ERV in encapsulated basement/crawlspace
Hi, I have a 3,600 sqft basement/crawlspace that I recently enclosed with 3” of spray foam from the floor to the rim joists. The areas are a combo of poured concrete and rat slabs. The area is pretty complex so adding some type of poly vapor is pretty difficult. The original buildings plan shows that a poly vapor barrier was added below the rat slab and poured area. I have a variety of mechanicals down there, hyrdoair oil furnace, hot water heater, 3 air handlers. There’s a mechanically controlled fresh air supply that fires anytime the furnace or hot water heater call for heat. I’ve got a Santa Fe force system that keeps the basement below 45 in the summer, and don’t see any water issues.
Since encapsulating my radon levels went up, so I had someone install a depressurization sub slab system with two drilled entry points that have lowered the levels. This system only covers roughly half of the basement space because of the length of the run, so the guy suggested to do a wait and see approach to see if we needed another one on the other side where the bedrooms are. The levels at the time tested at .7 and .8, and I’ve done a long term that averaged at .75 . I decided to retest this winter in the same spots, and have a digital unit as well, that both are now showing in the low 2s. Most people would say that’s not very high, but my family sleeps right above this crawlspace as it’s a 1 story ranch, and i don’t like the idea of the levels being on the fringe of potentially addressing, to “acceptable levels”.
My other concern is although the basement is nice and dry year round, there’s no sort of air flow in the crawlspace, so like it or not we get sort of a basement/crawl type of stagnant smell in the living space, and notice this more in the winter with the natural negative pressure, and the air essentially mixing with our living space.
I’ve done my own research and have a few options, I haven’t been able to find an HVAC professional that seems knowledgeable enough to provide a recommendation.
1. Install another radon system on the other side of the house. Not sure if that reduces it enough to be worthwhile, doesn’t solve the smell/stagnant air issue.
2. Install an ERV like this new Panasonic intelli-balance that would alllow me to balance or create slight pressurization in basement to partially help with the stack effect in Winter as well as dilute the crawlspace or smells, odors, while providing fresh air to space to also dilute or slow down the infiltration of radon. Would have slight energy penalty, but basement is so warm in Winter and dry in summer that heat in Winter wouldn’t have to work that much to offset 50-75 cfm of ERV filtered air, or dehumidifier having to work that hard in summer.
3. Maybe there’s another easier solution to reducing radon and dealing with smells. Is venting air from crawlspace out and creating a supply only from living space above into crawlspace enough to help with that? Or would I be creating a potential issue with all of my appliances that need makeup air. Plus exhausting would create negative air pressure which would allow radon to infiltrate the slab.
Please let me know if anyone has any words of wisdom or if they suggest anything that I’m missing? I though my about an ERV in the living space above, but I have 3 separate zones and the house is 31 years old with some insulation upgrades, but definitely not tight like a new house.
Thanks
Jason
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Replies
Jason,
1. According to my training, you want to test radon in the lowest occupiable level of the house. In your case, the radon level in your crawl space is irrelevant. What you care about is the radon level in your bedroom. That's where you need to test for radon. If it's above actionable levels, everything is good. (You don't live in your crawl space.)
2. You don't want to pressurize your crawl space. (That will just send odors upstairs.) If anything, you want to depressurize your crawl space with an exhaust fan installed in your rim joist. You also need a makeup air source; this is usually done by installing a grille in the floor above the crawl space. This allows conditioned indoor air to enter the crawl, replacing the air leaving via the exhaust fan. Details here: Building an Unvented Crawl Space.
3. Running a Santa Fe dehumidifier all summer is expensive. If this was my crawl space, I'd spend a day down there with polyethylene and high-quality tape to cover the exposed floor, even if the work is difficult. If you do that, you might be able to unplug the dehumidifier.
Hi Martin,
All testing was done on the main living space in our bedrooms and living room. We don’t inhabit the crawlspace or basement level so we don’t care what those levels would be. The EPA says to remove at 4 and above, but also suggests you monitor or consider removing between 2-4, while the WHO suggests remove at 2.7 . In either case we’d like to lower as much as possible. Most homes that read in the 2s on their lowest most occupied level will have levels half of that on their second floor where people sleep, so imagine that isn’t really a concern for them.
