I've just read a thread on insulating thermal breaks at ICF door sills (http://www.greenbuildingtalk.com/Forums/tabid/53/aff/4/aft/45365/afv/topic/Default.aspx). I think this is important to avoid heat loss but I confess I don't clearly understand the issue. I'm the house owner and I'd like to be sure the builder insulates thermal breaks if it'll avoid heat losses in my cold Ontario climate.

I'm using Durisol ICFs on a slab-on-grade foundation of a combined house and garage, just a single floor bungalow (all rooms are on the slab, no floors above). The garage is unheated and without attic insulation. My first question is: am I right that I don't need thermal breaks at the garage door sill and garage man door sill, since this 'room' is unheated?

More importantly, does this mean that I should be installing a thermal break all along the slab under the walls where the garage meets the rest of the house (kitchen in my case)? I'm using a passive solar design where all the house windows face south so that the exposed slab in the living area soaks up the sun's heat during the day and releases it at night. Will all this heat be sucked out the colder garage portion of the slab if there isn't a thermal break?

If so, should I be be pouring two slabs side-by-side with a few inches of blueboard in between them? Furthermore, should I similarly be isolating the concrete in the ICF exterior walls between the garage and the rest of the house, or is that overkill and does it risk weakening the walls?

Thanks for your help to this newbie. The builders in my area aren't very aware of green building techniques.
...Terry

Posted By terrynew on 05 Mar 2010 08:41 AM
in my cold Ontario climate.

That's the key consideration.

My first question is: am I right that I don't need thermal breaks at the garage door sill and garage man door sill, since this 'room' is unheated?

There's a lot difference between "don't need" and "good idea." The garage slab will undoubtedly be somewhat warmed by the earth heat underneath. There's no sense in losing that benefit by having that heat migrate to the cold outdoors. A thermal break and both doors will help keep natural heat inside the garage. Now, if you don't care if the garage drops down to 0°C, or however cold it might get, then don't bother. But that goes contrary to using ICF for the garage walls.

More importantly, does this mean that I should be installing a thermal break all along the slab under the walls where the garage meets the rest of the house (kitchen in my case)? I'm using a passive solar design where all the house windows face south so that the exposed slab in the living area soaks up the sun's heat during the day and releases it at night. Will all this heat be sucked out the colder garage portion of the slab if there isn't a thermal break?

If so, should I be be pouring two slabs side-by-side with a few inches of blueboard in between them? Furthermore, should I similarly be isolating the concrete in the ICF exterior walls between the garage and the rest of the house, or is that overkill and does it risk weakening the walls?

How do the costs pencil out if you use ICF for the wall between the kitchen and garage? That would give you the thermal break at the floor, provide a good fire barrier wall between house and garage, and keep the solar heat gain inside the house envelope. If not that, then I think you really should put a thermal barrier in the slab. Put the barrier entirely on the garage side of the wall line. If you use a wedge shape you can keep the slab strong at the break.

Posted By terrynew on 05 Mar 2010 08:41 AM
I think this is important to avoid heat loss but I confess I don't clearly understand the issue.

Sorry I didn't address this first. The issue simply is that heat travels from warmer to colder regions. An unheated garage probably will not be a freezing garage, which means it'll be warmer than the air and ground outside. How important, or how desirable, is it for you to keep as much heat inside the garage as feasible? An unheated garage will certainly be colder than the heated house space. Since you're counting on the sun to help heat the house do you really want to lose that hard won heat to a cold garage?

A competent HVAC contractor who uses a good heating/cooling load calculation program can help you get a feel for the numbers involved. Don't expect a builder to be an expert on that aspect of house construction.

Thanks for sharing, dmaceld. I follow all you say, except the suggestion to make the garage-to-living-area wall out of ICFs. Since that wall would rest on my single slab, the heat would still flow under the ICF wall. Or did you mean to have two separate slabs with an ICF wall between them?

Hmm, the thermal barrier in the slab is gonna cost. Maybe I'll be better off just to insulate the garage ceiling (since the slab and exterior ICF walls are already insulated). Although the garage won't have active heating, it'll be heated passively through the window and the thermal mass of the Durisol blocks, and the other three walls are below grade (walk-out basement style), so I'll get that earth mass heating. The garage will still be cooler than the living area but not by a lot, so not a lot of loss. Or am I dreaming? It sounds like I need that competent HVAC contractor you mentioned...

Thanks again,
...Terry

It is always a good idea to have a break between the garage slab and the main house slab. In addition, I prefer to lower the garage slab an inch or so below the main floor. This is done to prevent spilled fluids in the garage from migrating into the living space. This break does not have to be a thermal break but since the break will be there anyway, why not use 1/2" foam board.