Insulating basement walls with an interior perimeter and radon concerns
I have a 1950’s rambler style house with a mostly unfinished basement. I also have an interior perimeter drainage system which drains to a sump which the previous owners had installed due to water intrusion. The channel is open on the top with the purpose of pooling water coming down the block and into the channel. I still get damp block walls near the floor with heavy rain and I suspect this is partially due to the walls having been drilled below the slab to allow drainage into the tile. Enough water through the block and the water creeps it’s way up. I have never had actual pooling water though the block, just dampness.
In the next month I will be installing 2” XPS on the exterior walls for insulation and then framing.
My question is, I tested the basement radon levels about a year ago and it was a 13. I understand this isn’t incredibly high but I’m finishing the space for kids. Should I do what the EPA suggests and put backer rod and then urethane based caulk and cover the perimeter drain channel or would the foam board butted against the channel do close to the same thing? How would I install a radon fan go depressurize the slab if it’s already open on the perimeter?
I appreciate any suggestions. I’m trying to do this right the first time.
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You are correct to be skeptical about the effectiveness of the radon fan. The slab, sump, and walls have to be pretty air-tight to get any radon reduction benefit out of slab depressurization. Take big step back here- you have bigger fish to fry here.
Deal with the bulk water water first- doing it from the exterior, but you may get some relief (and radon benefit) out of interior side waterproofing of the walls. For radon abatement the below-grade walls also have to be air tight, and the waterproofing will help with that. But it may have to be waterproofed on the exterior to really get it right, or use an interior side dimple-mat and try to air-seal the dimple mat to the slab (which may or may not be possible.) It's tough to make that call even WITH a site visit, but nearly impossible to do well via web-forum.
https://www.greenbuildingadvisor.com/article/wingnut-testing-basement-waterproofing
It is also not possible to make XPS air & radon tight in a long-term reliable way. While it's still worth taping the seams, it will never be a good radon-barrier.
Keep in mind that XPS is perhaps the least-green insulation in common use today, due to the extremely high global warming potential of the HFC blowing agents used, a mix of predominantly HFC134a, worth about 1400x CO2 @ 100 years. Over a few decades as the HFCs slowly diffuse out the performance of the XPS drops toward that of EPS of similar density, or about R8.4 @ 2". The warranteed R-value of 2" XPS is only R9, not the labeled R10, but in 50 years it's likely to be below that level. See:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
When it comes time to insulate, use EPS, not XPS. EPS is blown with hydrocarbons, usually a variant of pentane, which only has about a 7x CO2 potential. Most of the pentane leaves the foam while still at the factory, and is recapured, not vented to the atmosphere. The R-value of EPS is not dependent upon the blowing agent, and is stable over time. At 2.5" EPS performs at R10.5 on day 1, and R10.5 on day 20,000 ( and beyond.)
Jeremiah, while I agree with Dana's comments, I would also consider 13 pCci/L of radon pretty darn high. The EPA says 4.0 is the safe threshold, and radon.org says it should be 2.0. Dana has written before that in Europe they use a limit of 8.0, and that lower thresholds are extrapolated from small data sets so it's hard to know exactly what is a safe limit. But the fact remains that radon is the #2 cause of lung cancer and should be addressed, along with your water issues. While not an approved method, I have had success simply using an exhaust fan in a basement with high radon levels, pending more permanent solutions.
> I have never had actual pooling water though the block, just dampness.
If you are going from uncovered walls to insulated walls, much more water will accumulate (because there it little to no drying to the interior).
You want to build a wall that a) continues to allow water from the wall to reach the drain and b) is airtight to the interior. This is doable. A drawing would help.
The dampness comes from the weep holes which were drilled in the block below the slab for drainage into the Interior tile system. The water traveling through the holes wicks up and around the block.
I should have mentioned I have a 12x12 or so bathroom with 2” XPS between the block and framed walls already in the basement and have not noticed any issues.
I don’t want to say water isn’t a concern, I’m stuck on addressing the radon issues.