If I should, do I vapour barrier the top or bottom of the joists? The house is a war time house in Welland.
I'm assuming the crawl space is un-insulated. If that is the case you will definitely want to insulate the main floor and that will include vapour barrier. Vapour barrier always goes on the warm (interior) side of the insulation so in this case it will go above the joists. There are several ways of accomplishing this. If the joists are exposed on the top then a 6 or 9 mil poly will be easily stapled to the tops of the joists. Then you can screw your sub-floor down on top of that.
If your main floor is finished and you don't have access to the tops of the joists, the alternative is to use spray foam insulation (not the canned version) between the joists. Spray foam is closed cell and will provide an effective vapour barrier if applied correctly. This is probably the easiest and most effective solution. But it is expensive.
Another alternative is to cut strips of 6 or 9 mil poly and install them in each joist space, up against the sub-floor. To be 100% effective you would have to caulk each joist on both sides where it meets the sub-floor using acoustic sealant (nastly black stuff that gets everywhere). Then you would install batt insulation between the joists.
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