Sill plate and the foundation wall

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Posted by: from London
12/31/2015 at 9:59:10 AM

I was just wondering if I was using PT on the basement floor with a sill gasket underneath. When I lay my 2x4 sill plate if I could measure 4.5" from the concrete wall leaving a 1" gap behind the sill plate and foundation? Or is this wrong?

Also is sill gasket and poly 6 together on the floor over doing it too?

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Mark from CMJ Renovations in Oakville
Date/Time12/31/2015 at 11:38:51 AM

Hello Paul

The air gap is recommended, the poly with sill gasket is not required, sill gasket is fine on it's own.

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Date/Time12/31/2015 at 12:57:27 PM


The other Mark answered you question. The main reason is to have a membrane between the concrete and the wood so as to prevent moisture from accumulating in the wood and causing rot (simple explanation). The space between the wall and the concrete foundation is to allow some air flow, again for moisture control. You are on the right track.

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Date/Time12/31/2015 at 6:39:15 PM

Regardless what is applied on top of the concrete the moisture will build and in no time bad odor and smell,if there is no air circulation to vent moisture nothing will help.Dry core most expensive material is just money grabbing, there in no different between regular plastic and most expensive, the moisture steal traps under neath. System I developed 8 yrs ago and wrote about it in construction association magazine is how to eliminate moisture and heath the floor at same time is the only system that works for our climate.

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Date/Time1/1/2016 at 10:29:35 AM

Hi Paul

Just as a secondary consideration; check with your local building inspector. I have been on a few job sites where the local inspector would not allow pressure treated wood inside the building envelope due to off gassing.

I have no experience in framing / insulation, just been there when the general contractor had to rip out and start over for putting a pressure treated sill plate in.

Different areas want different things.


John Kuehnl-Cadwell

Master Electrician

Datawise Solutions Inc

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Matthew from Matt of All Trades in Kitchener
Date/Time1/2/2016 at 2:19:30 PM

You are on the right track. Airspace is best if using bat insulation and 6mil plastic vapor barrier on wall. Spray foam is the best product to use and I still frame an inch from the wall to get foam in behind. PT lumber is fine but not required if using a sill gasket but is never a bad idea. Plastic/ gasket combo is not required. Either or...

Dri-core I agree is a very expensive way to install subfloor. Delta membrane rolls and 5/8" T&G OSB with tapcons is my preferred method. Be advised that it would be a great idea for ease and time savings to rent a rotary hammer drill for the 3/16" dia holes that will be required for the 1/4" tapcons that are needed to secure the floor. Screw head not socket head.


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