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Crawlspace Encapsulation Before or After Air-Sealing Returns

brian_wiley | Posted in General Questions on

We live in 1285 sq. ft. house built in 1958 in Boise, Idaho which is zone 4B. The crawlspace is ventilated to the outside, and has insulation on the foundation walls, but no vapor barrier; it’s just a dry, dirt floor with no ongoing moisture issues. The underside of the living space (subfloor/joists) has no insulation, for what it’s worth.

Several of the mechanicals are in the crawlspace, including the air handler, which is the beginning of the questions I have. I’m sure we’re suffering from a ton of air infiltration. While the ducts have mastic around all the joints (no insulation, though), the air returns that run through the crawlspace have several visible gaps/no mastic at the joints. I’ve held a smoke stick near them when the system is running and can see it pull it right in.

My knee jerk reaction is to seal the air returns first, and then encapsulate the crawlspace in a 4–6 months, but in thinking about it in longer, does it make more sense to encapsulate first and then turn my attention to the return air? My thinking is that if the crawlspace is totally encapsulated, any infiltration of the return air will be coming from the conditioned space, and therefore less problematic. Or, is there something I’m missing about bringing the entire air handling system inside the conditioned envelope that still requires all the ducts/returns to be sealed and insulated?

Thanks very much in advance for your thoughts!

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  1. walta100 | | #1

    I think you are looking at this the wrong way. You have 2 choices 1 is a vented crawlspace the other is a conditioned crawlspace. It you try to be some ware in between you are asking for a big moldy mess.

    Now that you have sealed the crawlspace you need to condition it by connect it to the living space with supply and return ducts. Sealing the ducts is not a good idea.


  2. user-2642926 | | #2

    Walta, with tons of respect to you and your input, I think you may have oversimplified his situation.

    It appears to me he currently has a vented crawlspace with perimeter insulation, no under-floor insulation, and the existing ducts are poorly sealed at best. I don't know if "ventilated to the outside" means there is some sort of powered vent duct installed or if the passive vents are still open, but I read it as the latter. If that's correct he effectively he has no floor insulation and is running his leaky ducts through the vented crawlspace.

    It also appears he's asking which direction to go first: seal the crawlspace or seal the vents. I'm just a layperson here, but it seems to me he should take some combination of these actions:

    1. Seal the crawlspace vents, complete the insulation for the perimeter walls (that may just be filling in the insulation where the open vents are now), install some sort of moisture / radon barrier and deal with radon venting if needed.
    2. Seal the crawlspace ducts.
    3. If the the crawlspace remains vented, insulate the ducts.

    It seems to me the lowest expense is likely having the ducts sealed and insulated, and coming back later to seal the crawlspace properly. But doing it that way just wastes the money of the duct insulation since it seems useless if the ducts are routed through a properly sealed crawlspace, while doing the ducts and not the crawlspace sealing means he's losing valuable heat through an uninsulated floor.

    Apologies to Walta and Brian if I misinterpreted anything. Brian, could you clarify for us?


  3. brian_wiley | | #3

    Walta; thanks for your input. Once things are totally encapsulated I plan on conditioning it with a supply/return duct.

    Mike, you hit the nail on the head. My situation is just like you described, and to clarify, the crawlspace is passively vented and the vents are open. Mine don't have the ability to close as some do.

    The cold floors and energy penalties are bad, but my primary concern is all the dirty air that I'm getting through the unsealed return air ducting.

    I'll be doing all the work myself (a bid for crawlspace encapsulation came in at $12k!) either way, but am wondering if I'm completely wasting my time in sealing the return air ducts if they'll be run through a completely encapsulated space?

  4. user-2642926 | | #4

    I can't imagine sealing them is a waste of effort or budget, but if you're going to fully enclose the crawlspace then they'll be within the conditioned space, so insulating would be a waste. (In the grand scheme of things, that doesn't sound like a significant expense though.)

    It seems to me the most complex effort is truly sealing the crawlspace. You seem to be in an orange zone here: I personally wouldn't just ignore that and hope for the best.

