Radon control and drain system for a frost-protected structural slab foundation
I am working on a project that will be PHIUS Certified. It is in Zone 6, incorporates a frost protected, structural slab (raft slab system, so no thickened edges or footings). My question is about the drain system and radon control. The lot is very level so there is no where to run a drain away from the foundation. I am trying to avoid a sump all together. The rough footprint dimensions are 28′ x 28′. The slab will be 6″ or 8″ thick, with 10″-12″ of EPS foam underneath, then 2″ of compacted fine fill with 4″-6″ of gravel under that. The soil on the site is HeB, sandy loam.
How can I (or is it possible to) provide necessary drainage without a sump?
Can I include radon control in the same system?
Where would the bottom of the drain be? At the bottom of the slab, foam, gravel?
Thank you,
Nick
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
In general, the drain pipe should be located alongside the footings, so below the foam and fill, and generally within the depth of the gravel. If the seasonal high water table on your site is well below your foundations, you do not need open-air drains or sump pumps. You may want to install a sump pit anyhow for insurance. There are sump pits with sealed lids, or you can seal a clear plastic lid on many sump pits so that you can inspect to see if any water is accumulating in the drain system and/or sump. A pump can be installed later if necessary.
If you do a very good job of air sealing the slab to the foundation walls, or some other form of air sealing, this will also help to limit radon entry. Is this a high radon area? if so, it would make sense to run a 3" pipe up through the house from the footing drains to the roof. This provides passive ventilation for any radon or other soil gases that may build up below the slab. This is often sufficient to reduce radon entry to acceptable levels. This pipe does create two additional penetrations in your air and moisture barriers, but you will have to weigh the risks. An alternate would be to run the pipe up from the drain system to grade level outside the house and cap it. If a subsequent radon test shows unacceptable readings inside the house, then the pipe can be extended up past the roofline with a fan installed in the pipe outside. This will use some energy, but no penetrations in your envelope. In general, the gravel bed and drain pipe are very effective as a radon collection system but you do have to give the radon a way to escape from it.
You ventilation system will also help to remove radon from the indoor air by dilution. This is not the best solution to air pollution, but it does help.
Hi Nick -
Great GBA resource here for you: https://www.greenbuildingadvisor.com/article/radon-and-a-passive-house.
Peter