Saturday, December 24, 2011

Stemwalls and Rat Slabs - Part I

As you'll note, it's been over a month since the last post. I (naively) thought that the stem wall would go rather quickly. And, realistically this *could* have been the case.

But I had a few decisions to make, and once the decisions were made, I had to finish up the work to make it happen.

The big decision was around interior of the crawlspace, and whether I should insulate the floor of the crawlspace now and cover it with a thin layer of concrete. (This thin layer of concrete is called a "rat slab" because it's not thick enough to be structural, it's just thick enough to keep rodents from tunneling up through it.)

The advantage of pouring the rat slab at the same time as the stemwall, is that I wouldn't have to have the pump truck come out a second time, and the pump truck costs roughly $600 per visit. The disadvantage is that I haven't picked out the type of insulation I want to use to insulate the floor. Normally, when insulating a floor like this, you place the insulation underneath the concrete (see above).

In the end, I decided I would go ahead and pour a rat slab when I pour the stemwall, but I wouldn't insulate underneath. The reason I think this will work is that I'll simply place the foam insulation above the rat slab and then lay down a plywood subfloor above to protect the foam.

Normally, this wouldn't work very well, but because the crawlspace will not be heavily used, the plywood should do a decent enough job of distributing the load.

Now that I've made the decision to pour the rat slab, I also had to figure out how I would support the house during the pour. Currently the house is supported on the interior with eight steel posts sitting on 1/4" steel plate. Unfortunately these supports are on the interior where the rat slab needs to be poured.

So I consulted with my architect and confirmed that I could support the beams with some of the left-over 2" pin pile pipes. I then used a laser level to cut the various pipes to the perfect length to bring the house to level.

Above, you can see the pipes after they were cut and welded to 1/4 plates on the bottom to help distribute the load. I also took the precaution of filling the pipes with concrete so they wouldn't be hollow, on the assumption that this would help strengthen the pipes.

I then jacked up the house a little bit, positioned the pipes underneath the beams and welded the pipes to the beams. I then welded the pipes to the vertical rebar, so that they would be supported laterally and keep the house from moving. And as you can see in the picture below, the posts were positioned within the stem-wall.

So I used a relatively "cheap" rotating laser when performing the measurements. According to the laser, the footings were sloped by about an inch over the length of the house. That didn't seem extreme to me, but when Joe the contractor came out, he was surprised. He was very exacting when he leveled the footings, and used his *very* expensive and *very* accurate laser. As soon as he said this, my shoulders slumped because I realised my laser was likely off and I had cut the pipes to the wrong lengths.

This led to him using his really awesome laser to help me figure out how far off each of the beams was from level. I then had to figure out how much I would need to shim each beam in order to bring the beams and house to perfectly level. This took about two hours, but I can't complain too much, because now the house should be level within 1/8". And because the foundation is sitting on pin piles, there will be virtually no settling. It is a little surreal to think that 50 years from now, this house will still be level within 1/4".

Now comes the ICF! ICF stands for Insulated Concrete Forms. The idea is that you used thick Styrofoam for the forms, and then after you fill the forms with concrete and the concrete cures, you leave the forms in place. The Styrofoam then acts as insulation. An added bonus is that they are pretty quick to setup as well!

The easiest way to think of ICF, is as large, hollow LEGO blocks. You stack them up and overlap the edges for strength.

Up next, the concrete trucks show up again and it'll be time for mud!


Jenelle said...

I like your neighbor's garden Zwerge (gnomes).

A rat trap subfloor (after you know you don't have excessive moisture). My folks' house was built on a hillside lot and subsequent construction adjacent lots exacerbated the seasonal "running water" under the house. In unincorporated King Co. where septic tanks contributed to that moisture. During the rainy season, my folks developed a 50 foot pond in the backyard and French drains had to be installed.

I live in a 4-plex, rated as "single family" dwellings; however, but the 2 middle townhomes share a foundation and vermin got in...and died in the wall. The rat trap sounds useful: but if a crawlspace is below ground level, wouldn't the FLOOR need to be insulated, rather than the crawlspace?

Jeremiah said...

So I'll have good drainage installed around the house. I'm not really worried about water in the crawlspace.

But my crawlspace will be "conditioned", which means it's included in the building envelope. So you can think of my crawlspace as a tiny little basement.

You are right, the floor will need to be insulated and generally you place foam insulation below the concrete. But I couldn't find a good enough deal on the foam and I didn't want to pay the concrete guys to come out twice, so I decided to put down the concrete, then the foam and finally a subfloor on top of the foam. The end result will be the same insulation quality, but an additional 1" of height in the crawlspace will be lost due to the wood subfloor on top of the foam.

Kay Strebe said...

I'm so impressed I think I'm a little bit turned on?!?!