Water management for a stepped foundation daylight basement
I am currently helping my builders excavate for a stepped foundation for my future house with a daylight basement. I’m not sure what to do for a footing drain with the stepped foundation or even if it is necessary. The obvious answer would be to get a pro to handle it, but I am in Japan and footing drains, exterior damp proofing, etc. are unknown to the pros that I have met with here.
From what I gathered from conversations with my contractor, there aren’t liquid water problems with foundations in the area. There aren’t many basements in the area either—just crawlspaces. A neighbor that does have a basement (with no damp proofing, footing drains, etc.) told me that although he doesn’t have a problem with liquid water, the humidity in the basement is high, so his basement dehumidifier fills up quickly. I don’t want that for my house so I imported some elastomeric waterproofing paint to coat the top of the footing and the outside of the basement wall.
From reading articles on GBA, it seems that all new foundations should include a footing drain, but is installing a footing drain worth it in my situation? (No one experienced with them, having to design, make, and install it myself with a drill and PVC pipe, a lack of known liquid water issues in the area, and several steps in the foundation to deal with.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Eric,
A footing drain is cheap insurance, and it makes sense for stepped foundations too. Where the foundation steps down, just install a couple of 45 degree or 90 degree ells.
If you can't get perforated pipe, it's pretty simple to drill out unperforated pipe. Drill 1-inch holes at 4:00 o'clock and 8:00 o'clock.
This source shows standard 4" SDR35 drainage pipe to have 1/2" (12 mm) holes on 5" spacing, 120 degrees apart (4 and 8 o'clock). That pipe conforms to ASTM D3034. From what I saw in a brief search, some other types of drainage pipe follow different standards and use 3/8" holes.
Dick,
I stand corrected. Thanks.
Thank you Martin and Dick. It looks like I will be drilling a lot of holes later this week. I'm assuming that the 1/8" difference doesn't make much of a difference, but to be on the safe side, should I go with the 3/8" or 1/2" holes?
I have another question related to the stepped foundation. I am waterproofing the exterior with an elastomeric waterproofing paint and plan to insulate the interior of the concrete portion with polyisocyanurate (XPS was unavailable). My contractor told me that a basement continues to dry for three years. If that is the case, is it unwise to put a moisture barrier on both sides of the foundation wall? Should I leave the exterior of the foundation above the ground level unpainted for drying? How about the slab—I planned to put polyiso over the slab as well but could that trap moisture?
Eric,
Q. "My contractor told me that a basement continues to dry for three years. If that is the case, is it unwise to put a moisture barrier on both sides of the foundation wall?"
A. No, it's not unwise. Damp concrete is strong concrete. The concrete will dry somewhat from the top, and somewhat by diffusion through the paint. There is no reason to worry if it dries very slowly. It's not as if the concrete is going to rot.
Q. "Should I leave the exterior of the foundation above the ground level unpainted for drying?"
A. You can if you want. I wouldn't worry abouy the issue.
Q. "How about the slab—I planned to put polyiso over the slab as well but could that trap moisture?"
A. The concrete will be fine. If you can find some EPS, it's a better choice for insulating a slab than polyiso. (If the slab ever gets wet, the polyiso can absorb moisture.) Ideally, you slab would have a horizontal layer of EPS under the slab rather than polyiso on top of the slab.
Martin,
Thank you for answering all my questions. EPS is widely available here so I will use that instead of polyiso for the slab as you suggest. I will talk with my contractor today about putting it under the slab before we pour.
Now I'm curious about what I should do about the thermal break between the slab and the foundation wall. The polyiso for the walls will arrive from the U.S. after the foundation slab is poured. Could I add EPS around the perimeter of the slab before we pour it for the thermal break?
Eric,
Q. "Could I add EPS around the perimeter of the slab before we pour it for the thermal break?"
A. Yes.