Thanks for the suggestion. This is uncharted territory for me so I hope you won't mind a couple of more questions. The counter is 3' high and approximately 8' wide. (I said 2' because the counter is screwed to the wall at 2' intervals and I assumed I would be using some kind of solid insulation.) Is spray the solution for a space this large? Will the spray go down to floor level and then slowly rise?
No, the spray foams up too quickly to get very far down. I have no experience with it, but have read there are pour in slow rise foams. Your Google would be as good as mine. But, once in there I would hate to remove those cabinets.
There are also sheets of thin insulation, but it would not be a tight fit.
Let me ask, why?
Also, there is perlite or vermiculite that can be poured in, as long as there are no holes where it can leak out.
This is an old house in downtown Toronto. Built in 1880. The outside wall abuts the neighbor's outside wall so it is inaccessible. It was never insulated and to do so would require a lot of demolition.
Our kitchen counter was screwed to the drywall but there is a gap between the back of the counter and the drywall, probably because the outside wall isn't level.
I've attached a picture. The gap is between the top of the counter and the rear wall.
I'm not sure there would be a big advantage of filling that gap vs just sealing the top. Air is actually a reasonable insulator and the transfer of heat from your cabinets to that outside wall through that gap is slow due to needing convection.
I did a little reading on pour in foam insulations and they warn about expansion. Plus, it is far too cold to be installing foam other than rigid.
If you are insulating that gap, it may be the "tip of the iceberg" because if the counter-top installer couldn't scribe it to fit the out-of-plane wall (wavy), that tells us the cabinets are also pulled away to line them with the worst cabinet base unit (closest to kitchen). You may need to buy a lot... add some foam outlet covers as well as insulation board (not foam) behind the dishwasher.
I would say spray foam, as it is excellent at getting into all nooks and cranny's companies don't normally come for such small jobs, but i used these guys for a small job (around the same size) and they had no problems doing it. http://torontosprayfoam.site/
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