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Community and Q&A

ERV vs. positive pressure vs. none

ars777 | Posted in Mechanicals on

Leaky 1800’s log home. Radon issue being worked on via traditional methods with mixed success. Radon issue improves dramatically when windows are open — I assume this is dilution and perhaps also pressure equalization?

Wondering if ventilating may help dilute radon w/o the windows open energy penalty, especially in winter. If so:

1. Positive Pressure Ventilation — perhaps a dehumidifier ducted to draw outside air — might the positive pressure help reduce radon draw and simultaneously dilute the radon issue?

2. ERV — does a balanced system ever make sense in a leaky home? The house also has some negative pressure devices (bath fans, boiler, wood stove insert). Goal here might be simply to dilute bad air whereas a positive pressure system may dilute but also prevent tendency to draw bad air in.

Is there a drawback to positive pressure in such a home?

Would you go with 1 or 2 ? or neither? I have a few follow-on questions depending on best course. Thank you.


  1. GBA Editor
    Martin Holladay | | #1

    It would be helpful to know your climate zone or the location of your home.

    In general, a leaky log house from the 1800s does not need a mechanical ventilation system. This type of house usually has way too much air leakage.

    By far the best way to deal with high radon levels in a home are traditional methods that focus on reducing the entry of radon into the basement or crawl space. For more information on the techniques used by radon abatement contractors, see All About Radon.

    To answer some of your questions:

    1. Yes, increased mechanical ventilation can lower indoor radon levels. This is an expensive way to lower radon levels, however, because of the energy penalty associated with mechanical ventilation.

    2. The main drawback to pressurizing a house with a supply fan is the risk of introducing moisture into wall assemblies and ceiling assemblies during the winter. At normal ventilation rates (60 cfm to 120 cfm), this risk is very low, because wind and the stack effect usually overwhelm ventilation at these low rates. If you anticipate higher rates of ventilation, you might need to consider this risk.

    3. I'm not aware of any studies that compare which type of ventilation (supply ventilation, exhaust ventilation, or balanced ventilation) has more of an effect on indoor radon levels. My guess is that these three ventilation have comparable effects on radon levels, because the main effect is dilution.

    Once again, my advice is to focus on traditional methods used by radon abatement contractors. If you haven't yet consulted a radon abatement contractor, you should.

  2. charlie_sullivan | | #2

    I have heard anecdotal evidence that adjusting the balance in an HRV system can have a dramatic effect on radon--in a tight house. The effect was presumably that with the exhaust exceeding supply, radon was sucked into the house through cracks in the foundation, despite this being passive-house or near-passive house construction. With improved balancing, the radon level went down. I would expect based on logic and based on that one data point that the best to worst ventilation strategies for radon would be supply only, balanced and exhaust only. The effect would be smaller with a leaky house.

    If I knew of a product that would do this easily, I'd recommend a supply-only fan with its speed controlled based on a real-time measurement of pressure difference across the envelope, in order to prevent depressurization when your other systems draw air out, without the risks associated with maintaining positive pressure.

    But I do think that traditional radon strategies should continue to be your first priority. I'm curious what you've tried so far.

  3. GBA Editor
    Martin Holladay | | #3

    The connection you posit -- that different types of ventilation systems have different effects on indoor radon levels -- is not supported by data. If any generalization can be made, it is simply this one: increasing ventilation rates usually dilute indoor radon levels.

    For more information on this issue, see Exhaust-Only Ventilation Systems and Radon.

  4. ars777 | | #4

    Thank you each for your reply:

    - House is in southeastern WI - Zone 6, I believe?

    - Martin answered one of my main questions: is radon reduction from open windows / mechanical ventilation primarily due to dilution or due to a change in pressure dynamics.

    - I agree that traditional mitigation efforts are preferred. I didn't want the purchase to be held up by what I knew would be a complex problem. Instead, I contacted who I thought to be one of the best mitigators in our area. I explained the specific challenges that make this home complex and I was assured he had never had a problem getting a home solved nor would he ever have a problem getting a home solved. With that, I went ahead with the purchase. Once purchased and he came by to inspect, he became "unusually busy" and we never heard back. We have had ~ 4-5 other contractors visit each of whom never so much as even responded with a proposal. We finally found a contractor willing to take on the challenge and he seems quite good. Even so, I have doubts as to how resolvable some aspects of this problem will be via traditional methods.