2. If I did an exhaust only system and provided make up air from above, wouldn’t I be creating a further negative pressure condition in the space? The air would be exhausted but wouldn’t that just pull harder from the slab below? If I pull 50cfm out of the crawlspace for example is that going to dilute the radon and smells that much that it won’t be simultaneously pulling in just as much radon from below?
3. I’d have to cover 3,600 sqft of space which is half on an angle and mixed back and forth between poured concrete basement space and rat slab that’s sits on bedrock and on an angle, there would be very little chance in getting a perfect sealed space. Doesn’t just one imperfection or crack in the area defeat the purpose? I’m less concerned with running the dehumidifier, it barely puts a dent in my electric bill and the space stays ultra dry. I’m more concerned about breathing in crawlspace air and radon levels.
What’s your take on adding an ERV down there besides having to run it? If plan on using between 50-75 cfm in the space which appears to take about 50-60 watts at most which isn’t very bad at all.
Thanks
Jason
Jason,
Q. "If I did an exhaust only system and provided make up air from above, wouldn’t I be creating a further negative pressure condition in the space?"
A. If you want to avoid odors in the space above, a negative pressure is desirable.
Q. "The air would be exhausted but wouldn’t that just pull harder from the slab below?"
A. Data on this issue are inconclusive. For a thorough discussion of the issue, see this article: Exhaust-Only Ventilation Systems and Radon.
Jason,
Q. "Doesn’t just one imperfection or crack in the area [of the polyethylene] defeat the purpose?"
A. No. When it comes to stopping moisture from migrating from the soil to the air above, the purpose of the polyethylene is to perform as a vapor barrier. The effectiveness of the vapor barrier is proportional to its area, so a polyethylene vapor barrier with 5% of the area consisting of holes is still 95% effective.
When it comes to a polyethylene layer used as part of a radon mitigation system, though, the purpose of the polyethylene is a little different. In that case, it's an air barrier. Your radon mitigation contractor should have been more concerned (it seems to me) with completing this polyethylene barrier when the radon mitigation system was installed.
Hi,
This is getting slightly off topic, I’m asking about radon and air quality, I’m not having issues with humidity. I like using the dehumidifier I have in place, I don’t want to add another layer to the ground which will cost a ton in that space just to potentially replace the dehumidifier..
My question was about air quality and radon. The floor was not covered with anything for the radon system, the radon specialist used a pressure type Fan and drilled two holes into the basement section and crawlspace area. Basement is poured concrete, crawlspace with rat slab, building plans show a poly barrier underneath both areas. The radon system uses suction below the surfaces.
Do you have any clarity to the negative pressure situation I asked about by exhaust only as well as using an ERV?
Thank you
Jason,
Exhaust ventilation sometimes raises radon levels, and sometimes it lowers radon levels, and sometimes it leaves radon levels unaffected, as explained in the article I linked to. On balance, the evidence that exhaust ventilation is problematic from a radon perspective isn't very convincing.
When it comes to air quality, the exhaust fan (coupled with a makeup air grille in the floor above) will improve the air quality situation.
Ok great that makes sense to me, thank you. Maybe what I can just do then is have a simple exhaust only fan in the basement, and if the radon levels go up then I simply have the radon guy add the other system to the other side of the house and be done with it. He had originally mentioned that because of the large footprint and different surfaces (concrete poured basement next to rat slab large crawlspace, leading to another concrete poured basement space, next to another rat slab crawlspace) that I'd potentially need another system depending on results. The house is broken up into 3 zones, kitchen/dining/office, main family room/living room/playroom, and bedroom wing. He was able to to one system with 2 suction points for the 2 living zones, couldn't reach the bedroom wing because of how long the run had to be. We can always add a second system to the bedroom wing so that the entire space is addressed and pulling radon from under the slab.
How much CFM of exhaust would you recommend?
Basement/crawl footprint roughly 3,600 sqft, 4-5foot average height across the space. First floor living space roughly the same, some rooms have 20 foot ceilings, others 9ft.
