Should I insulate the main floor over a crawl space?

Question Icon
Posted by: from Welland
3/21/2014 at 7:21:16 PM

If I should, do I vapour barrier the top or bottom of the joists? The house is a war time house in Welland.

User Icon
Brian from Brian Hagan in Hamilton
Date/Time3/21/2014 at 10:14:52 PM

Hi Greg,

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.


Search the TrustedPros directory and discover the best contractors in your area.

Find your home service pro
Great renovations start with a great contractor.

Since 2004, TrustedPros has been helping homeowners find the right contractor for their home improvements and repairs.

Post Your Project

Within hours you'll be comparing offers from top-rated professionals. It's free to post and you're under no obligation to hire.

Trustedpros Inc. does not warrant the accuracy, completeness, safety, legality or usefulness of any Content, or Whether Content is Current and up-to-date, and TrustedPros Inc. Shall have no liability whatsoever with respect to your use or reliance upon any content or for content being removed or otherwise ceasing to be available. Please refer to the terms and conditions of use of this websites for more details.

It appears you may be located in the US

TrustedPros is designed to deliver results targeted to your location.

Get Free Estimates

Post your project and compare offers from top-rated pros.