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Insulation Under Slab On Grade?

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Posted by Doug on September 24, 2003, 10:34 pm
Hi All,

We're building a slab on grade house near Ottawa.  The footings are
conventional in that they are down below frost line (or on bedrock).
The walls are insulated concrete forms and start on the footings.  The
slab will sit on ~ 5' of infill above the footings.  This infill is
insulated from the frozen ground by the ICF walls.  Given that there
should be very little loss of heat at the edges, should I insulate
under the slab or should I let it rest on the sand infill?  The slab
will be radiantly heated with water.  The house is oriented east-weat
and has lots of south facing windows.  The slab will be the thermal
mass for the house.

Would it make a difference if we have the builder put the plastic
barrier down before the infill goes in - keeping the sand dry?

In previous posts, the consensus was to insulate but perhaps the ICF
walls makes a difference.


Posted by Ecnerwal on September 24, 2003, 10:54 pm
 dpreece@storm.ca (Doug) wrote:

I have a frostwall insulated to the footings on the outside - similar to
your setup, just not done as ICFs. Would have been faster to do it that
way, in hindsight. Every radiant heat person I talked to was strongly in
favor of 2" foam under the slab, regardless of exterior wall insulation.
I ran some numbers myself and noted that even if it was losing heat to
ground that was much warmer than the air, the sheer size of the floor
meant that floor insulation paid off pretty quickly when I ran the
numbers, and I could not get any reliable numbers for dry earth
insulation (not to mention those would depend on the earth being
reliably dry).

I admit that I was tempted by the idea of the dry sand under the slab
and inside the frostwalls being an insulator and a heat sink, but given
the cost&annoyance of going back to insulate it if it doesn't actually
work out, I went with the insulation. I'd still put the plastic down to
keep the sand under the insulation dry. When somebody pays me to build
another shop just like it without the underslab insulation, we'll have a
science project - until then, it cost me less than $000 US to make sure.

Also, if you have footings on bedrock, meaning the bedrock is less than
5 feet down, that bedrock is probably something you want to be inuslated
against, as it will conduct the cold from outside the walls in under the
walls (or the heat out to the frozen earth, as you prefer - no

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Posted by Scott Willing on September 26, 2003, 3:26 pm
 On 24 Sep 2003 15:34:22 -0700, dpreece@storm.ca (Doug) wrote:

Not an expert myself, but everything I've read is strongly in favor of
decoupling yourself from the giant thermal sink known as planet Earth,
at least in a heating climate like Ottawa. (My home town, BTW.)

Makes intuitive good sense to me. Sure, the delta-T is going to be a
lot less than between the interior and the outside air in February,
but it will still be there.

And, as another poster pointed out, you can't really change your mind


Posted by Doug on September 27, 2003, 8:40 pm
 Thanks, all.  We'll put the insulation down.

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