Renovating Crawlspace with Poured Concrete Floor

Question

My newly-purchased 1950s home has a below-grade crawlspace with a poorly-poured concrete floor. Specifically, it is very coarse and looks like it was poured and spread with a garden rake. In some places, there are large holes that seem to go down to the dirt. Not good. Also on the list of "not good": we have slightly elevated levels of radon in the home.

Ultimately, we want to use the crawlspace as a clean, dry, conditioned space that is an extension of the basement used for storage.

My plan is to remove the door that separates the crawlspace from the basement, then have my HVAC guy run a supply line or two to the crawl. The supplies supply conditioned air, and the open entry to the crawl will serve as a return. I am also going to seal the outside walls of the space with insulation - but I will save the insulation details for another post.

In order to make the space clean and usable for storage, I obtained a quote for a poured concrete floor. The contractor's intent is to pour a couple of inches of concrete right over the existing "concrete" floor without chipping the old floor out, and then trowel/level it nicely.

My questions are:

1. Any concerns with pouring concrete right over the existing "concrete" floor?

2. Assuming that I am OK to proceed with pouring over the existing floor, do I need to stipulate that the contractor lay down a continuous sheet of polyethylene as a vapor barrier? To be clear, if I have the contractor do this, this poly sheet will sit between the old "concrete" and the newly-poured floor. Or is the vapor barrier a lost-cause (or, worse, going to cause a problem) if I do not remove the old floor?

3. I haven't lived in the house very long, but it looks dry (contractors have agreed). Also, for the record, I do have some water supplies and drains that run through the crawl to the upper floor. That said, how important would it be to install a floor drain? If I were to have a plumber install a drain, would this drain flow into the sewer? To do this, I think that I would have to have the plumber bore a hole through the foundation wall, which sounds like a huge job. So unless it's highly recommended or a code issue, I would rather skip it.

Regarding the radon, I am going to have the concrete contractor install a vent pipe into the floor (both the new floor and even down through the old floor) for sub-slab suction. My radon mitigation contractor will connect this vent pipe to the radon mitigation system.

I would love to hear any comments from the experts. Thanks in advance!

-Frank

Answer 1

It sounds like you already have a couple of experts working with you. Without seeing the proposed new space, I can only speak to a couple of your concerns.

There is no easy way to install a sub slab vapor barrier without removing the old slab. You do not want to sandwich plastic between the old and new concrete if you are going to only put in an inch or so of concrete to level the space. I'm sure your contractor will prep the old slab by cleaning etc to assure a good bond with the new concrete. One or two inches of new concrete should be fine, especially in a space with low, lightweight traffic.

I see no reason for a drain in this area unless excessive leakage or standing water is a potential problem.

If radon mitigation is required, the collection pipes should be installed under the existing slab via cutting trenches and tied into your venting before you pour the new concrete. Again, your radon contractor who can see the space should be able to advise you properly and make all the connections to your existing system.

Answer 2

Just to circle back on this... my radon contractor stipulated that a polyethylene layer is needed for radon mitigation, and my crawlspace contractor agreed.

The crawlspace contractor broke the concrete in a pitchfork-shape (to get good radon coverage and get around some concrete footings that are supporting the steel I-beam). He then dug a trench, put gravel down, and put slotted drain tile pipe in the trench covered by a silt sock. The drain tile pipe was then connected to a 3" schedule 40 PVC "T" that the radon contractor will tap into. And all of this, along with the remainder of the old concrete floor, is covered by the poly barrier mentioned above. They poured cement right over the poly.

My crawlspace contractor spotted a couple of foundation cracks - for good measure, I had him do an injection-repair on these. We are going to try and keep the space waterproof without installing a drain.

Answer 3

Concrete can always sit on old concrete. Will it bond properly? Not without some sort of connection piece. We use rebar "L"s to properly bond the new concrete to the old. Drill about 3-4 inches down into old concrete, insert short side of "L" and other side of L should be a little off the floor. I have seen others use rebar with mesh kits and a few different methods. No matter what chemicals these guys use the concrete will never "bond" so you need to have a common stabilizer.