    A quick search on this site for "radon crawlspace" yields several good articles on the subject. If you're up for tackling the encapsulation issues, it seems like that significantly mitigates the problems created by leaky and uninsulated ducts.

  5. brian_wiley | | #5

    Thanks, Mike. It sounds like it may be worth sealing things up now and then tackling the encapsulation during the summer when I can cut long runs of the liner in my driveway. I can survive one more winter with cold floors.

    As for the radon, that is something I definitely plan to test for with both a short and long-term test. I've been told that the levels in Boise can swing wildly within the same neighborhood. It definitely complicates things in terms of truly sealing things, but the upside is it gives me something to shoot for in terms of noble-gas-tight :)

  6. nshirai | | #6

    Your crawlspace doesn't know who it is yet. But it yearns to be a closed crawlspace. Having perimeter insulation is fantastic. Having open vents and exposed dirt is bad. Seal the vents and put down a sealed vapor barrier.

    Next turn your attention to the duct system. You have some decisions to make.
    1. Seal, or don't seal? Some have mentioned that once the crawlspace is enclosed duct leakage doesn't matter. I think it does. A sealed duct system delivers a predictable and measurable amount of air to each room that it serves. A leaky system does not. Sealed ductwork aids greatly in controlling and balancing air delivery. Sure, it matters more if the ducts are outside the envelope. But putting them inside does not eliminate the value of a sealed system.
    2. Insulate, or don't? Wait and see how things go once you get the perimeter sealed up. If your energy bills and comfort are satisfactory to you, it may not be necessary to spend the money on duct insulation. If you feel like something is still amiss, don't do anything until you have a HERS or BPI certified home performance specialist analyze your situation through testing.
    3. Give the crawlspace HVAC supply, or supply and return, or a dehumidifier, or some combination of all that, or not? For my money I tend not to prefer crawlspace air to mingle with living space air, even if the crawlspace is conditioned. I'd put a dehumidifier in the crawlspace with a permanent drain to the outdoors and let the HVAC system continue to serve the house only. If you do feel like letting the HVAC system serve the crawlspace as well, make sure you have someone who understands pressure and balancing handle the strategy there. Disclaimer: I say this as someone living in the mixed-humid south where dehumidification is always needed and where even the best conditioned/encapsulated crawlspace is always a little musty. Not sure what your specific climate is like. YMMV.
    4. What about radon? Keep an eye on it with testing. Continuous radon monitors can be obtained for around $200. If you have an issue, depressurize the crawlspace by introducing a pipe with inline fan to put the underside of the vapor barrier under suction and blow it to the outdoors. And/or consider keeping the crawlspace under constant negative pressure with respect to the house.

    Most of your gains will come from sealing those vents and putting the sealed vapor barrier down. Everything beyond that is incremental improvement. Therefore I'd start with those items as first priority (with radon monitoring going hand-in-hand), then move to the duct system from there.

    Another vented crawlspace bites the dust. You'll never regret it.

  7. walta100 | | #7

    When I see the word encapsulate it pushes my button. This is the language of the spray foam hucksters and we should avoid using whenever possible.

    The word implies that the space will be self contained and disconnected from the living space and that never happens.

    Most crawlspaces will be damp and the moisture must be removed to prevent rot and mold. Vent or condition if not the moisture will collect and make a big mess. There is no free lunch where the crawlspace will magically stay warm and dry without spending energy to keep it that way.

    It is painful to put the resister in parts of the house that are not living space. Many times the old ducts will leak enough that you can get by just the leaks. The other option is a dehumidifier that will keep the mold at bay mostly by heating up the space.


  8. brian_wiley | | #8

    Thanks for everyone’s input. I appreciate the perspective.

    I’m going to be encapsulating the crawlspace and will report back with the methods and products used in case it helps anyone else out in the future.

  9. brian_wiley | | #9

    I decided that I'd return to this to give a bit of an update. Hopefully it helps out should anyone be in a similar situation.

