I have the ground floor apartment in a 100 year old triplex on the plateau in Montreal. It's an 800 sq foot apartment with baseboard electric heaters and I'm currently using over 110kwH per day to heat the house. Having compared the bills from the previous 2 years which I got from the last owner I can see this is not just down to how the heating is being used. The 2 apartments above me have significantly cheaper heating bills.

In the 18 inch crawl space under my floor I can see there are 3 baseboard heaters. I have turned the thermostats on them down but having researched online it seems the recommendation is that the crawl space should be insulated.

The ground is rough and unfinished and last year we had a new moisture barrier installed. There are water pipes in the crawl space but no machinery. I have looked at options for insulating and it seems people are saying it's better to insulate the crawl space rather than just the floor.

To insulate the crawl space I am thinking I need to use rigid foam insulation on the walls, and on the floor above the membrane. Does that sound right? Is it ok to use rigid foam over the membrane/vapour-moisture barrier? Is it reasonable to crawl on the rigid foam without it compressing too much?

I can't find much information about insulating the ground but as it will always be below room temperature it seems like it is necessary. Interested to hear any thoughts about how to go about this and what thickness of rigid foam I should be looking for.


  • 1
    110kWh per day? really, that seems so high.
    – Solar Mike
    Commented Jan 21, 2020 at 7:30

1 Answer 1


I would insulate the outside walls with the rigid foam board insulation like you described first, but do not do the floor at this point in time. Then see what kind of improvement that provides.

I'm not sure what I would do about the existing vapor barrier that's on the walls. If you leave it in place and put the foam board on top of it, then it's possible that the moisture in the air in the crawl space will permeate through the foam board and condense on the vapor barrier that's against the cold, outside wall. So you might want to consider putting the foam board behind the vapor barrier, right against the wall, then the vapor barrier.

When insulating walls to crawlspace and/or basement, I have not found a definitive answer as to where a vapor barrier should go. I have even read some recommendations as to not use a vapor barrier at all; use foam board insulation that has some amount of permeability and just let the wall breath.

I would like to hear from others what their thought are and experiences have been.

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