High watertable crawlspace suggestions?
Putting a bid together to build a conventional 1800 sq ft home for some folks. This ranching community ( Ft. Klamath, OR) is very small. There have been a few additions in the community but there haven’t been any new homes built here for many years.
Irrigation ditches surround the town and so the water table is very high. (No need to irrigate yards throughout the whole summer.) Sand Filter septic system will be required.
Frost depth is 24″ Soil type is considered __________( Not sure but there is significant amounts of pumice from Mt Mazama’s eruption 7000 years ago)
I have experience pouring conventional crawlspace foundations (ie: 8×15″ footing and 8×24″ stemwall) and would probably use this approach unless there is a better cost effective option.
My main concern is that the crawl space will hold moisture.
The one option I have seen (on paper) is to have the excavator scrape off the sod (6-8″) and cover with 4″ of gravel. The bottom of the footing is then placed on top of the gravel. Footing and stem wall are conventional but a vapor barrier (6 mil visqueen?) is layed down over the gravel in the crawl space and a 3″slurry slab is poured over it. This slurry slab runs from footing to footing covering the entire crawlspace. As I understand it the idea is to seal off the crawlspace from the high water table.
Perimeter of house is backfilled and compacted to achieve 24″ frost protection.
Is this a viable/ cost effective solution? I t seems to me for this to work it would require that 1: there aren’t any punctures is the vapor barrier, 2: that all seams are sealed, and 3: edges stay above water. (wrapped up and fixed to inner founation wall?)
And the purpose of the slurry slab is what? To weigh down and protect the vapor barrier?
Your thoughts and ideas appreciated
Thanks!
Joel
Replies
Welcome to BT Joel, see this is your first post. Good first post.
A neighbor who is a contractor built a slurry floor as you describe for his crawlspace. Included a sump and sump-pump. Seattle area.
When the house was being built, there was a foot of water in the crawl space, but have not heard of any problems so it is apparently working well..
My house is only about 8 feet uphill, with a 7 ft basement, but I can drain to gravity with footing drains, 3-1/2 concrete floor over 4" of gravel and poly, no problems in 35 years, nice dry basement. **
Since you are in one of the better areas in the US for true geothermal energy availability (think many of Klamath falls houses are heated by hot ground water)
Have your customers considered drilling a true geothermal well?? Would make the ideal radiant heat system as next to nil energy cost. Or is Ft. Klamath too far north to tap the field?
edit ps** I did not take any specific measures to seal holes or wrap the poly. When I was digging the foundation, in March there was water literally pouring out of a few small 'aquifers' at the 6 ft level, but it all seems to stay at the bottom of the gravel bed.
edit 2 - Ft Klamath too far north, http://geoheat.oit.edu/pdf/tp24.pdf
Edited 11/8/2007 8:04 pm ET by junkhound
Edited 11/8/2007 8:15 pm ET by junkhound
It's easier to raise the house's foundation well above the water table, but you can do just about the same with a perimeter drain system. If water is at all a concern it's cheap insurance to put in the permeter drain and some extra washed gravel to keep the house high and dry.
If it was my personal home I'd go for a slab on grade and nix the crawlspace all together. A major side benefit other than lower cost to build, a more comfortable living space (more consistant interior temps), is how easily the slab can be fitted for radiant heat.
In many flat areas it's not a bad idea to raise the house so surface water flows away from the house. We've done that a lot when clay soils make a mess if any water is allowed to soak in.
Cheers
Beer was created so carpenters wouldn't rule the world.