Concrete Floor or Encapsulation for Crawl Space?

4 years ago

We'd like to hear your comments about putting a concrete floor in our crawl space prior to framing versus encapsulating the crawl space after the building is dried in. The concrete floor version of the crawl space will have about 42" of headroom. The encapsulated crawl space will have 48" of headroom.

We are building a new, single-story home ~1900 square feet in the SE U.S. The terrain lends itself to a crawl space but we didn't see the need for basement. We do plan to locate the HVAC system in the crawl space. The contractor added exterior sealant (tar?) to the block wall and used dimple sheet and drain around the perimeter of the exterior. The contractor indicated this is the method used in our area for basements and thinks this will be suitable "waterproofing" for the house.

The crawl space is 2800 square feet in total because the crawl space also includes 900 square feet under the concrete front porch - raised concrete porch floor on deck pans. The crawl space of the house and crawl space under the porch are interconnected via two passages through the block wall at the front of the house. The crawl space it will have a dehumidifier regardless of whether we have a concrete floor or encapsulation.

The estimates we've received for encapsulation make us think we may prefer to pour a concrete floor in the crawl space. The cost for both is within $2000 (or so) of each other.

If we we pour concrete it will have a 2" layer of #57 gravel, a 15mil vapor barrier, followed by 3" of concrete recommended by the contractor. The concrete floor would be done now prior to any framing.

Initially we had planned to encapsulate the crawl space. That's typically performed after the building is dried-in and the trades (HVAC, electrical, plumbing) have finished their work.

Questions:

1) Concrete versus encapsulation? Which do you prefer and why?

2) If concrete, is 2" sufficient or do we need 3" as recommended by contractor. The cost for 2" (17 yards) is $2600 vs. 3" (25 yards) is $4000. The $1400 is not trivial but not a major factor in the overall cost of building this home.

3) Other comments?

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Comments (4)

Seabornman
4 years ago
I have a 3" concrete slab in my crawl space and I highly recommend it. It's just so clean and easy to get around to access stuff. I use it to store building supplies. I don't see how 2" of gravel works, but maybe that's what is done in your area
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Charles Ross Homes
4 years ago
Whether you have a crawl space or a basement you need to control moisture in the space.

Sounds like your builder has implemented a strategy to minimize potential moisture entering through the perimeter walls. That leaves the crawl space floor as the other potential source of moisture. Concrete, by itself, does nothing to control moisture-- it's the soil moisture vapor retarder under the concrete that does that. The concrete simply provides protection for it.

A good, reinforced vinyl soil moisture vapor retarder is a good investment. If you want to protect it, topping it off with some concrete is a fine solution.
nvestysly thanked Charles Ross Homes
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just_janni
4 years ago
FYI for that vapor retarder, I used Stego wrap - stops enough that it can actually be called a vapor barrier. Highly recommend. And the tape they have is AWESOMELY STICKY

https://www.stegoindustries.com/products/vapor-barrier-20-mil
3onthetree
4 years ago
last modified: 4 years ago
Just to make a huge clarification - "encapsulation" is the process of closing off a crawlspace to the outside air, closing off the floor to moisture, insulating the walls, and air sealing. You are essentially making this unoccupied area a part of the house, just as you do a basement. Whether you add a "mud slab" (thin, non-structural concrete floor) on top of the vapor retarder is inconsequential to the act of "encapsulation."

Your contractor has gone above and beyond (apparently unsolicited from you) for a crawlspace by installing footing drain tile, damp proofing, and a dimpled membrane. That was an excellent decision and gives a hint of the level of house you may be building. So since you have paid extra for that, and now have a properly drained foundation, IMO it would make sense to pay additional ($4K on a $500K home?) for a mud slab to match that level of build. It is not required to seal off the floor from moisture, but provides good "housekeeping" (easy to clean & keep clean) for the crawlspace. You will have HVAC there, be accessing it, and probably store some things too.

Nuts/bolts and other comments:

- Just as FYI, 2" of gravel doesn't really do much. It won't really compact well (nor does #57 do so to begin with), which is contrary to what you need when under a concrete slab. If you are doing 2" thick concrete, then the slab will be harder to float smooth as the 1" size of minimally-compacted stone will jut under the vapor retarder, creating little pockets into the 2" concrete thickness, so potentially your slab may not look as smooth as a patio, and you may get some cracks over the thinner areas. With 3" there is more concrete cover and the slab should perform and look better. Typically a gravel base is 4" thick, max 3/4" size. The differences aren't critical for a crawlspace, but just giving this info in case you have preferences for how your crawlspace looks and can evaluate the cost choices you have.

- What you do have with the gravel layer is a drainage plane. As a bonus, this allows good movement of radon gas. Even in the SE, like Georgia, radon is prevalent. It makes sense now, for very little cost, to install corrugated pipe around the perimeter of the crawlspace. Provide a stub up through the concrete to allow you to connect to it in the future.
nvestysly thanked 3onthetree