First time poster, but have enjoyed reading the posts here for quite some time. We recently moved into a house built in 1984 in southeast Pennsylvania. Radon testing showed an initial level of 5.5 pCi/L and we installed a subslab mitigation system. Post-mitigation level came back at 3.9 pCi/L. While this is just under the EPA limit, it still strikes me as a bit high, and I'm pondering the next actions to take. The basement is a block wall with a floating slab. There's a sump pit installed, but not a perimeter french drain. The slab edge was mostly sealed with backer rod during mitigation, but some areas were inaccessible due to finished walls. We also have moisture issues in the basement (high humidity, significant efflorescence around floor edge and walls). I suspect the subslab aggregate is too dense to allow adequate flow of gases under the slab. It's also possible that the few spots where the slab edge isn't sealed is limiting the effectiveness of the mitigation system. What's the best way to further reduce the basement radon level? We're planning an interior french drains to address the moisture issues. Would that help improve the effectiveness of a subslab radon system? If the material under the slab is too dense to allow adequate soil gas flow, does it necessitate removing the slab and repouring over new crushed stone? Would it be more effective to use create a 'bathtub' with air-sealed dimpled mat to address the moisture and radon issues? This method was suggested by Building Science Corp for rubble stone foundations. Thanks in advance.

does it necessitate removing the slab and repouring over new crushed stone?

I expect that plastic sheet, dimpled mat and then a wood floor would be cheaper, but would only handle small amounts of moisture.

They should be able to measure the underslab pressure at various points to verify that that the system is working as intended. It could be modified to draw from multiple places to reduce the need for underslab flow. Also consider ventilating more with a HRV or pressurizing (or at least balancing) the basement (vs depressurizing underneath the slab).

Slab edges covered by walls might be able to be air sealed by sealing up the wall (ie, air tight drywall techniques). Consider that radon could also be coming through the block walls.

Radon is nasty stuff and technically, there is no safe level.

I don't know how they did your "mitigation system", but it's obviously not enough. Radon can be somewhat contained by concrete (although it will leak through cracks) so if there are openings around the edges, they will short circuit your "system" and must be fixed. In addition, I'm unclear if your planned french drain will be sealed from the living area but it needs to be. The right way to do this is, as you said, remove the slab, excavate some amount including a deeper drainage trench, install washed stone, a drain or sump pump, insulation (2" Styrofoam or EPS), a heavy, taped, poly vapor barrier which runs up and is fastened to the walls & then a concrete slab. Add a loop of 4" drainage pipe which will tie into the radon vent, then pour your concrete. Should be much lower level after all that.

It is unclear what sort of sub-slab mitigation system you had installed.

Concrete is porous to water vapor. Needless to say, radon will go right through it.

Thanks for the helpful responses.

It could be modified to draw from multiple places to reduce the need for underfloor flow. Also consider ventilating more with a HRV or pressurizing the basement (vs depressurizing underneath the slab).

The basement is round 23'x23' and the current system draws from two points located against the outer walls at basement midline. It uses 3" pvc connected to an RP145 fan. It's rated for 166 cfm at 0". I don't know what the actualy system pressure is. I've considered an HRV to further dilute the radon, but would also like to make sure I make the most of the currently installed system.

Bob I:

In addition, I'm unclear if your planned french drain will be sealed from the living area but it needs to be. The right way to do this is, as you said, remove the slab, excavate some amount including a deeper drainage trench, install washed stone, a drain or sump pump, insulation (2" Styrofoam or EPS), a heavy, taped, poly vapor barrier which runs up and is fastened to the walls & then a concrete slab. Add a loop of 4" drainage pipe which will tie into the radon vent, then pour your concrete. Should be much lower level after all that.

I think a partial short circuit of the subslab vacuum is occurring because there are a few places where the perimeter backer rod was placed between the wall bottom plate and the foundation wall, and not between the slab and foundation wall. This is a relatively small portion of the perimeter (maybe 10 ft?), but I'm not sure how much it's contributing. The planned french drain would be placed next to the footer and include dimple mat about one foot up the wall. It will be sealed and the walls eventually insulated with EPS that is sealed at the floor junction and the top of the wall. The french drain should improved air flow under the slab, and sealing it as described should be an improvement over the current state. My fear is that we go through all that trouble (french drain only), and it's still elevated. Replacing the slab as you describe sounds more reliable, but also significantly more expensive. Is there a way to estimate costs for replacing the entire slab and laying new stone? Is that done by a waterproofing contractor?



As jonr mentioned, the dimple mat bathtub would handle limited moisture. But shouldn't this accomplish air tightness similar to the approach outlined above? I'm not sure I can insert a link, but there's a similar description at the site below. It was also described on the Buliding Science website, using closed cell foam instead of dimple mat on the walls.

http://aarst.org/2014/1_Freeman_SOLVING-THE-CONFLICT-BETWEEN-BASEMENT-WATERPROOFING-BEST-PRACTICE-AND-RADON-MANAGEMENT-IN-THE-UNITED-KINGDOM.pdf



The walls of the basement are damp about one foot above the floor. I'm not sure how much moisture the dimple mat system can handle. We don't plan on finishing the basement, but would like to insulate.

I appreciate the help.