    The first thing I did was I tested for radon, and it was very, very low in the crawlspace and non-existent in the living space, so a mitigation system didn't seem necessary.

    I sealed the crawlspace using Stegocrawl 20-mil liner, and it is now conditioned. The liner runs up the walls to the mudsill, and is attached to the wall with Stego's butyl tape and Ramset mechanical fasteners every 24". I had originally purchased the Stego Term bar, but it was totally unnecessary in my case.

    After the liner was up, I put r-15 EPS on the walls around the perimeter. I would have gone with polyiso if it had been available, but it was surprisingly difficult to find. Only the orange big box had it (two other local building suppliers didn't carry it, and a third local supplier told me they had it only to drive down and discover that they only had XPS. But I digress...) but in 1" thickness. The EPS was made to order, so I was able to get it in 2' widths to save cutting down 4x8 sheets.

    For what it's worth, I likely wouldn't have run the liner up the walls if I had known I was going to use EPS. Because I anticpated using polyiso, I wanted the additional moisture protection (which was also likely overkill) of running the Stegocrawl up the wall. I would now consider using a foil-faced EPS with taped seams on the walls as the vapor barrier and just attaching the Stegocrawl a few inches up the wall.

    The Stegocrawl liner is compatible with Great Simple foam products, as well as OSI sealants like 3000 and Quad, so I used a combination of them to seal the EPS to the liner near the mudsill and at the base.

    I have a post/pier foundation in the field of my crawlspace—32 in total—and detailing them was laborious. I didn't have the capacity to jack the posts up, so I made 'boots' in advance to slip around the circular concrete footings. In retrospect, I probably should have tried harder to come up with a system to raise the posts enough to slip a piece underneath. All in all, it seems to have worked even if it wasn't super efficient.

    Finally, I can't say enough about the benefits. The energy savings weren't huge, but the comfort gains were. Our house no longer smells musty, and the floors were also much warmer in the winter, with the temperature now within a couple degrees of the other surface temperatures in the living space. The crawlspace itself now tracks within about 8 degrees of the living space, and is a similar relative humidity. The other noticeable difference is the amount of dust. Our filters would last just a couple of weeks before they were a dark grey; now they routinely last months before they need changing. I have to believe that's had an huge impact on our health, particularly this year when we're at home so much of the time.

    1. nshirai | | #10

      Glad to hear this update! A big part of your original question had to do with the duct system - what strategy did you settle on for the mechanical system -- vis a vis conditioning the space, dehumidification, a combo, nothing at all?

      1. brian_wiley | | #12

        That's a good question. I've left the supply side alone after consulting with a mechanical engineer. I have plans to insulate those ducts, but haven't done it yet. I've also left the return side alone because it will need a bit of rework to accommodate a new air handler and air-source heat pump that I'll be installing in the next few months. After that's in place, I'll tighten up any of the residual pieces of return side.

        As for the actual strategy of conditioning the crawlspace, I opted for a bit of a 'wait and see' approach. There were a couple of things I was hoping to understand a bit better before I chose an approach; the first is that I have a heat pump water heater in the space, and I was curious how that would affect both the temperature and humidity. The other factor was to re-test for radon (more on that in response to Steve's question below) to see if coupling or exhausting the space would be best.

        The HPWH seems to cool the space between 7–10 degrees, and the RH tracks almost identically. Right now it is 65.7°F/39.1% RH in the living space, and 56.5°F/37.7% RH in the crawlspace. Those are pretty close to the year long averages as well, although over that time span the temperature differential tracks more towards 5°F. Because of that small differential, I'm more likely to couple the spaces through a supply and return between the spaces, assuming there are no increases in radon that are discovered. If I do discover an increase in radon, I'll likely pivot to a continuously operated exhaust ventilation.

    2. stevedavis | | #11

      That all sounds great! Nice to hear a follow-up. Having done a similar project, I feel your pain on the custom taping at the post and piers. It is laborious and tedious.

