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First time home owner of 1 year old house in Southern Ontario.

South facing basement wall (50% above grade) has condensation forming between polyethylene barrier and pink batt insulation.

What could be a common cause of this?

example of condensation

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    Can You arrange a simple picture of what and in what order are wall's barriers (including wall) placed? Jul 14, 2015 at 6:32
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    All concrete walls below grade are going to have condensation. This is from the wide variance in temperatures and the water in the ground. A good basement insulation system will allow that condensation to evaporate quickly. It sounds like you may be trapping the water.
    – DMoore
    Jul 14, 2015 at 16:07
  • Related: diy.stackexchange.com/a/8644/1209
    – DA01
    Aug 13, 2015 at 20:49

1 Answer 1

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It's the combination of polyethylene and fiberglass batt insulation. What you have there is a mold-and-rot-machine. For more info, see http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements/

You need to remove the poly sheeting at a minimum. Preferably, you would redo the basement wall entirely to have rigid foam insulation against the concrete walls and spray foam in the stud bays of the aboveground part.

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  • Thanks. It's unfortunate that is how the builder choose to do the basement. My assumption was moist warm air is entering from the outside, reaching the cool basement barrier, and then condensing.
    – Tom
    Jul 14, 2015 at 20:06
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    Indeed it is unfortunate. And since Canadaian building codes still wrongly encourage and in some cases require these plastic vapor barriers, the builder is legally in the clear even though this is a terrible, dangerous practice.
    – iLikeDirt
    Jul 14, 2015 at 20:48
  • Yes, this is a frustrating, ongoing problem in North America. Most code enforcers are still woefully behind the times. In MN I had to argue the case before they finally relented and agreed that the Building Science method is the proper way to handle basement insulation.
    – DA01
    Aug 13, 2015 at 20:49

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