    The specific dynamics of the home and what we have done thus far include:
    - Essentially 5 "basement systems" -- two basement foundations, two dirt crawl spaces, and a slab on grade room.
    - We have sealed the dirt crawl spaces -- one quite well, one fairly well (incredibly limited access)
    - We have sealed a large open sump pit
    - Sealed various cracks in the basement floor
    - We have an active system tied to the sump pit / drain tile and one of the basement foundations and also the two crawl spaces.
    - A separate active system for the slab on grade room. This room was seemingly the most problematic -- it had readings that would approach 20-30 if you closed the door isolating it from the rest of the home. The room was built on top of hard clay, without gravel underneath. The contractor wasn't sure how good of a draw he would be able to get from this room, but the system appears to be working quite well. Unfortunately we won't know for certain how solved this room is until weather conditions change.

    The second basement foundation he was unable to get a good draw from thus far. The two basement rooms connect to one another above ground. The readings aren't off the charts -- they seem to be in the 5-12 range typically. I do wonder how high they will spike as cold weather increases the stack effect and also as the boiler runs and draws additional air. I cannot tell if the entirety of the radon is coming from the room he couldn't get a draw from -- and migrating to the other room -- or if both are problematic. I suppose I should attempt to isolate the two rooms as best as possible to help determine.

    Another challenge with this home is that the foundation is a ~ 3 foot stone wall. A new block foundation wall was built inside of the stone wall. I assume the stone wall has numerous cracks and radon could be coming up the wall.

    I think a huge part of our problem was the slab on grade room -- hopefully solved. The crawls didn't seem to be too big of an issue and they are probably as solved as they reasonably can be. I assume a big part of our problem is the basement room he was unable to get proper draw from and/or the stone foundation walls. We are still working on this from a traditional approach.

    We also have some air sealing to do in the attic that may slightly help reduce the stack effect, but I doubt that's a fully solvable issue either.

    With regard to ventilation, I just noticed that when I open some windows, the readings seem to dramatically improve. I was trying to determine how much ventilation equates "some windows open" and how viable that would be to replicate via mechanical ventilation. I don't love the energy penalty, but I also don't mind it if it brings the problem down to manageable levels. I am more concerned about introducing a new problem -- mold, etc.

    I was also torn on the best way to go about this. Some of the thoughts I had include:
    - Positive pressure to the basement -- might that help prevent draw from stack effect / boiler / etc.
    - Positive pressure to main floor -- dilution. Perhaps dehumidified air from outside.
    - ERV on main floor perhaps with a supply to but no draw from the basement -- would this slightly pressurize the basement? Issue is, might this simultaneously depressurize the first floor (potentially causing issue with the slab on grade room)
    - Simple ERV setup with just a supply near the slab on grade room and a draw from opposite end of house to provide dilution throughout.

    Can a simple ERV setup as the last provide dilution that would benefit the home house-wide ? I assume air gets around -- or would it need to be ducted throughout. We have no existing ductwork.

    I feel that with traditional methods and ventilation we would likely be OK. I just can't determine the best approach regarding ventilation so as to solve this problem without introducing others.

    Also wondering how much benefit we might get from reducing the negative pressure systems in the house -- boiler, bath fans, dryer, wood burning stove, etc.

    Thank you again for thoughts / expertise.

  5. charlie_sullivan | | #5

    Data supporting the importance of ventilation balance on radon is here
    Only one building but clear evidence that it matters, and can even outweigh the dilution effect.

    I don't know of data that would indicate how common that is, but on the other hand, it was in a supposedly balanced system that was off a bit.

  6. GBA Editor
    Martin Holladay | | #6

    Thanks for the link. That's an interesting case, and the account is particularly detailed and well written. The homeowner consulted the right person -- Marc Rosenbaum.