If my mechanicals are on one side of the basement next to the make-up air supply vent (blows fresh air from outside next to mechanicals when they call for heat) should I have this exhaust fan on the opposite side of the space? How far away should this make-up grill be from the exhaust fan? And is it alright to do one of those transfer grills that only allows the air to go one way? I'm guessing that in the winter the stack effect would out-way this exhaust only system and push air up into the living space?
Jason,
Q. "How much CFM of exhaust would you recommend? Basement/crawl footprint roughly 3,600 sq ft."
A. Once again, I'm going to refer you to the article I mentioned in my Response #1. Here is the link again: Building an Unvented Crawl Space.
The article notes that the relevant section of the building code requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” So you need an exhaust fan rated at 72 cfm.
ok thanks, last question as it doesn't mention where in the article the actual grill should be placed in relation to where the transfer grill is. I'm guessing we need it to be a certain distance from the transfer correct? so that it doesn't just grab air directly from that area correct?
Jason,
The exhaust fan and the makeup air grille may well allow you to get rid of your dehumidifier. That would save energy.
The location of the floor grille isn't critical -- but far away from the exhaust fan is probably better than right above it.
That would be great if it did remove the need for it. Am I able to rely on the fact that the floor structure above the crawlspace isn't exactly sealed, in that nothing is air tight, so if i'm pulling 72 cfm out of the space, its going to get enough just by the imperfections above? Or do I specifically need to have a new cut out and grill large enough to match the CFM being drawn? What about locating the actual exhaust fan? I'd put it on the other side of the space from the oil furance and hot water heater, but if I put this exhaust fan on the opposite side where my two air handlers are, would that mess with those systems? I wouldn't want them to draw air or mess with the exhaust fan running 24/7, so i'm guessing I need to space these from those units as well?
thanks
Note that makeup air from above will do nothing good most of the year and when the AC is running, it will increase your AC bill. There is no "free dehumidification".
The goal is to create a negative basement pressure to stop upwards flow. A makeup air grille in the floor above will only mean you have to waste more energy to get to the same pressure. Barring a humidity problem, flow from above is a bad thing.
If basement ducts are adequately sealed, this alone should solve your problems.
Don't worry much about negative room pressure undoing the radon mitigation system - the latter creates a much more negative pressure under most of the slab.
How many CFM - whatever is needed to get a pressure less than the space above.
I'm fine to continue to use the dehumidifier, my goal isn't to reduce its use, so if that make-up air is simply allowing the crawl to exhaust and allow less air to infiltrate upstairs then I think i'd be solving my overall concern.
Martin,
By exhausting the crawlspace and adding the transfer grille from the living space above, aren't I also putting negative pressure on the living space above? Wouldn't I have the chance of pulling in smells from the chimneys into the living space or have issues if I'm usnig other appliances? or is the amount of CFM i'm exhausting not enough to be an issue? My house is 31 years old and isn't the tightest construction so its probably less of a concern compared to brand new construction.
Jason,
A fan that is rated at 72 cfm isn't moving much air -- it's about the same as (or a little more than) a typical bathroom exhaust fan. It won't depressurize your house the way a 1,200 cfm range hood fan would.
Especially if the fan is installed in a 3,600 basement / crawl space, it's not going to affect the pressure upstairs very much. Most basements have lots of cracks, so much of the makeup air will come from basement cracks, while some will come through the floor grille.
The idea is to encourage crawl space smells to travel somewhere other than the upstairs rooms.
Ok great thanks again for the advice. Will pick up one of those smaller variable speed exhaust fans and give it a try.
As far as the transfer grill..would it be alright to have one added into a built in cabinet floor? I have this wetbar that’s built right into the wall that has some cabinet doors, it would be very easy and unnoticeable if I just popped it in there. The doors aren’t sealed tight or anything, would that not be ideal? Would prefer to not have to put a whole in my floor or another hole in the wall if I could. Just to note, it is about 8 feet from the fireplace which may not be a good idea, but as you said it’s not that much force.
Jason,
Ideally, the grille should allow an airflow of 72 cfm with the fan running. I think that with the cabinet door closed, the grille you propose would have too much static pressure to work well.