Why is frost growing on basement concrete wall behind the fiberglass/vapor barrier?

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Posted by: from Burlington
3/6/2015 at 3:52:16 PM

Too many different opinions, need a straight answer. Frost growing on my basement concrete wall behind the fiberglass/Vapor Barrier and the RIM Joist is saturated with moisture. Is it simply because I have removed some of the builder insulation and I am in the middle of the project and I get condensation or do I have another problem.

Now I am stuck not knowing if I can continue. I want to insulate the wall, due to depth restriction, I selected the blue foam insulation 1/2inch, it is insulation and vapor barrier altogether. I want to ensure that once the wall is dry I can glue the foam on the concrete, tape it/spray foam, frame, insulate, vapor barrier, Drywall, etc...

Please let me know. This is an outside above grade wall, west exposure and exposed to elements. The house is concrete foundation, waterproof membrane, and brick exterior.

Please let me know.

Ontario Building Code applies.

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Josh from Welch's Drywall in Portland
Date/Time3/6/2015 at 4:12:29 PM

Hi Concerned!

You have created this problem yourself. When hot air meets cold air, condensation is created. When you removed the insulation from the wall, warm air on the hot side of the wall meet with the cold concrete on the other side of the wall. These two don't like each other. When the insulation is put back in place this will be your thermal brake to divide the two and everything will be alright.

If a lot of condensation is visible, put some big fans down there by the wall to dry up water on the plastic and wall. When this dries up, have your insulation and whatever else you are doing and seal it up before the hot and cold air do the same thing all over again.



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Date/Time3/6/2015 at 4:35:35 PM


I agree with Josh. Another explanation is that there is excessive moisture in that area. This can sometimes happen if there is too much water accumulation on the outside of the wall and the weeping tile isn't working as it should. (weeping tile is a perforated flexible pipe or tubing that runs around your house and is supposed to drain excess water away from the foundation). If this is the case, it is a temporary issue that may have been caused by repeating sessions of melting and freezing. Once spring arrives, issue will be gone.


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Date/Time3/8/2015 at 1:52:18 PM

Yup, you are causing issues. The joist ends and the walls have to have every gap insulated and about 1/2" min. from touching concrete then vapor barrier must be solid. If you have leaking windows or concrete walls it will come back regardless, so fix this 1st if any issues. This is why people should hire reliable contractors. Costs more but we do it right and then costs less in long run :)

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Barry in Irricana
Date/Time4/3/2015 at 7:03:50 PM

Hi Celine,

I have had the pleasure of working in five out of the ten provinces in the construction business as a general contractor/carpenter. The building code for each province varies a bit with climate taken into consideration. But the physics of water vapour & condensation rules stay the same across the country.

If you have condensation it is like Josh said in his post. Warm air is meeting cold surface. The space between your floor joists is one of the worst spots in a house for heat loss. The rim board is only 1 1/2" thick separating you from the out side world. Plus the top couple feet of foundation wall that is above grade is a frost conductor.

Spray foam in this area works really well to seal & insulate the joist space. The foam insulates & also acts as a vapour barrier. It also helps to run your poly vapour barrier on the inside of the wall all the way up in between each joist. The least amount of cuts & holes in the poly the better. It helps to tape the edge of the poly to the sides of the joists making it air tight.

Another way to slow down frost conduction is to stop it on the outside. Apply rigid foam insulation to the exterior side of the foundation. Dig down below grade level then apply. This can be covered wwith many different cosmetic dresses of your choice.

I hpoe that gave you some more insight.

Ardent Carpenter

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