Hi- I live on Long Island, and I'm in the process of having my 325 sq. ft. crawlspace insulated and waterproofed. Part of the work involved pouring a four inch concrete slab on the floor. The two attached pictures show that the concrete was not poured all the way to the edge, and it tapers to far less than four inches. Is this typical of a concrete pour in a crawl space or did I just get a sloppy job--the waterproofing company had contracted out for the concrete, and I was told that this is the standard way it's poured. I have not paid for it yet, so I would like to know whether or not I should be asking for this to be fixed, and if so, what can be done to improve it. Any advice would be appreciated. Thanks, Smallhouseguy

was the insulation installed flat and level and the concrete sloped down, or did the insulation taper up to the wall and the slab poured flat and level? Ideally the insulation would have been level and the slab also poured level. One positive feature is that the concrete doesn't come in contact with the block wall, so there is a good thermal break there, and you have room to install sheets of Thermax on the blocks to insulate the walls. Could it be that they were leaving a low area as a drain channel in the event of leakage?

Have them come back and pour the right thickness and make it to wall to wall. A rat slab only works if it is complete. Looks like some Einstein can't do length, times width, times height. They just didn't order enough concrete. Amazing!

Posted By Bob I on 07 Feb 2015 11:01 AM
was the insulation installed flat and level and the concrete sloped down, or did the insulation taper up to the wall and the slab poured flat and level? Ideally the insulation would have been level and the slab also poured level. One positive feature is that the concrete doesn't come in contact with the block wall, so there is a good thermal break there, and you have room to install sheets of Thermax on the blocks to insulate the walls. Could it be that they were leaving a low area as a drain channel in the event of leakage?

Thank you for taking the time to reply. The slab was poured directly on dirt--Looking at it now I believe the dirt tapered up the wall slightly, but I should have paid closer attention at the time. I will look into the Thermax--thanks for the tip.

Posted By smartwall on 07 Feb 2015 11:17 AM
Have them come back and pour the right thickness and make it to wall to wall. A rat slab only works if it is complete. Looks like some Einstein can't do length, times width, times height. They just didn't order enough concrete. Amazing!

I suspect this is the case--thank you for the advice. Is there any problem pouring new concrete on recently poured concrete?

First a vapor barrier, over the existing slab if necessary, and then a 2" cap would do. No weight will be held.

On LI (US climate zone 4) it's worth putting 2" of EPS under the new rat-slab, with the poly vapor barrier on top of the EPS, and using 2" /R12 of Thermax on the wall. If you do it without the floor foam & vapor barrier, take care to keep the cut edge of the Thermax off the slab, since it can wick ground moisture. If this is a site susceptible to storm surge flooding (remember Sandy?), use EPS on the walls, not Thermax.

IRC 2012 code-min for crawlspace walls for US climate zone 4A (that's all of LI), would be R10 continuous insulation, which would be 2.5" of foam if EPS.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

For the sub-slab foam, 2" EPS (R8) is still in a financially-sane range, according to Table 2 on p 10 of this document:

http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

It's more than just an energy-use payback, it's buying resilience. By putting foam under the slab and insulating the wall it lowers the mold & rot risk, both winter & summer.

Is that the footing that they poured up to? It could be the right thickness and have just a sloppy edge job, which really doesn't matter in a crawl space. The only way to tell now is to bore holes with a hammer drill in spots to figure out the true thickness.