Hello again, folks! I just got the radon test back on my new (to me -- built 1925) house, and at 6.5 pCi/L it came in 1.5x higher than the actionable level. The test has an uncertainty of +/- 15%. The test location was a room in the finished basement where my wife plans to have an art studio. Since the same ductwork serves upstairs and downstairs, I have to assume some of the radon is circulating upstairs as well.

If I were not a former energy auditor, I would slap on a radon mitigation system that exhausts air from the foundation of the house 24/7 and not think one second about where the make-up air comes from. But since I am, I'm trying to decide where to add ventilation to bring in filtered fresh air to replace the radon.

Before I go down that road, though, I wanted to ask whether any of you have had documented success mitigating radon with an HRV or ERV rather than an exhaust-only system. Is that a reasonable approach? Or should I just suck air from the foundation like everyone else?

Thank you in advance for your advice.

Tags: HRV, radon

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Hi Ben, you ask a complex question that depends on factors beyond the information provided. Has the house been blower-door tested? If not, do you have a sense of how leaky/tight it is? A reading of 6.5 in a tight house is very different than 6.5 in a leaky house in terms of how one approaches radon mitigation.

Also, it's not clear if the foundation you refer to is beneath the finished basement room(s), or if it's a partial basement on a slab with an adjacent crawl space below the main level.

Another important consideration is whether any return ducts are routed through the foundation area.

I think whether or not you need to provide makeup air to the living space depends on the extent to which the foundation communicates with the living space, which is something you should be able to control to some degree. Zonal pressure diagnostics would be a good first step. Do you have access to a digital manometer?

Hi, David. I posted a few months ago about how my house tested quite leaky but has high CO2 and VOCs (according to a consumer-grade sensor) when my wife and I are in the house. The house as a whole is leaky and I haven't sealed any leaks yet because I was waiting on the radon test. Also, the utility bills have been very reasonable, so it's hard to justify spending a lot.

It's a full basement, so a traditional radon mitigation system would be below that, about 6 feet below grade. The ducts are all in the basement sealing. There are a few cracks in the poured cement, but nothing I'd suspect of leaking air. If I understand what you're getting at, this means that makeup air from an exhaust only system would be coming from outside the house, not inside. Correct?

I don't have a blower door or manometer anymore, but I know a guy.

So now I'm confused. If the house has a full basement, what foundation are you referring to? Is your basement not built on a slab? I've heard of basements built on wood foundations, but I've never actually encountered one!

The basement is on a slab foundation. The house is fully on the basement. Sorry for the confusion.

It appears the confusion is on my part (semantics). In my mind, a slab in of itself is not a foundation. The foundation consists of your footings and/or thickened portions of the slab, and basement wall to the extent that it's load-bearing. So when you refer to exhausting radon from the foundation in a basement home, I was expecting there to be an adjacent crawl space, or a sub-space beneath the slab, as there would be with a basement built on a wood foundation. Otherwise, you would be looking at some serious demolition!

Of course, radon mitigation systems are typically installed sub-slab. But in an existing home built on a slab, I'm guessing what you're actually talking about is using a ventilation system (exhaust or balanced) to remove the radon after it passes through the slab and below-grade walls into the rooms (as opposed to "sucking air from the foundation"), right?

What the mitigation companies around here typically do is drill or dig a hole beside the basement to below the level of the basement floor and depressurize that sump hole with a fan, to try to get the radon out of the ground before it passes through the slab into the basement.

But what I'm talking about in this post is whether an HRV or ERV inside the building envelope is effective at addressing radon. The EPA's own publications on radon mitigation list it as one of the approved methods, so I'm not making stuff up here, I've just never seen it done, so I'm wondering if anyone here has experience with doing that.

To clarify my original question: Has anyone attempted to mitigate radon along with other air quality problems using an HRV or ERV, rather than tackling the radon as a separate issue? Did it work?

Not to my knowledge & I don't think it would be to wise to try it - especially with an ERV. An HRV might be somewhat acceptable (no mixing) but you will never get all the pull you need / what you would get with an underslab style.

Of course part of the question is just how often is this room used now / what are the readings throughout the rest of the basement - long term or only short term?

We only took readings on the one room because we didn't know it was going to be over the limit, and it was a 90 day test. Currently it's only being used an hour or two a week, but ideally it would be more like an hour or two a day, and in any case air circulates to the rest of the house.

Why can't you work on sealing the cracks in the slab to slow down the infiltration of radon?   Is this not the standard approach?

If there are any cracks in the slab or the walls, I haven't found them in 4 months of living here.

Is the slab sealed around all penetrations?  Is the slab visible or covered with flooring.


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