Moisture and ice on top of concrete walls?

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Posted by: from Ottawa
5/16/2014 at 9:51:46 PM

I left a one inch space between the the R21 Roxul insulation and the concrete wall and found that ice buids up on the top of concrete wall. Full height insulated wall and very well sealed with 6 mill poly.

Reseached on internet re; ontario building code and central morgage and housing are both saying builders should not leave any space between insulation batts and concrete wall. There should be a 1/2 inch space for the wood framing only and not the batts. Any small space will allow for convection looping which will eventually lead to mildew and or mold.

What do the pros think? Planning on correcting this soon.



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Date/Time5/18/2014 at 7:20:38 AM


Not sure why this gap would have been left. In essence you have created a cold space between the insulation and the concrete. Not only is the concrete protected from getting any warming effect at all but this would be a prime location for the growth of mold.

The best solution would be to use foam insulation directly adhered to the concrete then strapped for your drywall. Not sure about the code requirements of ON but I always put my vapour barrier directly between the batt insulation and the concrete(and seal all around with acoustic sealant) and none between the drywall and the batts. This provides a moisture barrier to protect the insulation from the concrete which emits moisture.

Hope this helps!


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Date/Time5/18/2014 at 11:54:09 PM

The 1 inch gap could be the result of an adjustment made to keep all walls level.

It is hard to give advice without seeing the job.

There could also be a water leakage from the eaves troughs etc. Because the ice means water freezing.

However you may also spray foam all the necessary areas.

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Dave from 360renos in Navan
Date/Time5/19/2014 at 10:13:10 PM

Hi Mike,

You are correct. No air space as it will create a convection loop. You do want the framing off the foundation wall so there will be a thermal break.

Doesn't take much of a space or gap for warm air from your basement to get behind the vapour barrier and and condense upon contact with your cold basement foundation wall

Did you also acoustic seal the bottom of your vapour barrier to the basement floor?

Air sealing is very important including a proper air barrier over header plates at the top of the foundation wall all the way to the slab

Some great reading here Mike to help you out -

Let me know if I can answer any further questions for you.



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Date/Time5/20/2014 at 7:16:21 PM

Hi Mike,

The best way to insulate a basement and avoid any air space for moisture condensation is to install foam panels glued to the foundation itself. Now there are about five different manufacturers of these products, and I like the R10 blue board myself.

We then frame and add robuxel insulation in the frame. You end up with about R28.

There is a more expensive method, I use if space is a concern, and that is having one of my sub-contractors come in and spray foam to insulate. You pick up about 1 1/2"s on each wall.

Be sure to insulate against the sub floor outer frames above the concrete as well.

Hope you find this useful.

James Fram

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