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Spray-foamed basement / vented crawlspace – Now have radon and IAQ concerns

Jason R | Posted in General Questions on

Hi All,

I bought a 1 story raised ranch which has a basement and crawlspaces all interconnected. House is located in zone 5, 3,600 sqft , 3 zone hydro oil, 30 years old. Hot humid summers, cold below zero winters (at times). The basement mechanical room is about 6 feet high, 800 sqft with concrete slab, connected to that is a large crawl space with 6mil poly and a 30 yr old rat slab on top about the same size. Cracks and exposed Rocks and boulders everywhere. Small basement type mechanical room next to that with two other air handlers, then another large crawl space with rat slab, sloping from 2 feet to 6 feet height. In there is the well pump mechanicals.

When we bought the house the crawl space was vented, fiberglass in the ceiling, moldy drywall on the ceiling, well pump and 3 air handlers are all broken, ruined, and rusted. We bought the house in the summer and it was totally humid and hot down there.

When we originally tested the radon the house was at 1.4 .

So after reading a ton about insulation products from this site, I took the advice of many and did 3″ of closed spray foam on all of the walls, from the sill to the floor on all walls. The Fiberglas was removed, air scrubbers came in, removed all of the drywall, and we installed roxul safe n sound for sound insulation in the mechanical room ceilings. What we didn’t do was touch the floors, and maybe that’s why I’m having the issues..

The space is now dry and warm in the winter. As a backup I bought a Santa Fe Force for the summer months to keep the humidity in check. All sounds great but now I’m having some issues…

I originally did a blower door test, I was at a 10 ACH50. Besides basement sealing , I airsealed the attic space, and did 18″ of cellulose. I wish I could do all alternatives to foam, but it seemed like the only right choice for the basement. Waiting to do the blower test but I was told I should’ve been in the 6-7 range once all done. So not exactly a tight house, but not drafty and we’re fine with it.

The problems are:

Radon was retested and now we’re at a 4-4.5 in some areas, others around a 2. I know times of the year are different with radon, and I know it can vary, but we were at a 1.4 beforehand and now more than doubled in the living spaces. I know it’s not extremely high, but I’m taking the ” any amount of radon is bad” approach, and the areas we sleep and live in mostly are 4s, and I want it under 2 like it was before we did this work.

Besides the radon, the basement space seems a little stuffy now. I really don’t want to be breathing that air upstairs in the living space if it’s not all cleaned, or filled of radon. I’m having my HVAC guy install a make up fan in a can to provide combustion air for my furnace and hot water heater because he thinks it’s too tight down there, so when the units call for heat, I’ll get some make up air from outside. He did a drafting test and the furnace seems to work, but I don’t want to run into issues of creating extra negative pressure in the basement if I can avoid it:

Here is what I think the problem is…it used to be a vented crawlspace, so radon escaped naturally outside on its own. Now it’s all sealed up down there and probably negative pressure, it’s sucking more air from soil below and the radon has no where to go but up. Maybe it will improve a little when I get some make up air for the mechanicals? But is it now a problem that my crawlspace is tightly sealed? Code said to add exhaust to the basement, but that would just increase radon levels because air will pull more from the ground because of the negative pressure. What about an ERV down there? I’m told that wouldn’t do much and just waste energy that I just spent money on sealing up. Maybe the idea is to get a radon mitigation system, but I’m afraid that it’s going to require a more expensive complex system because I have basement, then crawl, then basement , then crawl. The crawlspace areas have a ratslab but it’s all broken up and old, so maybe my best bet is to have it resealed? Or have a nice 12 or 20mil vapor barrier installed on top to seal the floors all up? I know the radon isn’t coming from the sprayfoamed walls, so it has to be coming from the ground, I just figure that I’m looking at a major project now and I just spent $10k doing the basement walls which is probably the cause of my issues now!

Code says I should either exhaust the crawlspace which I know will increase my radon, and the other option is to add fresh air vents from my air handlers above. Should I add two 50 cfm supply air inducer vents on either end of my crawlspace from the ducted systems in the living space? Essentially add a little supply from my air handlers above and run a little constant positive pressure to the crawl space? I have new ecm variable speed fans in the air handler so I can essentially provide 24/7 air, but I wouldn’t want to add returns because then I’d just be pulling all of the radon up even more.

