How to protect a monolithic slab from frost without the 4′ insulation in front of it
Hi GBA,
I am proposing a one storey extension to an existing building. I am opting for a monolithic slab to limit the amount of excavation. the propose extension has to be on the lot line. That being said, I cannot add the 4′ extra insulation that goes in front of the footing of the slab because I obviously can’t excavate in the neighbour’s yard. I spoke with a foundation wall repair expert who told me I could add an 6″ compression resisting insulation underneath the footing and a 4″ insulation under the rest of the slab. That solution seams compelling but a second opinion would definitely be appreciated. I am also adding a 2″ insulation in front of the wall. The project is in a Montreal suburban area (Canada) = climate zone 6.
Thanks in advance.
Regards,
Pascal
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Replies
Pascal,
Unfortunately adding more insulation under and against the slab makes frost damage more not less likely, by keeping the surrounding ground colder.
You have two related problems. One is practical - that is a solution that would work. The second is what solutions are available in the perceptive paths for foundations in your building code. That second one is where I'd start. I suspect you will find you will need an engineer to come up with something acceptable.
Thank you both for your comments, much appreciated! There is indeed a professional structural engineer on the project who is going to design what is concrete related. The reason why i'm not extending the foundation wall down the frost line is because I want to limit the amount of excavation needed; it's a narrow space where the implication of a digger truck would require a partial demolition of a small garage and that's not really in the plans. But i know it was going to be a probability. My second option is to go with insulated concrete forms but before going with it do you have any other recommandations ?
thank you
Pascal,
Just as with the first plan you posted, insulated concrete forms won't solve your problem. You need a solution that keeps the soil under your foundation warm enough not to freeze. Insulating the concrete doesn't do that.
If you don't want to extend the footings down below the frost line, the two solutions I can think of are:
- Move the foundation back from the property line and cantilever the framing above so that you don't lose any floor area.
- Consult a geo-technical engineer to see if they can design a substrate of materials that are free-draining enough not to freeze.
That design would work reasonably well from heating and structural viewpoints, but it does nothing to keep cold air from reaching the soil under the footing, so no, it's not a good solution. Is there a reason you can't extend the foundation walls down to the frost line? If that risk undermining the existing building, I would involve a structural engineer. They can use the ASCE 32-01 document to design a custom solution that balances the depth and width of the foam and foundation.
Thank you both for your comments, much appreciated! There is indeed a professional structural engineer on the project who is going to design what is concrete related. The reason why i'm not extending the foundation wall down the frost line is because I want to limit the amount of excavation needed; it's a narrow space where the implication of a digger truck would require a partial demolition of a small garage and that's not really in the plans. But i know it was going to be a probability. My second option is to go with insulated concrete forms but before going with it do you have any other recommandations ?
thank you
As Malcolm noted, ICFs won't do anything for you. As I noted, ASCE 32-01 is a guide for engineers to use for designing frost-protected foundations that aren't covered within the prescriptive code. There is a chance your engineer doesn't have a copy or experience using it; I would ask them specifically to use that guide to design a system that will work for you.
Best practice when adding to a building is to match the foundation type, although there are times when this is impractical. However, it looks as though a frost protected foundation is the best bet here.