Hello:

I was speaking with a ICF supplier and he said that my biggest concern should be the fact that my 'potential' ICF walls will be sitting directly on bedrock with only about 1 to 2 feet of soil on the outside.

His concern is frost, the house being 80 miles north of Montreal, Quebec.

He suggested placing foam on the exterior bedrock around the perimeter of the ICF wall extending out about 4 feet and then back filling.

I am trying to understand just how serious my problem is.

Has anyone had experience with ICF walls on bedrock with very shallow grade?

Thanks for any input on this.

Rob.

The rock your building on I assume is the Canadian Shield, same on we have here in Ontario. Your supplier is worried about something that doesn't exist. The rock goes many many miles deep, frost will not be a problem. You can build directly on the rock without using the shallow footing detail.

Thanks Chris. :)

Perhaps he was concerned about 'the cold' passing into the insulated crawl-space, under the ICF walls, through the bedrock.

Would this be an issue, and if so, would you have any suggestions?

Thanks,

Rob.

The cold passing through is no worse then the cold of the rock itself.

His concern is frost protection against heaving of frozen ground, the rock will not heave

Rob - your ICF supplier is right on the money. He is not concerned about frost heave. He is concerned about the thermal bypass of the rock under the wall. With only two feet of frost protection outside the wall the frost will travel under the wall and freeze anything in close proximity to the wall. If you have plumb right next to the wall it might freeze but more to the point why go to the trouble of doing an energy efficient house and allowing such a huge bypass at the footing?
If the rock is granite it will have more conductivity then concrete. If limestone, then somewhat less.
Bob

Thanks Bob.

I believe that you have explained the ICF supplier's concern much better than I.

Is there a solution to this or is outside perimeter insulation my only option?

Thanks again for your time.

Rob.

Rob - only three solution come to mind. The wing insulation as you have described is one. not the best but better than nothing.
Secondly you could lay down insulation under the footing and over the complete rock slab with a bit of sand for levelling. But you need to make sure it can't slide.
Third - do your stem walls. Fill the complete "crawlspace" with granular. Lay insulation over the complete slab area including the stem walls and pour a slab floor over every thing than frame up the house. A version of this might include a reduced crawlspace where the depth is greater if you need the space for utilities. Sort of like an insulated moat around the perimeter and than say 1/2 the space down to bedrock. Of course you would want to insulate the bedrock at this area as well but there might be usable space. If I remember your picture correctly there was not much useful space (less than 18" ) at the high side of the lot. Blocking that off and filling it would save you from having to crawl around down there and it would beat the frost.
Maybe someone has other ideas.

You can do a frost protected shallow foundation monolithic, thickened edge slab. Normally this would have wings of foam extending outward, but in this case, you could alternatively dig out all the frost susceptible soil underneath the slab and replace it with gravel. Ie, no wings needed in this case. It will be completely surrounded by foam with no thermal bridging.

Out of curiousity. What are the arsenic levels in areas of bedrock like that? Out here in AZ areas of bedrock always have high arsenic and radon levels. Per the hydrologists this is common.

If you pour the concrete core directly on bedrock I'd worry you could achieve ground coupled ICF in a bad way. There are lots of lightweight concretes out there: foamed, perlite, volcanic, eps beads, and in Europe expanded clay. http://www.liapor.com/en/group.php Happily, Quebec is a climate where pouring insulating concrete inside of insulation would not be overkill. You'd want a concrete company with experience, and you'd pay more but construction would be straight ahead. Set the forms; spray foam over enclosed rock. Good to go.

Use enerSLAB. Engineered raft slab. Works on most any soil and is designed to eliminate frost heave which can occour in the space below slab in layer of soil between rock and slab

I'm in the process of doing what your asking about. Northern Ontario, building on solid granite, building official had no problem with it, and saved time not having to form footings.

It looks like that cold granite is now thermally bridged to the center of the ICF. If so, that reduces the insulating value of the lower blocks. You can get foams that will support the weight of a house (even without footings) and that would have created a thermal break. The rebar would also take some of the downward force.

Like Jon says, you could have saved the cost of the ICF because the core will be cold anyway.

Great choice on the ICF. Unless anyone has thermal imaging eyes, there is little more than speculation to the thought that attachment to bedrock will cause the concrete core to be cold. The heat sink created below your slab will definitely help in the equation. Variables such as how much and what type of fill is placed under slab and around perimeter will impact heat loss/ heat gain ect. You have made a sound choice and if you ever want to qualify your decision, find an energy auditor in your area with a thermal imager to scan the intersect of your walls to rock. I bet they cannot find a measurable difference even on the coldest day

Thanks Cold Weather ICFer, the outside has between 3-5' of sand backfill, and the inside was filled with 95 ton of washed gravel, and topped with 2" foam under the concrete. With mostly south facing windows all triple glazed my heat gain/loss calcs came in at 37KBtu.

In the end, I built the house on a standard 8" cement foundation resting on 16", stepped footings which were pinned to the bedrock. The footings vary in thickness from about 6" to 16".

On the inside, I had the footings, foundation walls and rim joist spray foamed.

Boringly traditional.

Rob.

Where an I get info on enerSlab?

Thanks,

Rob.