Need some suggestions. My 70s house is post and pier over a 4′ crawlspace. It is built on a modest grade, with the high end at the back of the house. Very high clay content soil. The foundation is 8″ x 30″ poured concrete wall over a 12″ x 12″ footing. There is a (poorly installed) 4″ perimeter drain on the outside of the foundation that is sometimes setting on top of the footing and other times is 4″ lower, with its top even with the top of the footing. At the back of the house, grade is about 2′ above the footing. The issue is that the crawl space was excavated to about 6″ BELOW the BOTTOM of the footing at the back of the house, for some reason, and never back filled. Over time, of course, water has found a way to drain into the crawlspace in the winter, when the water table rises (Portland, OR). I have dug a temporary sump and installed a pump, but I want a permanent fix. There may not be enough grade to dig the exterior out and reset the perimeter drain to the bottom of the footing and still get enough slope to run the drain to the street. Also, since the grade of the crawlspace is so low, and the post footings were poured to be about level with the foundation footing, the moisture barrier was draped over the 40 post footings and over time has become pretty well shredded. So I’m thinking I’d like to backfill the crawlspace so grade is basically even with the footing, with or without an interior perimeter drain, and replace the barrier. I can use the founadtion vents to get material in. I thought about aggregate, but that would be one brutal job. Then I thought of CDF. Much more expensive, but fast and easy. And the CDF would stop the water draining into the crawlspace, whereas the aggregate would still allow significant water inflow. But maybe there are other, better ways of back-filling a crawl space. Anyone out there had to do something like this? Is there something I haven’t thought of? Is there a better way?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
The FHB Podcast team weighs in on Building Science career questions.
Featured Video
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
If you use flowable fill you will still need a drainage system and a sump pump because the fill will not give you a water tight joint where the fill is against the footing. So plan for a drainage system. You might want to investigate placing board insulation under the fill and up the sides of the foundation wall.
Dr. Joe Lstiburek, the chief engineer of the Building Sciences Foundation, did something similar in retrofitting the foundation of the home he lives in that has a dry laid stone foundation. He used spray foam and rigid foam board along with a membrane to seal the foundation and limit the infiltration of ground water. His solution included a sump pump (see second attachment-Rubble Foundation). I have attached some information about crawl spaces.
I understand about the drainage. My thinking was that if I re-set the exterior perimeter drain where it SHOULD have been installed, so its top is level with the top of the footing, then it would drop the water table at the footing to 4" below that. Then I could use flowable fill to raise the level of the crawlspace to something close to the top of the footing (and 4" above the water table) on the inside. That would keep flowing water out of the crawlspace, although the fill might still be damp from wicking. That would be controlled with the vapor barrier. I would LIKE to insulate the floor of the crawlspace, but the ground is SO uneven (not to mention the 40 post footings) that it doesn't seem realisitic. Unless I did TWO CDL pours, one to level the floor, followed by laying down insulation, then a second lift to get the floor to the level I want. Then the crawlspace will be better built than the house itself! Anyway, the real question: is there a better option than CDL to raise the floor of a crawlspace with very limited access?