FPSF ICF’s andRadiant insulation Detail?
I am about to build a house in Vermont, from the footings up, with ICF’s. The house is cut into a slope, so the footings in the back will be just below the slab and 8′ below grade. But, the front will be a walk out, and the garage door on the right front side. We are looking at using FPSF (Frost Protected Shallow Foundation) in the area in the front and around the side until we are deep enough below grade. The slab on grade in the house will have, radiant heat. I normally put 2″ of insulation under the slab, which is now at odds with the principles of the FPSF. The slab is going to be poured up against the ICF’s so there will be very little edge loss. Has anyone worked with combining these two Green Methods, that seem to be at odds with each other. I thought about leaving the insulation under the slab about an inch from the wall (ICF) to allow some heat to leak down, or is there just enough downward loss through the insulation to keep the footings warm enough?
Thanks for your thoughts
DavidS
Edited 3/2/2009 9:58 am ET by DavidS
Replies
David,
If I understand correctly, you intend not to insulate under the slab where it overlays the footing. Is that right?
Why not just deepen everything six inches or eight inches (half the height of whatever block you are using) so the slab will be poured on two inches of foam and four or six inches of stone? The slab is better off not supported on the footing anyway.
In addition, the exterior insulation protecting the footing from frost will be better protected if it's a little deeper in the ground and a bit less likely to become a roof for a massive ant nest. They tend not to go very deep. There should be at least a foot of cover over that exterior foam.
The rule of thumb where I live, where it doesn't get quite as cold as Vermont does, is that an inch thickness times a foot of width of extruded high density polyurethane is the equivalent of an foot of dirt. If you take my suggestion and pour the footing at a depth of about 1 1/2 feet below grade, then you will have to add 3" of foam three feet wide to end up with the equivalent frost protection of four feet of dirt.
Myself, I think it's usually easier to just pour a footing and frost wall (which, with ICF's, you can do in one pour), fill the inside with clear stone and carry on.
Ron
ThanksI was planing on the footings to be 18" below grade in the front. In all cases I would use insulation under the radiant slab, the issue is at what point will it impact the FPSF, and the footings will appear to be in a unheated house, which is not a recommended practice.Thanks again
DavidS
David,
The idea of the shallow frost -protected footing is simply to substitute insulation laid nearly level on top of the footing for dirt so that the undisturbed earth under the footing will not freeze and heave. The heat of the earth itself keeps it from freezing.
What is the frost-free depth in Vermont?
The heat from inside the house is not relevant except that you can't let your lowest level go much below freezing. But you know that already.
It's really nothing to worry about, especially with an ICF house. The temperature of the footing will be pretty close to the temperature of the ICF core. That is unlikely to ever get below freezing as long as you are maintaining a living temperature indoors.
Ron
Thanks for your response, we work with 48" as the frost depth around here. I was coming to the same conclusion about the core temp of the ICF', and that there should be enough heat conducted from the core to the footings. Thanks again