pouring concrete floor in crawl space

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swimmer_spe

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Part of our basement is a 4 foot craw space that is dirt floor and block walls. There is an issue with moisture in the house, of which I think it is from this area. I was thinking of putting a layer of concrete in to cap it and keep the moisture out.
It is too low for me to get in there and dump concrete at the far end. Can I simply make up a bunch of self leveling concrete and dump the mix into the crawl space, letting it go to the edges?
How think should I be aiming for?
I know concrete releases heat during the curing process. I was thinking doing it during the winter so that the heat isn't wasted..
 

joecaption

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Just lay down a vapor barrier.
There's so little heat given off that last thought is not a great one.
 

bud16415

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In that case I would smooth it out put down a vapor barrier and tape all seams. Then lay cheap 2’x2’ patio blocks starting at the front and working back as far as you wish to store things.
 

68bucks

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Have you searched crawlspace encapsulation? Thats probably what you would want to do for the moisture issues. Moisture will work through concrete. If you go the concrete route make sure you put vapor barrier down first. Also when it cures it will give off a ton of moisture so you'll need fans and/or dehumidification for a while, some number of weeks I'd guess. You can pump concrete, that would be the best way to place it if it's large enough area/volume. Also don't forget to to put expansion material all the way around wall and piers if there are any. I have also thought of doing what you propose to my crawl.
 

swimmer_spe

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Have you searched crawlspace encapsulation? Thats probably what you would want to do for the moisture issues. Moisture will work through concrete. If you go the concrete route make sure you put vapor barrier down first. Also when it cures it will give off a ton of moisture so you'll need fans and/or dehumidification for a while, some number of weeks I'd guess. You can pump concrete, that would be the best way to place it if it's large enough area/volume. Also don't forget to to put expansion material all the way around wall and piers if there are any. I have also thought of doing what you propose to my crawl.
After posting the original, I googled it. I then learned the sheer amount of concrete I would need. I have no easy way to pump concrete in, and it would take a massive amount of bags to get a thick enough layer.
 

mabloodhound

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the OSB plywood is NOT a good idea. Ir will become moist and then moldy over time. the paver blocks over 6 mil poly is the best idea.
 

BvilleBound

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I agree with 68Bucks. DO NOT try to install a "concrete cap" on the 'floor' of your crawlspace. Moisture travels through concrete, as anyone with a basement and concrete walls knows. (Google "how to create a dry basement".) Plus it will emit a ton of moisture as it cures. Plus it will be very difficult and costly to pump and place concrete in your crawlspace. In short, definitely not a good idea. I also agree with MaBloodHound. OSB is particularly sensitive to moisture -- as many people with OSB sheathed walls have found. It would quickly degrade in your crawlspace.

The ONLY solution is to 'encapsulate' and isolate your home from the moisture in your crawlspace. One approach is to lay thick poly sheet down on the floor of the crawlspace, bring it up and seal it to the top of the foundation wall. We prefer a different approach: attach and seal thick poly sheet to the bottom of the joists, and seal the perimeter. This is easier to install and seal. DAP 230 or 3.0 are good solutions to seal and glue the perimeter edge. Use the 'Window and Door' version of Great Stuff spray foam for larger gaps.

If you live in an area with cold winters, this is also an opportunity to improve the insulation under the floor. Fill the joist bays with rock wool batts, which are more effective than fiberglass and deliver R-4.1 per inch of thickness. Use Simpson Strongtie insulation wire to hold the batts in place. If you have very cold winters (or exposed plubing between the joists), add 1" of poly faced polyiso foam board over the rock wool, screwed to the bottom of the joists. The poly faced foam board will also create an effective moisture barrier -- eliminating the need for the poly sheet. Be sure to seal the joints and perimeter of the foam board with DAP sealant.

All of the products noted above are available from Home Depot and Lowes, and can be installed by the typical homeowner who can crawl around the crawlspace with some tools.
 

swimmer_spe

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I agree with 68Bucks. DO NOT try to install a "concrete cap" on the 'floor' of your crawlspace. Moisture travels through concrete, as anyone with a basement and concrete walls knows. (Google "how to create a dry basement".) Plus it will emit a ton of moisture as it cures. Plus it will be very difficult and costly to pump and place concrete in your crawlspace. In short, definitely not a good idea. I also agree with MaBloodHound. OSB is particularly sensitive to moisture -- as many people with OSB sheathed walls have found. It would quickly degrade in your crawlspace.

