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jay cronin Posted: Sep 06, 2008 03:06 PM	crawl space again newbie Hi Lots of crawl space questions but I couldn't find an answer to mine, first question have a crawl space that is dirt, no insulation in 1st floor, We were planning on spraying 2" of 2lb on the foundation walls and box beam, lay down plastic on the dirt as a vapor barrier run up the walls alittle and spray that to seal it up. Problems? Why can't I forget the plastic and just spray the walls and dirt? Other crawl spae is new has a vapor barrier under concrete and we plan to spray the same material on the concrete walls and box beam. Both spaces will be sealed up both in New york. Any info would be great. Thanks
mason Posted: Sep 07, 2008 08:45 AM	Moisture from the crawl space typically would travel upward into the house. The amount would vary depending on the water table. A closed cell foam would reduce the rate, water vapor would flow into the house and by itself may provide enough of a vapor retarder element to prevent condensation in the house. But, it may not. Hygrothermal modeling can be conducted on the proposed design that would let you know if there is a potential for condensation. Computer programs such as WUFI (developed by the Department of Energy's Oakridge National Labs) are good tools for predicting potential condensation problems. Or to be safe use the plastic on the floor before sealing up the space with foam. (note: this solution is predicated on there being a higher moisture content in the crawl space than in the house. If the houuse or building has a higher relative humidity than the crawl space, the vapor drive would travel into the crawl space. In most situations, the crawl space would have the higher moisture content.) You should be all right with the other crawl space that has a vapor retarder under the concrete.
jay cronin Posted: Sep 07, 2008 10:18 AM	thanks for the info. any advantage to spraying foam over the plastic?
mason Posted: Sep 07, 2008 12:35 PM	I don't see the point if you insulate the underside of the floor and the walls.
jay cronin Posted: Sep 07, 2008 08:21 PM	I was never planing on insulating the 1st floor. The last few jobs I've done the insulator (fiberglass) only insulates the foundation walls from plate to 2' below the finished grade. And thats it. I was planning on doing the same.
mason Posted: Sep 09, 2008 07:15 PM	OK, I get the picture now. Depending on how cold the floor gets, it could help to insulate the underside of the floor or foam the plastic (If not for saving energy, for comfort's sake) I don't typically like to foam on plastic because sprayfoam doesn't stick to plastic and it has a tendency to bubble up from the exothermic heat reaction and expansion of the foam.
jay cronin Posted: Sep 09, 2008 07:38 PM	Thanks Mason What I plan to do is cover the dirt with plastic, run it up the sides a little, spray the walls and down onto the plastic alittle to seal it up. and the other one with the vapor barrior Im just going to spray the walls from plate to 2' of grade. all will have no ventalation. other then being conected to the rest of the basement. Sounds good?

Author

Comments

jay cronin

Posted: Sep 06, 2008 03:06 PM

crawl space again newbie

Hi
Lots of crawl space questions but I couldn't find an answer to mine,
first question
have a crawl space that is dirt, no insulation in 1st floor, We were planning on spraying 2" of 2lb on the foundation walls and box beam, lay down plastic on the dirt as a vapor barrier run up the walls alittle and spray that to seal it up. Problems? Why can't I forget the plastic and just spray the walls and dirt?

Other crawl spae is new has a vapor barrier under concrete and we plan to spray the same material on the concrete walls and box beam.

Both spaces will be sealed up both in New york.
Any info would be great.

Thanks

mason

Posted: Sep 07, 2008 08:45 AM

Moisture from the crawl space typically would travel upward into the house. The amount would vary depending on the water table. A closed cell foam would reduce the rate, water vapor would flow into the house and by itself may provide enough of a vapor retarder element to prevent condensation in the house. But, it may not.

Hygrothermal modeling can be conducted on the proposed design that would let you know if there is a potential for condensation. Computer programs such as WUFI (developed by the Department of Energy's Oakridge National Labs) are good tools for predicting potential condensation problems.

Or to be safe use the plastic on the floor before sealing up the space with foam.

(note: this solution is predicated on there being a higher moisture content in the crawl space than in the house. If the houuse or building has a higher relative humidity than the crawl space, the vapor drive would travel into the crawl space. In most situations, the crawl space would have the higher moisture content.)

You should be all right with the other crawl space that has a vapor retarder under the concrete.

jay cronin

Posted: Sep 07, 2008 10:18 AM

thanks for the info.
any advantage to spraying foam over the plastic?

mason

Posted: Sep 07, 2008 12:35 PM

I don't see the point if you insulate the underside of the floor and the walls.

jay cronin

Posted: Sep 07, 2008 08:21 PM

I was never planing on insulating the 1st floor.
The last few jobs I've done the insulator (fiberglass) only insulates the foundation walls from plate to 2' below the finished grade. And thats it. I was planning on doing the same.

mason

Posted: Sep 09, 2008 07:15 PM

OK, I get the picture now. Depending on how cold the floor gets, it could help to insulate the underside of the floor or foam the plastic (If not for saving energy, for comfort's sake)

I don't typically like to foam on plastic because sprayfoam doesn't stick to plastic and it has a tendency to bubble up from the exothermic heat reaction and expansion of the foam.

jay cronin

Posted: Sep 09, 2008 07:38 PM

Thanks Mason
What I plan to do is cover the dirt with plastic, run it up the sides a little, spray the walls and down onto the plastic alittle to seal it up. and the other one with the vapor barrior Im just going to spray the walls from plate to 2' of grade. all will have no ventalation. other then being conected to the rest of the basement.
Sounds good?

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