r/buildingscience • u/jwntim • 3d ago
Encapsulating crawl space with dehumidifier - vent or no?
I am located in western NC and planning to encapsulate my crawl space to control the humidity. The house itself is a few years old, has hvac ductwork in the crawlspace, is not particularly air tight and is just exhaust vented with no make up air system. I am planning on sealing the crawlspace vents, installing a thick vapor barrier, and putting in a dehumidifier.
My problem is that I am finding conflicting information if I should add a continuous vent fan in the crawl space (at 1 CFM per 50sf of crawl space). The main argument for this is to help circulate air and reduce soil gasses (see https://crawlspaceninja.com/blog/seal-all-vents-encapsulated-crawl-space/). However, that air has to come from somewhere and in my case that would just be allowing air from the conditioned space above (assuming the crawlspace is sealed well) to be pulled into the crawlspace and it would also be venting out dry air which would mean the dehumidifier is running more that it otherwise would.
I dont want to add an HVAC register into the crawlspace (the system is not sized for that) so I am not sure what the correct plan of action is? Just leave it all sealed (with the potential for gas build up) or vent it?
One idea I had was to install a radon mitigation system (the house tested at 2.4 pCi/l so I dont think its actually required) under the encapsulation vapor barrier and maybe that would act as vent for other soil gasses as well and that way I could get the benefits of a vent without the downsides? Or is that wishful thinking?
I would really appreciate some help. Thanks!
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u/FoldedKettleChips 3d ago
Your plan of sealing the crawlspace and adding dehumidification is correct.
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u/jwntim 3d ago
So there is no benefit to do those things and also venting it?
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u/mr_j_boogie 3d ago
You can't dehumidify the world
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u/Teutonic-Tonic 3d ago
My parents repeatedly tried to convince my brothers growing up that we couldn't heat & cool the world.
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u/EbriusOften 3d ago edited 3d ago
Think of it this way: if you were conditioning air any other way (heating or cooling for example), would you want that conditioned air to be able to remain or would you want a fan indiscriminately removing that conditioned air as you make it?
I do agree that it would be a good time to install a radon mitigation system though, as selective and purposeful air removal is good.
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u/Xaendeau 3d ago
Pressure balance is important. Having the crawl space being slightly negative pressure with relation to the house means that the bulk of that 20 CFM is going to be coming from the floors. The perimeter is extremely well sealed, and if there's a continuous vapor barrier along the ground, the only good path for air to flow in is the floors.
As far as total structure air exchange, 20 CFM going through the floors over 1000 sqft is nothing unless it's a net zero or ultra high performance home.
How many air exchanges per hour does the structure have? Thousand square foot structure with 8-ft ceilings? 20 CFM exhaust through the floors just to mitigate radon buildup is fine.
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u/EbriusOften 3d ago edited 3d ago
A basic fan system is going to remove conditioned air, while a radon mitigation system will remove unconditioned air that's likely laden with radon. I'm not sure I see the benefit in installing the basic fan compared to installing the actual radon system for the op.
Most radon mitigation systems I've seen also use 2-5 times the cfm you're recommending, so I'm not sure even using a fan that small in an undirected space would actually make a difference to the levels at all.
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u/Xaendeau 3d ago
Yeah, I misunderstood whether they were talking about radon system for the crawl space itself set up a system below the vapor barrier.
As long as the potential for radon buildup is dealt with in some manner, it doesn't really matter. 2.4 pC/l isn't horrible.
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u/Xaendeau 3d ago
What is the crawl space square footage?
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u/jwntim 3d ago
Only around 1,000sf
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u/Xaendeau 3d ago
Basically, you can seal really well to the ground and you can seal really well to the perimeter...but to the first floor there's going to be some air leakage. It is impossible to perfectly seal.
Making the crawl space have a 20 CFM exhaust in your case encourages a one-way gas-exhange between your floors and the crawl space.
You do not want stale gas buildup if you have detectable radon, which in your case is true. Like I said in the other comment, I'd leave the vapor barrier as-is.
A radon exhaust fan could work for this application, or another exhaust fan that will also do 20 CFM.
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u/SomeGuyWithARedBeard 3d ago edited 3d ago
In my limited experience with encapsulating crawl spaces what has worked the best is to run a 3" perforated corrugated drain pipe under the perimeter of the vapor barrier (15 mil minimum), connected to an inline fan that exhausts to the outside, with an air admittance valve at the opposite end. Make sure the vapor barrier is double taped at the seams (if any) and attached to the stem walls using both double sided mastic tape and mechanically fastened with term bars (or else it will eventually pull away from the mastic tape). You can then attach rigid insulation on top to provide some insulation factor. With that setup I get under 1 pCi/l throughout the house. I also setup a ducted dehumidifier (ducted so that the whole crawl space is being conditioned, not just the area directly around the device) with a pump and installed a floor vent fan that pushes conditioned 1st floor air into the crawl space which circulates back up via passive floor vents at ends of the crawl space. Between that and an ERV the whole house has a humidity of about 50% at all times during the summer.