Will just adding supply vents from above help circulate the air, balance out the pressure a little, and potentially help with radon? I basically just want to keep the crap air in the basement and crawlspace out of the living space, and deal with the radon. All mitigation guys in my area only guarantee down to 4, so I’m a little hesitant to go spend money on something that may not get me anything better. Any suggestions? I may try the approach I had above and retest, but if it’s not advised to do the suppy vents I’m a little lost on what I should do.


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  1. Jon R | | #1

    > exhaust the crawlspace which I know will increase my radon

    You can add a plastic sheet on the floor and exhaust air from under it (to the outside).

  2. Jason R | | #2

    That's doing the vapor barrier approach in the crawlspaces and then doing the pipe system underneath the barrier connected to exhaust fan to outside right? If so wouldn't I be concerned that my basement floors wouldn't be apart of this, or are you proposing that the basement floors get covered as well? this would be done by a mitigation professional?

  3. GBA Editor
    Martin Holladay | | #3

    First of all, there is no evidence that an exhaust ventilation system will increase indoor radon levels. To learn more about this issue, see Exhaust-Only Ventilation Systems and Radon.

    Nor is there any correlation between airtightness improvements and higher indoor radon levels. To learn more about this issue, see Radon and Airtightness.

    For an overview of methods used to lower indoor radon levels, see All About Radon.

    Although you have posted lots of information, the situation is not as complicated as you make it sound. Your house has high radon levels, so you need to consult a radon mitigation specialist. I imagine that the radon mitigation specialist will recommend the installation of a sub-slab depressurization system, in conjunction with repairs of the cracks in your rat slabs.

  4. Jason R | | #4

    Hi Martin,

    Thanks for the article links and information. I read the exhaust only article in depth and just have a few follow up questions. The examples in the article all talk about homes that use EOV's, but not specifically EOVs in the basement or crawl. If my basement has a negative pressure because of stack effect, and I add an EOV to try and dilute the concentration or separate the air from the living space above, how could that potentially cause issues in my living space?

    - exhaust only in basement can cause slightly negative pressure in the house, so my infiltration or make up air would be coming from above, instead of below. That is a good thing right? I want air coming from above grade intrusions instead of below where radon gas is more prevalanet?

    - causing potentially negative pressure in the basement would be pulling air from above, but as long as my mechanical make up air from the fan in a can (350cfm) outweighs the negative pressure caused by the EOV in the basement (100 cfm) then I don't have to worry about those backdrafting.

    Let's say I didn't have a radon problem and I wanted to just simply separate the air quality in the basement from the space above, shouldn't a EOV in the basement only separate these two spaces and improve IAQ above? Do I lose much energy performance as a whole if I'm just looking to clean up the air below? Don't I need some genera air circulation or ventilation in the crawlspace or basement regardless? Or if everything is fixed with radon, leaving the space below nice and sealed is best to leave as is?

    I'm meeting with a radon specialist on Monday so I'll see what he has to say.

  5. GBA Editor
    Martin Holladay | | #5

    Installing an exhaust fan in your crawl space, and installing a floor grille in the floor above the crawl space to provide makeup air, is one of two options (code-mandated options) for ventilating a sealed crawl space. For details on these requirements, see Building an Unvented Crawl Space.

    It makes sense for you to go ahead and follow the recommendations in that article (installing an exhaust fan and a makeup air grille in the floor above) first -- and then test again for radon. If the results of that radon test meet your goals, you'll save the expense of installing a radon mitigation system.

    All of that said -- it makes sense to try to seal the cracks in your rat slabs.

  6. Jason R | | #6

    Ok thanks, I agree I think the rat slab definitely needs to be addressed. As far as sealing cracks, I was thinking that it may be worth having my contractor just lay down crawl space felt that is available for any of the rough jagged areas, and then a 12mil thick vapor barrier on top of that for the entire crawl space areas. For the poured concrete basement areas just some crack sealer. Would this be overkill, or would this be the right thing to do to truly encapsulate a crawlspace the right way?

    As far as installing a floor grill to the space above..wouldn't I just be introducing the air in the basement to the living space above? Or if i'm running an exhaust fan 24/7 in the basement i'd be pulling air from the space above which would be in one direction (downward) and not freely moving back and fourth between the spaces? I'm just trying to avoid having to co-mingle the air from the two spaces as much as possible, or rather having the air from down below infiltrate up. The basement is air sealed for most of the main gaps but isn't exactly air tight as is.