      It sounds like you tested for radon before encapsulation? Personally, I would test again after encapsulation. What radon there is will likely be far worse. Yes - it is now under plastic but let's be real, no tape job is perfect. And now with the vents are sealed, the air just sits down there allowing the radon to accumulate. Furthermore, any leaks in your ductwork are likely the easiest pathway for for radon to escape which means going into your house rather than out vents.

      I sealed my crawlspace then tested for radon which there was. Then I ended up putting in an exhaust fan with a speed controller. Using an active radon sensor, I was able to "fine-tune" the speed controller to exhaust the minimum amount of air while keeping my radon levels very low.

      1. brian_wiley | | #13

        That's a good point about sealing; and my tape job is far from perfect for as hard as I tried. I tested for Radon before the project (a rapid test), and am in the middle of a long-term test in crawlspace right now. I had intended on testing around October of last year thinking that the house would naturally be more closed up due to the cooling weather, but with Covid got a late start and didn't begin testing until January. I'm hoping to send it in to the lab in a few weeks.

        In the event that I have increased levels of radon, I'll move to a ventilation strategy with an exhaust fan. I bought a Panasonic WhisperValue DC Ventilation Fan (FV-0510VS1 for anyone that wants specs) along with a timer that will cycle it on/off every 20 minutes to meet the CFM needs per the space requirements that are outlined in the code.

        While I'm sure the timer will work well enough, I'm super interested in your system that's tied to an active monitor. Can you share more about how you set that?

        1. stevedavis | | #14

          Hi Brian,

          Sure! Though its not really that sophisticated. The operation is manual.

          I ended up buying a radon fan Fantech RN-1 and then wired a Fantech "Varispeed" WC 15 Speed Controller to it. I wasn't sure if I was going to do submembrane exhaust or just continually exhaust the crawlspace itself. The fan and controller combo allowed me to do either approach. Simply exhausting the crawlspace per code only required 33 CFM, which the controller allows me to throttle down to. In the end, I just ended up exhausting the whole crawlspace rather than doing submembrane. It took care of the radon problem and I found it only needed a very low CFM to do so. Much easier than running a perforated pipe everywhere.

          I use an AirThings Wave Plus to actively monitor the radon levels. I can just look at my phone to see the radon levels and it can also alert you if they go over a certain level. There isn't an automatic connection between the radon sensor and the fan. It just allows me to fiddle with it to get the radon down to a reasonable level without exhausting a ton of air seemingly defeating the idea of a closed crawlspace. (I've been averaging about 0.5 pCi/L which seems fine).

          My summers are usually somewhat humid with a dewpoint of 60F. (enough the dehumidifier runs often). Occasionally we get dry air and a lower dewpoint. I'm yet to test this, but in theory, I could manually crank up the exhaust during these times to bring in some dry air and give the dehumidifier a rest. We'll see if this works but it should theoretically. I had plans to add a supply fan that would automatically turn on if the outside dewpoint was lower than the crawlspace but I've been unable to find a sensor to do this other than Atmox (very pricey) or building my own with an Arduino. Maybe someday but too much else to work on in the meantime.

          1. brian_wiley | | #15

            Thanks for that, Steve. Those monitors look pretty great; I've been on the lookout for a IAQ monitor and hadn't considered one that does radon as well. This is probably a dumb question, but is your monitor located in the crawlspace or the living space?

  10. stevedavis | | #16

    Hi Brian - the IAQ sensor is located inside the house. That's all I really care about since that's where I breathe :-). I did spend sometime testing out different rooms in the house and found the radon levels all to be about the same. It is a single-level house though so no big suprises there.

    Before I bought an active sensor, I did do a one-time test. When I did this, mainly out of curiousity, I did two tests - one in the house and one in the crawlspace. I think I basically wanted to confirm that if there was radon, that is was coming from the crawlspace. I found that the radon level was about 3 times as high in the crawlspace than in the house. So the radon was coming from the crawlspace. Looking back on it, this was a no-brainer but it helped me to make sense of things.

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