    It's an anecdote, but an interesting one. I continue to stand by my assertions, however: (a) It's hard to predict how indoor radon levels will react to changes to a ventilation system, but in general, dilution usually lowers radon levels; (b) The only way to determine indoor radon levels is to measure them; (c) Tight houses as well as leaky houses can have radon problems.

    The anecdote also adds more evidence to several other conclusions: (d) It's hard to measure the very low air flow rates in ventilation ducts; (e) Paying for a Zehnder HRV is no guarantee that you'll get a good ventilation system (or even a system that uses a reasonable amount of electricity): and (f) Passivhaus homes can have radon problems.

    To read about another Passivhaus home with radon problems, see Radon and a Passive House.

  7. charlie_sullivan | | #7

    Thanks Martin. I agree 100% with your assertions a-f.

  8. ars777 | | #8

    It does not surprise me at all that an imbalanced H/ERV could make a radon problem worse. That is part of what started me down the positive pressure consideration. Even if I had a perfectly balanced ERV, I still have several negative pressures going on in the home -- stack effect, gas boiler, dryer vent, fireplace insert, bath fans, etc. I could remove some of these by going with a condensing dryer, direct vent boiler, balanced bath fans, etc.

    Even my anecdotal evidence would convince me that Martin's comment that dilution usually lowers radon levels is correct. I can open a few windows and watch levels drop. I tend to believe that exhaust-only ventilation may not solve the problem or may indeed make it worse for drawing more bad air in.

    Agreeing that traditional abatement methods are preferred, if it does come down to dilution in my case, does anyone have an opinion on which of the following you would try:

    a) ERV - does it need to be ducted or is just a simple supply and draw to the first floor sufficient? Can we trust one supply to help dilute throughout?
    b) ERV - w/ additional supply to slightly pressurize the basement. I fear if we slightly pressurize the basement we may be simultaneously slightly depressurizing the 1st floor slab on grade room.
    c) Whole home dehumidifier with a fresh air intake. - I believe these are designed to return from a central area and supply to a few areas. I have a good basement dehumidifier, but do feel that a whole home system would prove beneficial. There are units with ducted fresh air supplies.

    Given dynamics of the home, any ducting would be less than perfect. I can reasonably duct to most areas of the 1st floor. In order to duct to the 2nd floor as well, I'd have to install mechanicals and duct work in an unconditioned attic / knee walls. I'd like to eventually add A/C -- which is getting into another dilemma altogether.

    I guess it comes down to moisture concerns re: positive pressure vs. wondering how well an ERV can even function in a leaky home.

    One contractor suggested a central air system in the attic w/ an ERV tied to it. Hesitant to do this for any number of reasons -- attic is unconditioned, ducts would need to run under sloped roof to unconditioned knee walls to supply first floor, and I'm not sure how well ERVs ducted to A/C work in the first place vs dedicated ductwork or a simple one supply / one return system.

  9. GBA Editor
    Martin Holladay | | #9

    You wrote, "I tend to believe that exhaust-only ventilation may not solve the problem or may indeed make it worse for drawing more bad air in." I understand why you tend to believe that, but there is evidence that exhaust-only ventilation systems lower indoor radon levels. Here is a link to an article on the topic: Exhaust-Only Ventilation Systems and Radon.

    Whole-house dehumidifiers are notorious energy hogs, so I don't suggest you install one.

    You should go ahead and install an HRV (not an ERV) with dedicated ventilation ducts. Running ducts in an old house can be tricky, but even one supply register and one exhaust grille should provide some dilution of your radon levels. If you can manage two supply registers and two exhaust grilles, even better.

  10. charlie_sullivan | | #10

    It's hard to tell how much difference there would be in your case with the different types of ventilation systems, but all indications are that H/E-RV would be the best option. It would provide dilution with little effect on radon entry, and would lose less heat than other ventilation systems.

    For dilution purposes, first-floor only is fine. If you were stuck without a good way to put in ducts, the Lunos ductless system would be a good option to consider. It seems like ducts for the first floor is not going to be too hard, but in case you want to check out the Lunos system, see

    I don't think pressurizing the basement is a good idea. For one thing, you can't count on that to reduce radon entry--for complex reasons. And for another, that could help push air from the basement into the house, increasing radon in the rest of the house.