    What about an exhaust only fan in the basement space with repairing the rat slabs, and then using something like a "supply air inducer" which would tap into my supply ducts above and provide 50 cfm of air per register which has a built in backflow damper to keep bad air from entering the system?

  7. GBA Editor
    Martin Holladay | | #7

    Your long-term solution is to replace all of your atmospherically vented combustion appliances (a category that includes water heaters, furnaces, and boilers) with sealed-combustion appliances.

    You say you have a "hydro" system -- since you have air handlers, that sounds like a hydro-air system. It sounds like you have a 30-year-old oil-fired boiler supplying hot water for this system. If that's true, now is a good time to replace it. Choose a heating system that has a sealed-combustion burner.

    Tight houses shouldn't have any atmospherically vented combustion appliances.

    Your Fan-in-a-can device (providing combustion air) confuses things. If you can get rid of the Fan-in-a-can, the system suggested in my crawl space article (a small exhaust fan in your crawl space, coupled with a makeup air grille in the floor above) will keep crawl space air from entering your house.

  8. Jason R | | #8

    Correct we have 3 zones in the house, all are fed/connected to an oil-fired hydro boiler, but the boiler was replaced less than 15 years ago, so i'm not ready to replace it just yet. The previous system was 3 separate furnaces. Once its ready to go I'll certainty replace it with a sealed combustion alternative.

    The fan-in-a-can device hasn't been installed yet. The house was sprayfoamed about a month ago and we haven't moved into the house yet beacuse I was hoping to get this radon issue resolved. The appliance make up air idea was recommended by my HVAC contractor, and noted as a concern by my insulation contractor, though they don't really have any way of explaining to me why I need it, or how to test if I need it. He also suggested an ERV in the main living air for good air circulation, but as I said above i'm getting about 6-7 air changes as is!

    I'm not going to replace my boiler and hot water heater now, so is there any way to test to see if I need that device for my mechanicals in the first place? He already did a draft test on the oil burner and said it was fine, i'm not sure if that has to do with make up air though.

    For the exhaust fan in the crawlspace area - the whole space is about 3,200 sqft, so I was planning to do a 100 CFM fan, is that overkill? As far as the make up grill, how far away from the exhaust fan should that be, and is it necessary if the ceiling above isn't air tight?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "Is there any way to test to see if I need that device for my mechanicals in the first place?"

    A. Yes. The test is called a "worst-case depressurization test," and that may be the test your HVAC contractor has already performed. If not, you can hire a home performance contractor or energy rater to perform the test for you. If your atmospherically vented appliances passed the worst-case depressurization test without backdrafting, then you don't need the Fan-in-a-can.

    According to the formula in the building code, a 100 cfm exhaust fan would be overkill. You only need a maximum of 64 cfm for your crawl space / basement. The formula can be found in my article, Building an Unvented Crawl Space.

  10. Jason R | | #10

    Thanks for all of your help Martin. Last few question for you..

    If I go with the exhaust fan route, is there a recommended distance to put the floor grills in from the fan? If I do the fan i'd like to keep it as far away from the mechanicl area as possible. As far as the grille between both areas, is this necessary if the ceiling itself isn't extremely air tight?

    If i'm exhausting 64 CFM out of a 3,200 sqft basement will I have enough of cracks and crevices in the ceiling already that will allow the air from above to come down? I'm assuming that doing 64CFM on a house that is a 7 ACH50 wouldn't cause any real negative pressure from above, but just enough to keep the air from below coming up?

    Thanks again

  11. GBA Editor
    Martin Holladay | | #11

    Q. "If I go with the exhaust fan route, is there a recommended distance to put the floor grille in from the fan? If I do the fan, I'd like to keep it as far away from the mechanical area as possible."

    A. That's not the way pressure dynamics works. If you depressurize the crawl space, you will be depressurizing the crawl space -- no matter where the grille is located. The answer to your question is: No, there is no such recommended distance.

    Q. "As far as the grille between both areas, is this necessary if the ceiling itself isn't extremely airtight?"

    A. As far as the code is concerned: yes, it is necessary. But if you want to experiment, be my guest.

  12. Jason R | | #12

    All I can seem to find are crawl space exhaust fans that have a minimum of 110cfm and have smart thermostats to shutoff under 40 degrees. If my basement walls are sealed don't I want this thing to run 24/7 by code? What about just a bathroom exhaust fan with a duct leading outside?

  13. Jon R | | #13

    If you want to do it optimally, measure the pressure in the area being exhausted and make sure it is always more negative than the space above it. Pressure is what matters for radon, the CFM needed is a highly variable side effect.

