Moisture control and insulation for crawl space

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Posted by: from Ottawa
8/21/2015 at 12:36:48 AM

We have an enclosed crawlspace underneath our addition. It is enclosed with concrete walls, which are insulated with extruded polystyrene. It has a dirt floor with a vapor barrier that has never been taped down. The floor joists are also currently not insulated. The vents of the crawlspace to the outside have been sealed off. There is one heat register open to the crawlspace but it has been sealed off.

We have noticed: 1) a bit of mold under the vapor barrier on the floor 2) musty smell 3) appearance of moisture absorbing upwards on the original foundation of the house 4) cold floors of the addition.

We are planning to spray foam the floor joists and tape down the vapour barrier. Is this a good plan to keep the addition floors warm AND to prevent any moisture/mould issues? Should we be concerned about the appearance of moisture absorbing upwards on the original foundation?

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Brian from Next Restoration in Rockwood
Date/Time8/21/2015 at 8:37:28 AM

Hi Stella,

The vapour barrier doesn't have to be taped down although the vents should be opened up the vapour barrier is there to help prevent moisture from coming up thru the ground just make sure it covers the whole area and open the vents spray foaming the floor joist is a good idea.

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Date/Time8/21/2015 at 10:26:23 AM

Hi Stella,

I have attached a link from CMHC and they have some decent drawings of what should be done and include some studies that they have performed on vented crawlspaces.


Wayne Balliet

Project Manager

Builtru Construction

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Date/Time8/21/2015 at 11:30:47 AM


First of all the vents should not be closed.nPeople do this thinking that it will keep the cold out but the reason for the vents is to let some air flow and reduce the moisture. The others have answered the questions as well. The insulation under the floor will help retain the heat in the addition. Keep an eye on the moisture but, as mentioned, the opening of the vents will definitely improve this issue.



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Date/Time4/21/2017 at 12:59:30 PM

Hi there,

Insulating the floor joists is a great idea. yes, open up the vents and create some air flow, which will help prevent further moisture/mold build up. if your concerned about the appearance or mold, you could remove the polly, clean the wall and reapply new polly.

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Date/Time5/16/2017 at 10:56:00 AM

If all else fails, then install a fan with a moisture activated sensor. Similar to what is available for bathrooms.

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Dennis from Custom Touches in Winnipeg
Date/Time3/13/2021 at 2:09:54 PM

This is an example of incorrect construction and in experienced people trying to correct the problem. There are a lot of good things in place at present. Insulated walls are good. Vapour barrier on the earth floor is good other that requiring sealing at the perimeter.

Air circulation is the problem if the other two items are in place. The crawl space needs to be treated like another room in your home. Warm air in and cold air return out. So you need at least one warm air duct going to the far end of the space and a cold air return intake at the opposite end taking air out. Your problem will take care of itself. You will want to prob replace the old poly with a heavy 6 mill poly. Also take care of destroying the mold.

Do not insulate your floor joists above. The warm crawl space will naturally keep your floors warm like in a 2 story home.

Try this test. Hold your hand against your wall in the winter. It feels cold, just like your floor will if insulated. Hold your hand on the floor in a regular part of your home. Compared to the insulated wall your floor will be warmer and you should feel the difference.

Remember. A sealed insulated crawl space and warm air in (heat) and cold air return out. It's called treating your building as an envelope.

That exterior vent needs to be blocked of and insulated and sealed.


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