    You could put supply and exhaust vents from the HRV in the basement to increase the dilution there as well as dilution in the house, although if the basement is colder than the house, that would mean you'd have cooler exhaust going through the HRV and the incoming air would be warmed less. From that point of view a separate basement HRV would be better, but that's a pretty small effect.

  11. ars777 | | #11

    Thank you, Charlie. Very helpful.

    I had seen the Lunos system previously - a really neat system. Thank you for reminding me to consider. My issue with Lunos is that I'd have to go through 12" - 18" logs to install. I'm not certain, but ducting a traditional system may be easier even if a hassle.

    Speaking of brands, most of the contractors here seem to prefer Renewaire. Does anyone have experience / insight regarding RA or other brands? From my brief research it seems that RA may be one of the better brands at handling extreme temps, i.e. WI winters.

  12. ars777 | | #12


    Thank you for the reply and the link. Science debunking intuition. I suppose I could try some small tests in my home -- running the bath fans often to see if any effect on radon good or bad over time.

    Out of curiosity, if anti- whole house dehumidifier, is A/C alone the preferred method of dehumidifying?

    I have been trying to come up with a strategy on this front. I think I could handle the heat if not for the excessive humidity.

    I have read conflicting reports on the ability of Mitsubishi ductless mini splits ability to dehumidify. Installing those would be one consideration.

    The only other way to easily set up a whole home system would be as I mentioned before -- air handler in the attic with ducts to the upstairs rooms -- ducts running along roof to drop into unconditioned knee walls to supply the first floor.

    Based on other postings, I assume the above is a terrible idea? What do you suggest for dehumidifying this home?

    I thought maybe a whole home dehumidifier would allow one to run an A/C system sufficiently less often (and at a higher temperature setting) to offset or at least diminish the energy penalty.

    Thank you.

  13. GBA Editor
    Martin Holladay | | #13

    A house in Wisconsin does not ordinarily need a whole-house dehumidifier. If your house has high indoor humidity, you need to find the source of the moisture. My guess is that the moisture is coming into your home through your basement and crawl spaces.

    If you house is indeed humid, it provides more evidence that you need to double down on your efforts to install polyethylene vapor barriers over your basement floors and crawl space floors. These efforts at air-sealing and reducing vapor transmission should not only reduce radon levels; the efforts will also reduce indoor humidity levels.

    Moreover, operation of a radon mitigation fan usually dramatically reduces indoor humidity levels. So it's odd to hear that you have high indoor relative humidity (RH).

    In your climate zone, high indoor RH during the winter is addressed by operating a ventilation system.

    High indoor RH during the summer is addressed by operating an air conditioner.

    If these methods aren't working for you, and if you are operating your radon mitigation fans 24 hours a day, I'm really surprised. You need to inspect your crawl spaces carefully, and excavate soil from areas that are hard to access until you find out what's going on.

  14. ars777 | | #14

    We have done a number of things to hopefully keep humidity lower in future summers -- encapsulated the crawls, sealed the large sump crock, basement dehumidifier, and radon system.

    The RH we experience is likely self-induced. Without A/C, we try to open the house up as much as possible. Summers here are quite humid. When it gets too hot outside and we close the house up, the logs seem to do a decent job of keeping the house temperature tolerable, but the humidity is often an issue at that point.

    I guess the answer is to install A/C, but the question then becomes ducted (in the non-ideal manner described above) versus ductless....

  15. Expert Member
    Dana Dorsett | | #15

    Ducted systems have the potential to pressurize or depressurize a room (or the whole house) relative to the outdoors. possibly changing the radon dynamics, whereas ductless systems do not. Either would be able to control summertime humidity in a SE-WI climate, assuming moderate ventilation rates during high dew-point days.

    High outdoor dew points correlate highly with higher outdoor temperatures, so air conditioning isn't necessarily a bad thing, especially if you have some rooftop PV to cover that load.

  16. anonymousfarm | | #16

    My home is like this 186? 3 brick thick with radon problems in northwestern IL. I'm considering PPV HRV plus underfloor heat to minimize stirring up the radon with air from the basement and just avoiding the basement level except to gather supplies from the freezer. Any comments?

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