob.
Interesting that no one else has responded already. My house is all ICF with concrete footings and dirt crawl space. I put insulation around the footings, poly sheeting on the earth covered with 3/4" XPS and on top of that about a 2" concrete 'rat slab'. So my crawl is much the same as yours except for the insulation under the rat slab.
My walls are continuous ICF from the footing to the roof so perimeter sealing was not an issue. I put my air handler for the heat pump in the crawl space and use the entire crawl space as the supply plenum. There is no insulation under the floor. This gives me a guasi radiant heated floor. The concrete slab works as a thermal mass to even out heating and cooling fluctuations.
So, with this in mind my first question is how much earth fill is there above the rock on the outside? What climate are you in? Does snow cover the ground all winter, or no? Are you heating or cooling predominant?
My thoughts are first, make sure the perimeter is well sealed. You will have to exercise particular care there since you have an ICF to frame interface. The ICF will provide adequate perimeter insulation. Make sure the ICF is sealed to the rock with spray foam. Then, condition the crawl space. If you want to make it part of the air system of the house, like mine is, then the foam will need to covered with a fire barrier like 1/2" drywall. Otherwise, duct some of the air into the crawl space to keep it at house temp and exhaust it outside through an HRV or ERV. Consider taking advantage of all that exposed bedrock as thermal mass.
No way would I waste money insulating the underside of the floor. And, don't spend money on poly over the rock, unless there are a lot of openings or cracks that lead down to earth. Moisture isn't going to migrate through rock. If you ventilate the crawl to the outside you don't need to be concerned about radon, if that is an issue in your area.
Now, if the bedrock is exposed outside the house so that you are faced with having a thermal bridge from outside to inside the crawl space then all the above needs to be rethought. In that case I would put foam on the rock, even at the additional cost. Just make provision of some sort to walk and crawl around on it, like raised walkways, without smashing it down so you can do maintenance on the house.