GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Using a Sealed Crawlspace for Return/Supply Air Ductwork

tvrgeek | Posted in Green Building Techniques on

Sealed crawlspace, Air handler in the space.  Why don’t I use the crawlspace as a plenum, either return or supply air?  Seems it woudl be more efficient, options for easier placement of either supply or return vents depending on which to remove.  I am in central NC, so loads are not too terrible.

Note: the Dehumidifier provided by the basement company has failed twice in three years. Including the crawlspace would eliminate both the failures as well as the additional energy cost to run it.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. walta100 | | #1

    Because most people do not want to fully condition the crawlspace and make it be the same temperature as the house. The leaks in the uninsulated ductwork is often enough to keep the crawlspace dry enough not to mold.


  2. tvrgeek | | #2

    GOOGLING all morning.
    My thoughts, as I have a sealed insulated crawl space, it is no different from a basement. It already tracks the house temp. I do have to run an auxiliary de-humidifier in the summer. (which seem to last about a year each) I think the job the company did sealing the sheets to the piers is leaky and will be addressing their bad , very expensive work. I do not believe we have a radon issue here, at least testing suggests not.

    But, M1601.4 says no as it precludes a gas line, waste cleanout and requires all non-combustible materials. This makes NO sense to me as in the typical occupied basement, you have gas lines and cleanouts with far more combustibles. A gas line and waste line had better not leak! Space from furnace outlet to structure makes sense, but nothing else does. If used as return, then the temperature is ambient. I am going to send an inquiry to the county engineer.

    However, I found in the same article as the code comments, descriptions of using the space as either supply or return, included by just circulation fan, or as part of the powered whole house exhaust system. If used as a return, seems it would be easy to add many more return grills for better, more even flow. My house is circa '92, so only two returns. Not one in every bedroom. I also have insulated flex supply ducts which looking in, seem to be restrictive and dust traps. One argument would be plenum supply to eliminate the bad duct work, the other the ease of adding sufficient return ducts.

    I did find for sure, the contractor should have provided a dedicated combustion supply to the furnace even though it is a 85%. ( Just needs the pipe extended to the outside) Put in a tankless DHW, so no longer is it a risk. When it passed inspection, it had a conventional gas DHW using crawl space combustion air. Should never have passed!

    I have not found anyone to do a pressure door test on this house. Back in MD we got them done for free, but not here.

  3. Expert Member
    ARMANDO COBO | | #4

    You need a duct system to deliver fresh, clean, filter and conditioned air to all rooms, regardless of conditioned attics, crawl spaces or basements; otherwise humidity, radon, gases, critters, etc. could get delivered in the livable space... oh yeah, it's code too!

  4. Jon_R | | #5

    Is there really code that says one must have ducts?

    Using the word "plenum" may trigger some issues. But afaik, there is nothing that prevents one from overheating a room and providing openings that allow airflow to other rooms. If said room is the crawlspace or basement, then it also creates a more comfortable radiant floor.

    Similar for putting a central return in one room.

  5. Expert Member
    ARMANDO COBO | | #6

    Building codes restrict the use of cavity spaces as supply ducts, but duct design is covered in M1601 section of the code; and there is a difference from cavities and open pace. I'm pretty sure the Fire codes will have a section too.

  6. tvrgeek | | #7

    Yes, with further reading, the word "plenum" is the only issue. So as long as I have mechanicals and or storage in it, it is "occupied" space and the silly requirements do not apply. It seems the code is more to prevent running a gas line within a duct, which I have seen many times!

    So as far as drops, it would be more efficient to use the CS as supply, but I think I prefer to feed through the filter so making it the return is a better choice. I can then easily add several more return ducts that would even out the overall air flow. If I add more returns, I can lower the he height of the two I have now recovering space badly needed for pantry ( my beer refrig actually) . Optimum would be smooth long elbow supply ductwork as a final optimum. Besides flow, the can be cleaned. Even with the MERV 15 filter in the air handler, dust gets through. I think 2X cross-section of the air handler intake is probably a good target.

    I would like to hear any experiences with downside. Talking to myself, this looks quite reasonable, but often there are catch-22's that are not obvious. Of course, code is supposed to protect us from that.

    I think, but could be mistaken, current code suggests a return in every bedroom. Enough return allows one to use the supply grills to modulate room by room. One more oddity is my TV/cable etc is in a closet and it gets that room pretty warm. I could easily add a thermostatically controlled return "booster"

    1. Jon_R | | #10

      A little known fact is that in low heating load homes, a radiant floor is reasonably self zoning. For example, a radiant floor at 75F automatically stops heating a room if solar gain heats the room to 75F. This is pertinent in a radiant crawlspace case where everything is in the same zone.

      1. tvrgeek | | #11

        All depends on where the thermostat is mounted. Same with any other technology, but that has little to do with using the crawl space for return air.

        FWIW, I was thinking about radiant heat in the master bath as I am about to re-do it, but I think an overhead IR spot is better. I have had them before and really nice. With a sealed CS, the floor is not that chilly to start with. If we set the house cooler at night, maybe it would be a bigger deal. I can see some of those folks who run 60 degrees at night might want the bath floor on a timer to be a tad warmer on bare toes. :)

  7. tvrgeek | | #8

    That confirms as M1601 is the section mentioned in the paper I referenced.

  8. tvrgeek | | #9

    Reading a bit more, I can see one issue. That is with light AC loads, probably not enough dehumidification. ( a problem as my Sani-dri has failed twice in three years) I talked to a few local HVAC contractors who all screamed not to do it, but all got very squirrely when asked why and had no answer. Careful inspection suggests the huge amount of money I spent on sealing the CS was not well spent. I need to mitigate a lot of their work. It is not as sealed as it should be. Tape failures, gaps around piers etc.

  9. tvrgeek | | #12

    More reading and a bit of contemplation on that location where ideas come from. You know, a poise like 'The Thinker"

    I think the "perfect" solution is an air to air exchanger intake for the exhaust from the CS, fresh into the return duct with an additional "leak" from house to CS. For some reason, almost every HVAC contractor or supply house gives me a dumb look when I mention one. They must be living in the last century. I will be searching this forum for more information on them.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |