Insulation Question - Crawl Space

davewgJanuary 5, 2006

Please let me know if there is a better forum for this one....

We have a 3ft stone over dirt crawl that is uninsulated. The prior owners never corrected a water issue, and while the walls of the crawl were insulated with some type of insulating board (house was built in 1972), it had to be removed due to mold.

We want to reinsulate between the floor boards and laydown a moisture barrier.

My question is this - could we use expanding foam in between the joists to insulate in place of wrapped fiberglass batting?

I've heard that the expanding foam is better at resisting moisture, etc.

Any opinions, advice are welcome.


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You can use expanding foam insulation, but building codes require that foam insulation in a crawlspace must be thermally isolated from ignition in a fire.

This means that you must place over the foam insulation either drywall, sturctural plywood, mineral wool insulation or other approved barriers to prevent the foam insulation from burning.

Since spray foam insulation already costs 3-5X more than using fiberglass batts you will increase these costs all the more by installing the required thermal barrier if you use foam.

Better approach is to seal the floor with a plastic vapor barrier and to insulate the floor joists with standard unfaced fiberglass.

    Bookmark   January 6, 2006 at 7:35AM
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This page suggests insulating the foundation walls instead of the floor. This puts your heating/cooling ducts and plumbing in an insulated area. Remember that the earth is already warm, and heat rises. It makes sense to me.

Here is a link that might be useful: Insulatiing Crawlspaces

    Bookmark   January 6, 2006 at 1:02PM
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Whle the earth may be relatively warm compared to outside in the winter, it is still colder than the house above.

    Bookmark   January 6, 2006 at 4:59PM
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Last house I owned I insulated the floor in the entire house using fiberglass and the wires that hold it up, and the side of the crrawlspace with the plumbing, the wall got it also. Just make sure you extend the plastic vapor barrier up the entire wall overlapping the floor plastic by 2-3 ft. fasten it good to the sill plate then put batts in. To keep them in place, I stuffed them up into the rim joist and stapled them into place. Friend of mine who had a water problem in his crawlspace put 1" thick blue styrofoam in instead and also insulated under the floor.

    Bookmark   January 6, 2006 at 11:26PM
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Thanks for all the great suggestions (keep 'em coming).

We have a contractor coming to review a bathroom renovation. I'll discuss these with him and get his thoughts.

    Bookmark   January 7, 2006 at 9:14AM
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I would recommend against insulating the crawl space walls of an older home that was never designed for a sealed crawl. Espeically if there have been water and mold issues.

Probably much better to lay down 10 mil poly on the crawl floor, to retard moisture rising from the earth, and then use FACED fiberglass batts under the flooring (like Johns Manville Comfort Therm or Owen-Corning Miraflex). These faced batts will still let some humidity through so the crawl won't get too humid. Also of course you'll need to open up any vents that were sealed on the crawl walls - so that the moisture coming up from the earth has someplace to go besides into your home.

One question: What is "3 foot stone over dirt crawl"? Three foot thick stone covering the dirt? Three foot high stone foundation? Stones that are 3 ft x 3 ft tiles?

    Bookmark   January 7, 2006 at 7:43PM
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sudsmaster -

Ha ha....note to self...reread post before submitting.

The crawlspace has a floor of stone over dirt. The 3 feet refers to the height of the space in the crawl floor to joist.

    Bookmark   January 7, 2006 at 8:01PM
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I should also add that the crawl space is a decent size and only has ONE foundation vent. Something else we need to rectify.....

As noted the crawl walls were originally insulated with fiberglass or foam sheeting. Not sure what exactly was used since they were removed as part of mold remediation before we ever saw the house prior to purchasing.

    Bookmark   January 7, 2006 at 8:04PM
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Correct the water issue firstoff. Ideally, the soil surface inside the crawlspace should be higher than that outside - water WON'T flow uphill. If you can't raise the inside level or lower the outside, at least install drains or make changes in drainage patterns in the yard to diminish the possibility of water accumulating and standing in the crawlspace.
Once you've remedied the water situation, I'd put down a 6-10 mil vapor barrier on the ground, lapped up 6" or so onto the perimeter walls, close the foundation vents - and never open them again - and have 2 inches or so of wet-blown cellulose blown onto the perimeter walls. That way, any leakage from your ductwork(I'm presuming it's in the crawlspace?) is contained inside the crawlspace, but is still able to help heat your floors.

