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nilsca 03-20-2009 10:12 AM

Insulating Foam *under* concrete basement floor?

I have a contractor lowering my basement floor (in Toronto) and he quoted me cad1900 for putting a 2" insulation (HD Polystyrene) between gravel and concrete.

Someone else recommended that solution to me but I'm not sure it's worth the money re: 'warm feet'. The basement got a new weeping tile, water membrane etc and otherwise sits on the soil 5-6' or so below grade.

Advice appreciated

1. add 2" insulation under concrete?
2. (bonus question) vapour barrier between gravel and concrete?

Thank you for your input


jomama45 03-20-2009 11:09 AM

6 mil poly sheeting (visqueen) is the most popular vapor barrrier around my area, & is very inexpensive. Definately use the vapor barrier, as it does a good job of controlling "sweating" in humid weather, odors, radon, etc.

The foam needs to be 25 psi rated minimum. That helps to bring the price up more. As a concrete contractor, we rarely use foam under unheated slabs, as IMO the initial expense outweighs any benefit.

Chemist1961 03-20-2009 06:58 PM

Greetings from SWO. Check our Ontario codes versus others re vapour barriers.
I have seen a commercial building where scrap window cutouts from 2" doors were used as insulation before pouring.
If you want to see what the value is here, price out rigid foam yourself at HD. It's not rocket science to lay it down if your contractor will let you.
I just tweaked my furnace ducts / dampers and sent more heat upsatirs with a new HE furnace and uninsulated basement. The floor is certainly cooler but the gas consumption also dropped over 30%.
Big question here is will you use that basement much? If so a naturally warm floor is a huge bonus.

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