Is a vapour barrier required under subfloor?

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Posted by: from Ottawa
5/20/2019 at 1:43:48 PM

I have a three season, unheated cottage on a cinder block foundation & dirt floor where the beams & joists have just been completely replaced. There is water under the floor for a couple of weeks in the springtime and then, due to good ventilation, it dries out. The cottage originally had hardwood throughout and I intend to replace this. Do I need to put a vapour barrier under my subfloor and over the joists and, if so, what is the recommended product? My contractor told me to put TYVEK over the joists.

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Date/Time5/21/2019 at 9:35:18 AM

This is what the Code reads on the subject (

Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces.

It would seem to be required only if the floor assembly is insulated in which case it would be 6 mil poly.

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Date/Time5/21/2019 at 9:44:11 AM


We recommend you use Resisto Joist Guard, this is waterproof protection for joists. Tyvek is a vapor barrier wont protect the joist.

Rona Doidge Building Centre Welland, ON

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