Crawl Space foundations

[rwalter]

I was checking out a new construction website, www.hgtvpro.com and watched a video on best practices for a crawl space foundation. I was always under the impression that the crawl space should be ventilated unless it was heated. They say the best practice is to seal a mosture out of the crawl space by placing a sill plate sealer on the sill and calking and gluing the rim joists and subfloor. They also use 10 mil poly on the foor with 2" of concrete on top of it and they insulate the crawlspace wall. What is everyones take on this. Is it overkill? If i have to pour a crawlspace floor I might as well dig the rest  the wasy and  put in a basement. I think the difference is on 2 or 3 courses of block.

[Amanda_931]

No ventilation was what was specified by TVA in the late 70's and 80's for houses in Nashville.  Mine was retrofitted very badly though.  Not sure if the previous homeowner had pulled out the poly film, or the contractor had never put it down.  Either was possible.  No concrete, though.

(That house had been made out of walls from a couple of box-cars decommissioned after WWI, with a couple of standard additions.  Always cold.  Way too close to the ground  Drill to try to hang a picture, you'd run into galvanized steel.  Even with the best will--and crew--in the world, the contractor would have had major problems.  And dealing with the homeowner at the time was nearly impossible--he tried to back out of the deal at closing--maybe I should have let him.  But those people retrofitting insulation, heat pumps, etc. got a bad reputation for truly crappy work)

Sounds like they're talking about digging down to frost line for footings and foundation but then  going up from ground level about three concrete blocks worth.  With undisturbed earth on the inside.  Poly film down,  then--should be--sand and  then pouring the two-inch slab.

Not at all the same, especially if you're doing it by hand, as taking all the area under the house down to the same level as the footing, i.e., a basement.

[rwalter]

Amanda,

I my situtation the frost line is 48" so they the contractor I am looking at was looking at lay 6-7 courses of 8" concrete blocks. If I folllow this "best" practice" then its only a few more courses of block and I'll have a full basement. Granted If I go the basement route I'd have to pour a 4" floor instead of the 2" floor.'

Here is a link to HGTVPro's best practice page. Just click on conditioned crawl space.
http://www.hgtvpro.com/hpro/pac_ctnt/text/0,,HPRO_20196_27960,00.html?cat=0

[glenn kangiser]

Unless you are going to drive a truck in your basement, a 4" floor doesn't make sense, but I suppose it has something to do with the code and the corporate enrichment program.

It has been demonstrated many times over especially in India that a 1" floor will easily support a rated 40 lbs psf with hessian (jute) reinforcement and plunger piers, but has been tested at 450 psf without failure - but that doesn't sell concrete, or cost enough to increase the permit fee sufficiently.   :-/

[John Raabe]

My home, the solar saltbox,( http://www.jshow.com/y2k/listings/55.html) has a sealed and insulated crawlspace, about 1/3 of which has a 2" dust slab over 6 mil black poly. The poly covers the rest of the soil and up the inside of the walls. There is 48" wide R19 batt insulation on the inside of the concrete crawlspace walls up to the subfloor and then draped and taped to seal the rim and then taped on the lower edge to the poly.

The crawlspace is very dry and the 23 year old wood structure is in good shape. There is a fan that moves air from the house through the underfloor plenum and keeps things somewhat even. There are three open floor registers and no insulation between the floor joists.

This has worked quite well for many years. The extra thermal mass gives the house a flywheel effect and it never gets too hot or too cold.

However, since I am in damp sandy soil I find that the crawlspace is always cooler than the house and pumping heat into the underfloor area gives me a less than a 100% return on the investment. In the coldest time of the year we don't run the fan and the crawlspace ends up being 10º cooler than the house above.

The crawlspace plenum works best in the summer when the fan sees much more action moving air into and out of the cooler crawlspace.

[Amanda_931]

"dust slab" ?

[John Raabe]

"Dust slab" is what a 2" slab like this is called in my area.

It is an unreinforced slab that is poured directly on the poly and not steel trowled to a smooth finish. It provides dry solid storage at low cost.