Exhausting Radon
I have a complex radon mitigation situation in an existing home. After reviewing posts on this site and elsewhere, and consulting a radon mitigation contractor, I need further education. Input on any of the questions below would be appreciated.
The house: Hudson Valley, NY. Original mid-1980s home plus 3 year old large addition. Original: 2 story plus partially finished walkout basement and finished attic. Post-and-beam, SIP walls. Concrete block basement walls, rough concrete basement floor. Part clay and part bedrock under & around house. Basement floor drain to daylight. New addition: 2 levels, above grade “basement” on same level as original house’s walkout basement. Crushed stone & some bedrock underneath, poured foundation walls, slab, and outside footing perimeter drain to daylight, and separate basement floor drain to daylight. Spray foam under slab, spray foam wall & roof insulation. ERV in this part of the house. Open floor plan between original house and addition (including on basement level). Work done to tighten envelope of original house during addition.
Continuous radon monitor shows very high levels on the floor of original home’s basement (25+ during cold weather), with lower levels in lower level of new addition and upstairs (between 7 and 11).
Contractor proposes a single active sub-slab depressurization system drawing from two points beneath new and original basement floors, with fan inside the basement mechanical room of the original house and exhaust at grade level—not extending above roof—for aesthetic reasons. He also proposed capping the floor drain to daylight in new basement with a cover designed for active sub-slab depressurization systems; and completely sealing up the floor drain to daylight in the original basement. And he intends to seal a crack and expansion joints in the new slab, but did not propose any sealing in the original basement.
Questions:
1) Given that there are and will be no children in the home, is ground level exhaust acceptable if greater than 6 feet from windows, doors, or frequently used outdoor spaces? Code compliance is not a concern here, just whether it functions acceptably & is reasonably safe. I found research that tested diffusion of radon from ground level exhausts which found under virtually all conditions, levels were indistinguishable from background levels in open air between 3 and 6 feet from exhausts. So I’m leaning yes on this, with the understanding that if the home is ever sold this may need to be changed.
2) May the fan safely be located inside the house? Does doing so require any special equipment or precautions (alarms, etc)?
3a) Sealing the original home’s basement against air leakage may be a hopeless task because the block walls and concrete floor are very porous and partly covered with finish materials. Given that, does depressurizing below the original home run the risk of sucking too much conditioned air out and creating backdraft from the wood stove (the only combustion appliance in the house)?
3b) Does this potential for negative pressure condition inside the house vary with outside temperature as the natural stack effect within the house gets stronger in winter and weaker in summer? If so, how do you size the fan to account for that seasonal variation?
4) Given the spray foam below the new addition’s slab, and the likely impossibility of sealing the original basement and the joint between old and new, is sealing cracks and expansion joints in that new slab really likely to matter?
5) Since radon levels are far higher in the original house’s basement, would it make sense to begin with a system that draws from one point below the original house, see what that does to the radon level, then only if needed add and tie into another point of draw below the new addition?
6) What’s the reasoning for sealing the original basement’s floor drain to daylight?
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Replies
I would check out the Canadian guide to radon mitigation. It made more sense to me to follow in a cold weather climate. My radon fan is inside my basement and my fan discharge is at ground level.
https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radiation/radon-reduction-guide-canadians-health-canada-2013.html
That’s very helpful, thanks! Much more useful than most US EPA documentation I found.
Do you have a heater in the basement. For me the biggest driver of radon in was combustion pushing air out, and no fresh air coming in. Adding an air intake was very helpful to cut down the levels - so the air came from the outside rather than the ground (from the negative pressure).
My heater is in the basement, but it’s an electric ground source heat pump—so, no combustion air. But I assume even without combustion, heating is driving stack effect movement of rising warm air from the cooler basement level up and out gaps higher in the house, so that negative pressure is part of what’s pulling radon up into the house.
People have posted on this site that had very good success dealing with radon by installing an ERV in the basement.
You mention you already have one for the addition, this is a good time to install one for the main structure.
Besides helping with radon, it will provide ventilation for the house and would be much cheaper to run than an exhaust fan. If you are doing major reno down the road, you can always plumb the ERV into the rooms.
The idea is to put the ERV stale air pickup near the floor in the basement and supply the fresh air from ERV to the living space above. This will also help with circulating conditioned air the the basement and exhaust any basement smells along with the radon.
I see how that strategy could be sensible as well. Although the original house and the addition are very open to one another and about equal in size, the existing ERV in the addition pulls in stale air from the main floor, not the basement. So adding another in the original house that pulls in from the basement could help.
And to follow up, we’ve just discovered the majority of the addition’s slab and the original basement are built directly on bedrock, so sub-slab depressurization is not going to work. Which makes the ERV solution suggested above the most viable option. I’ll read those threads and educate myself on that strategy.
It's a perfect time of year to test it --- put two fans in the basement windows, one for intake one for exhaust and see what happens to your radon levels.
Not as efficient as an ERV in the summer and winter, but should be close enough in the spring. And you can try different fan speeds to see how much air you need push to get the levels where you want them.
Here is the thread:
https://www.greenbuildingadvisor.com/question/erv-best-practices-for-ventilation-and-radon-mitigation
Normally I would say to install with a dual vent cap to save drilling/flashing two holes through your walls, but I think in this case it makes sense to have more separation between outside intake and exhaust.
Thanks for the link. Radon contractor’s ERV specialist is scheduled for a visit and I’ll talk through locations of intake and exhaust. Agreed that two holes widely separated makes sense (even though putting holes in this house is a pain—you go out through a 6” thick timber rim joist or through 8” thick SIP wall. Doable, but … “fun”).
TwoCoats, any update on your Radon mitigation? Did you end up using an ERV?
I live in the Hudson Valley my self and I’m running into a similar situation with my home.