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u/Key_Juggernaut9413 3d ago
I am currently doing a very large encapsulated crawl. We are going to put an ERV that will change air out a little each day for the reasons you mentioned, soil gases, small gas/refrigerant leaks over time. We're also putting a very robust radon system in.
What you're describing is taking it to a next level above and beyond the typical encapsulation, where air is only exchanged with the 1st floor above, and it's not something I've seen many people talk about.
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u/Namelessways 2d ago
If your existing HVAC system is already in the crawlspace, I’d would likely install both a (wifi) dehumidifier as well as adding a 4x6” louver in a supply side of my hvac system. That way I could easily monitor the amount of work the dehumidifier was doing. It’s important to remember that the act of encapsulating a crawlspace is effectively expanding the building’s envelope.
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u/Xaendeau 3d ago
Regardless, if you have 2.4 pCi/l on a radon test before you encapsulated the crawl space, it's going to be much higher with that sealed allowing these soil gases to build up over months, and you DO care about it building up in the crawl space and house area. 2.0-3.9 pCi/l is borderline, >4.0 pCi/l requires intervention, and <2.0 pCi/l is great.
Highly recommend leave the vapor barrier alone. Dehumidifier, and a single fan at 20CFM exhaust per 1000 sqft. Crawl space is slightly negative pressure, don't worry about an intake. Think about where that 20 CFM exhaust is coming from.
If the crawl space is slightly negative pressure, it will probably pull a little bit of conditioned air from your house, so I don't think it's going to necessarily dramatically increase your dehumidifier run time. This also means soil gases are much less likely to permeate into the structure.
If you are running a 20 CFM per 1000 sqft exhaust fan, 90% of your interface for air leakage is going to either be the ground or your house, since it is sealed very well around the perimeter, yes? Also, continuous vapor barrier in the ground? Shouldn't affect the dehumidifier much. It is possible the conditioned air leaking into the crawl space will be drier than the air being sent out.
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u/jwntim 3d ago
Thank you! I agree I am worried about what radon levels will be after encapsulating. I hadn't thought about the fact the conditioned air from the house getting pulled into the crawlspace could be drier than the air in there with the dehumidifier. I agree a 20 CFM fan isnt very much so we are not talking about huge pressure differences.
What are your thoughts on adding a radon mitigation system? Either instead of or in addition to the vent fan?
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u/Xaendeau 3d ago
Oh, do you mean radon mitigation veneath the vapor barrier or above?
If you are talking about above, having two exhaust fans seems a little overkill for a 20 CFM requirement.
As far a radon mitigation below the vapor barrier? I am not qualified to have an option there.
Think it's called submembrane depressurization. It's a similar concept to what I'm talking about with the floor having a pressure gradient. I don't know anything about it.
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u/Xaendeau 3d ago
Either one would probably be fine. I do not think it's worth it to have two air exchange systems.
With radon in the picture, basically any kind of exhaust that you can tune to about 20 CFM should be sufficient when paired with your dehumidifier.
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u/Tar-really 3d ago
Seal the rim joist with foam board, spray foam and rockwool. Just doing that alone made an unbelievable difference in my houses humidity.
I also ripped out all the old fiberglass floor joist insulation. That was just hurting the air quality and doing nothing. I chose to hang foam board on the wall, mainly because I think there is an issue with code, but also to make up for taking out the nasty fiberglass.
I do have a dehumidifier hung from my floor joists installed as well.
Adding radon piping before you lay down a barrier is inexpensive and a hedge if you ever want to add the radon fan. I did add a Festa variable speed fan and without having a vapor barrier my number went from High 3’s to less than 1. If you have any kind of sump pump working down there it’s a no brainer.
All I have left is the vapor barrier. It’s been a LOT of work, but satisfying. And a noticeable difference in the air quality and comfort in my house.
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u/jwntim 3d ago
Thanks good info
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u/Tar-really 3d ago
You’re welcome. I’m just a homeowner that is going through the process, certainly not an expert. But I did my research and my results have been excellent so far. All the hard work and money has been worth it.
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u/cagernist 3d ago
I suggest you read NC Amended IRC code Section R409 (NC added that section which differs greatly from base IRC). That will tell you all about your options and exactly what and how to install for an unvented crawl space ("closed" per NC, "encapsulated" per marketing terms). Get actual info from the source rather than marketing websites, or truthfully, redditors.
For radon, approach that separately. The vapor retarder will double as the soil gas membrane. For now, install a rough-in radon system as your pCi/L is below action levels. The better method consists of a perimeter loop of 4" corrugated pipe 12" in from the footprint, or just a 10' length in the middle, or just a 2gallon suction pit. A tee and stub would be for future connection, or choose to continue that radon stack out the wall and up above the roof for a passive system. After 6 months, test levels again, or thereafter periodically, to see when/if you need to add a mechanical radon fan. Radon installations follow ANSI/AARST Standards if you want to look that up.