  14. Jason R | | #14

    Hi, how do you measure the pressure?

  15. GBA Editor
    Martin Holladay | | #15

    Here is a link to an exhaust fan with adjustable airflow:

    The way you measure pressure differences is with a manometer. If you don't own a manometer, you can hire a home performance contractor or energy rater to help you.

  16. Charlie Sullivan | | #16

    You can buy a cheap manometer for less than it would cost to hire a professional, if you are willing to take a risk on a cheap electronic one:

    Or if you are willing to use a traditional fluid-filled one:

  17. Jon R | | #17

    For this application, you want something with 1 pascal or better resolution. For example, an Infiltec DM1. Maybe there are other methods - a smoke pencil near a deliberate hole?

    It sounds like you previously had acceptable radon results without doing anything in the basement areas. That suggests that you only need to address pressure in the crawlspaces.

  18. Jason R | | #18

    Just spoke to a few radon specialist. All were independently in agreement that adding a 65-70cfm exhaust only fan and makeup air from above would be a horrible choice if i wanted to reduce radon. The increased negative pressure id be adding to the space would pull some air from above, but also from the foundation and floors itself below increasing my concentration. Back to the drawing board i suppose. Looking at HRV units now, I'm curious to see what size they would recommend. Would the 1 cfm per 50 sqft be adequate? This would at least give me some fresh air exchange in the area besides help with radon, and I have the Santa Fe dehumidifier to keep the summer dry. Anyone know if an ERV would be the better choice if I had 90 degre humid summers at times, and 10 degree cold winter days?

  19. Jon R | | #19

    Why do you care what happens to the radon level in the crawlspace - as long as all of it then flows outside?

  20. Jason R | | #20

    I don't care if it just stayed in the crawlspace, but living space above an crawl space below are just separated by floor joists and hardwood floors. If it's currently negative pressure in the basement and I add more negative pressure aren't I going to be getting higher leves above as a result of the current state? The 65cfm isn't going to suck all of the radon out if it's already a problem now is it? The guy on the HVAC forum said add positive pressure or suppl down there, I would think that would bring more upstairs though wouldn't it? I can't win. What do you do when half the people to tell you exhaust it, the other side says supply it?

  21. GBA Editor
    Martin Holladay | | #21

    Let's recap. In my first response, I advised you to contact a radon mitigation specialist and to install a radon mitigation system. You have followed half of that advice. You called up the specialists -- but you didn't tell us what they recommended. Nor did you follow through by installing a radon mitigation system.

    Instead, you just told us what they told you not to do.

    You proposed an alternative approach: instead of assuming the high expense of installing a radon mitigation system, you wanted to see what happened if you install an exhaust fan. I said, "Sure, give it a try -- see what happens." But you didn't try that approach either.

    By the way, there really isn't any evidence that an exhaust-only ventilation system raises radon levels. But I already told you that.

  22. Jason R | | #22

    I contacted 4 places, 2 told me to test long term, and said don't use an exhaust only system for the crawl. The 3rd company came out, the guy said my basement to crawl to basement to crawl set up was too complex to make a subslab system effective as is. Even if he was able to drill the holes in the crawl spaces with the rat slabs, the slabs themselves are old and unsealed so he advised me to have 2" of concrete reporured for those areas which is 10-12k from a few quotes.

    A submembrane system wouldn't work in the same areas according to him because of how the crawlspaces are all connecting between basement to crawl to basement. He mentioned an ERV but said they only get 50-70% reduction. If I did one of those I'd have a bunch of maintenance. My levels are hovering around the action limit so if I could get them 60-70% lower in the winter I'd be thrilled , so that's one option. The others don't sound promising. Waiting to now speak to another place today for someone to come out. I don't want to go cover up the rat slabs with plastic if it's not going to be effective, and i don't want to cover rat slabs with 2" of concrete if they can't properlly get under the areas. So yes, I wish I could just simply have one giant square basement slab that someone could drill into and be done with it but of course it's not that simple. So far does any of that sound promising?

    I didn't try an exhaust only approach because my HVAC guy wants $1,100 to put in a 100cfm fantech, dial it down to 65cfm, and Y duct it in the areas. Why go spend that if radon companies are telling me not to do it?

  23. GBA Editor
    Martin Holladay | | #23

    It sounds like you've researched your options. Now you need to look at your budget and make a decision. Good luck.

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