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  • Dave B

Fixing Condensation Behind Basement Insulation

A great question a from a homeowner in Alberta

Hi all, This is our second winter in a newly built house. The basement was insulated with a layer of poly against the concrete and framing done about 2-3 inches off the wall. One layer of fiberglass batting is running horizontally and set in between the concrete/poly and the framing. A a second layer of batting is then running vertically between the framing for a total of two layers. There is then another layer of poly on the inside sealing it off. So it is concrete - poly - horizontal batting - framing - vertical batting - poly.

The problem is moisture has found a way in and there is considerable condensation between the poly and the horizontal batting. There is frost formation and it has condensed enough to seep out the bottom (which is how we noticed the problem).

I've opened up a few areas to assess, but am wondering what the best way to resolve this is? The ground will still be cold for another month or two (we're in Winnipeg) so I'm not sure if it's best to wait for things to warm up and the frost melts, or if this is urgent enough where I should start trying to rip out insulation now? A lot of insulation is actually frozen to that layer of poly on the concrete so it'll be messy. Thanks for any advice.

This is a very common problem even here in Ontario

Below is my response to him.I would leave it for now. let it warm up, then tear out all the insulation out and start fresh.

Not sure of the National Building Codes for your area so you should find out before replacing materials to ensure it is completed correctly. I'm in Ontario so we use the OBC.

You want an air barrier membrane against the foundation wall not a vapour barrier.

The poly vapour barrier only goes on the warm side of the insulation. What you currently have lets any warm air that enters the insulated area to condense and form water droplets and then freeze to the foundation wall. The warm air has nowhere to escape and gets trapped between the two layers of vapour barrier.

One area that is quite often missed and most likely is a cause for issue is to ensure when the new vapour gets installed that EVERY SINGLE entry point for warm air is either taped, and/or acoustic sealed. This includes the header area especially. No warm air behind the vapour barrier means no moisture/water damage.

A little reading that may help with understanding some correct ways to insulate

Hope this helps answer some questions.


#basement #insulation

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