In a dry crawlspace, with no standing water, opening & closing foundation vents CAUSES moisture problems. You close them in the winter, to keep your pipes from freezing & your floors from getting so cold, right?
But, then you open them back up in the summer? I'll bet it's nice and cool in the crawlspace in the summer, so all that hot humid air comes rolling in through those foundation vents, to condense on your floorjoists, etc., just like moisture condensing on the sides of a glass of iced tea.

I'd insulate my perimeter walls, and NOT under the floor - but not until you eliminate the water in the crawlspace problem.

    Bookmark   January 16, 2006 at 3:07PM
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I don't like the fg batts under floors.
Over time the fg will rest on the chicken
wire creating a space between floor & insulation.
As this area reaches dewpoint, condnensation forms
between flooring & insulation. This creates 'soft spots'
which is wet flooring which will eventually rot the flooring & joists.

My suggestion would be to ask the experts.
visit Building Science's site (
email directly with all the
info & specific location.
While waiting for his reply (and he will reply, when he
can) read up on the info on thier site on insulating
crawlspaces, vented crawlspaces & unvented crawlspaces.

I have the same situation with a client nearby.
We are going to remove the insulation & chicken wire
seal the openings in the floor & attach foam sheathing
boards to the undersides of the joists. To make the
sheathing air tight we will tape the seams, & caulk
the outer edges of the sheating.
On another house, the home owner decided on foam
insulation, 3 foam insulation companies later, we
finally got the install that was correct. Overselling
of performance is a problem here with foam insulation.

Best of luck with your project.

    Bookmark   January 18, 2006 at 11:15AM
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similar situation here.
house over 3 foot unheated crawlspace, dirt floor.
It has been suggested to me that I should cover the dirt floor with plastic sheeting to prevent my floor from becoming damp.
My concern is that this will trap the ground dampness between the ground and the sheet, with the sheet preventing the ground from ever drying out or getting fresh air. Will this not create mould and/or smell?
House floors and walls of crawlspace have been sprayed with foam insulation. There is good ventilation to the outside.

    Bookmark   February 3, 2006 at 10:32PM
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The plastic sheeting on the ground is standard practice and is what is recomended by every proffessional to rid moisture from a crawlspace. I had a house with a crawlspace for 20 years and had plastic on the ground to block the moisture. I also had it lapped up the sides of the crawlspace wall and sealed it to the top of the wall to prevent moisture from getting out.

I never had a mold problem under the plastic. Always hasd moisture come up to the plastic pool under the plastic at different times of the year and then subside.

The reason i never had a mold problem because there was nothing under the plastic for the mold to live on. just soil. Mold needs something to eat to live and grow,something organic like wood. It cannot survive growng on stone or on top of the soil.

    Bookmark   February 4, 2006 at 1:14PM
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My problem is slightly different. I have a rubble foundation, just lose rocks. I cannot insulate the walls because I do not want to trap moisture and heave the rocks. I also have standing water in the basement. I am addressing the water through french drains on the outside, and will put plastic on the floor, NOT up the walls, and a sump if needed. I will then place polysicurnate(sp?) sheets between the joists. I do not want to use fiberglass because I want to remove them easily to fix rot as it occurs. I hope this rotting will stop due to my efforts. I will most likely leave the gaps unsealed so I get some air flow. I only want to slow down my heat loss, I think in my setup it would be unreasonable to try and stop it. I would create as many problems as I solve!

At some point, I plan to jack the house and repour the foundation, then I will go to a conditioned space. That is years away however.

    Bookmark   February 7, 2006 at 5:06PM
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Oh, and I used Icynene in a seperate crawlspace that was about 15 inches high. I replaced 30 year old fiberglass that was mold and animal infested. I did not use the foam boards because it is to small to work in.

    Bookmark   February 7, 2006 at 5:09PM
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We were advised to lay 2-3" of pea gravel down before laying down plastic as it allows any moisture between the plastic and the dirt to percolate out through the gravel and go away.We have a stem wall foundation that steps drastically because we are building on sloped property.Code allows for 18" of stemwall above grade max. In order to bring everything level, we have 2x8 pony walls. The foam company says that by code , they can only foam the underfloor, as it is an unconditioned space meeting conditioned. I would like to go with the advise of our plumber and insulate the pony walls only, along with the pea gravel/ visquine,and leave the underfloor uninsulated, because of the underfloor plumbing.We have plenty of vents,(code), that we can plug in the winter. The plumber stated that using this method on a crawl space, the temperature never gets down below 43 degrees in the worst of cold and has done it that way on his own home with no problems.We are building in the sierra nevada's, gets a little chilly in the winter.Any thoughts would be appreciated. Mark

    Bookmark   February 7, 2006 at 7:47PM
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