The ONLY solution is to 'encapsulate' and isolate your home from the moisture in your crawlspace. One approach is to lay thick poly sheet down on the floor of the crawlspace, bring it up and seal it to the top of the foundation wall. We prefer a different approach: attach and seal thick poly sheet to the bottom of the joists, and seal the perimeter. This is easier to install and seal. DAP 230 or 3.0 are good solutions to seal and glue the perimeter edge. Use the 'Window and Door' version of Great Stuff spray foam for larger gaps.

If you live in an area with cold winters, this is also an opportunity to improve the insulation under the floor. Fill the joist bays with rock wool batts, which are more effective than fiberglass and deliver R-4.1 per inch of thickness. Use Simpson Strongtie insulation wire to hold the batts in place. If you have very cold winters (or exposed plubing between the joists), add 1" of poly faced polyiso foam board over the rock wool, screwed to the bottom of the joists. The poly faced foam board will also create an effective moisture barrier -- eliminating the need for the poly sheet. Be sure to seal the joints and perimeter of the foam board with DAP sealant.

All of the products noted above are available from Home Depot and Lowes, and can be installed by the typical homeowner who can crawl around the crawlspace with some tools.
Only 1/4 of my basement has the uncapped crawlspace. The rest is a full basement. This is due to a massive rock under the house that at the time they decided not to blast away. Sealing the joists will do nothing.

Here is my plan:

I have left over 6mm poly vapour barrier from another job. I will start at the far end, laying it down and taping it to the walls and taping the seams.
Then, starting at the closest entrance, I will lay 2x4s, about 12'' on their sides. I will then screw 2x4s on their sides perpendicular every 6 inches. I will lay pieces of plywood as needed to walk around. The rest will be left open. I will use that to store plastic bins of personal stuff (Christmas decorations, camping stuff, etc). This should seal the area. It should also prevent adding more moisture to the area. Finally, it should reduce the materials mold and mildew can grow on.

Does this make a good plan?
 

oldognewtrick

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Instead of regular ply, you might want to consider using advantec. Just my 2 cents.
 

Sparky617

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In addition to the problems of the OSB getting damp, it will be a termite magnet. The builder in my neighborhood installed a PT wood deck in parts of some crawlspaces. I have a full walk out basement with a vapor barrier under the poured concrete floor.

I like the idea of a concrete floor, you'd probably want to ventilate the crawlspace for several weeks after the pour and run a dehumidifer once it is sealed up. There is a lot of info on the web about conditioned crawlspaces and how to do it properly.
 

mabloodhound

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I have left over 6mm poly vapour barrier from another job. I will start at the far end, laying it down and taping it to the walls and taping the seams.
Then, starting at the closest entrance, I will lay 2x4s, about 12'' on their sides. I will then screw 2x4s on their sides perpendicular every 6 inches. I will lay pieces of plywood as needed to walk around. The rest will be left open. I will use that to store plastic bins of personal stuff (Christmas decorations, camping stuff, etc). This should seal the area. It should also prevent adding more moisture to the area. Finally, it should reduce the materials mold and mildew can grow on.

Does this make a good plan?
Too many 2x4's. 16" on center is plenty strong enough; it's only for storage, not a dance floor.
 

swimmer_spe

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Too many 2x4's. 16" on center is plenty strong enough; it's only for storage, not a dance floor.
It isn't for strength. It is to make a lattice so that I can put stuff on them without worrying about how it sits.
 

Sparky617

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I wouldn't use regular plywood on top of the plastic that is directly on top of the ground. Not sure you need the second layer of 2x4's. I'd look to use 5/4" PT decking instead of plywood.
 

swimmer_spe

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I wouldn't use regular plywood on top of the plastic that is directly on top of the ground. Not sure you need the second layer of 2x4's. I'd look to use 5/4" PT decking instead of plywood.
I didn't think pressure treated wood inside the home was a good thing due to offgassing.
 

Sparky617

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I didn't think pressure treated wood inside the home was a good thing due to offgassing.
I'd be less concerned with outgassing than termites. Any wood in contact with concrete in my basement is pressure treated, granted this is more than a PT floor but any outgassing will be pretty short term. If the wood is dry from the PT process any outgassing is going to be pretty much done.
 

swimmer_spe

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I'd be less concerned with outgassing than termites. Any wood in contact with concrete in my basement is pressure treated, granted this is more than a PT floor but any outgassing will be pretty short term. If the wood is dry from the PT process any outgassing is going to be pretty much done.
Termites are not an issue here.
 

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