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Community and Q&A

Why is there condensation on the return ducts and mold in the attic as well 60% humidity throughout the home?

emsent04 | Posted in GBA Pro Help on

This home has since been remodeled, including all new metal duct work and insulation. The issue that we need to resolve is the high humidity in the house at about 60-65% even though the unit is running efficiently. There is quite a bit of condensation on the return ducts in the attic as well. We have had two reputable Heating and Cooling Contractors come to assess the situation to no avail. The owner attempted to contact the original Mechanical contractor, but has not been able to speak to anyone yet. The owner is 9 months pregnant and afraid the mold will be extremely harmful to a newborn. I am now are reaching out to you for your assessment of what you believe the problem may be. Additional information that may be germane to your analysis includes:

· 1959 Ranch style home in Tallahassee, FL
· 1746 SF
· R19 insulation in the rafters
· R13 insulation in the walls
· Metal roof with styrofoam underneath R15-20
· Soffit vents
· Crawl space has 20 mil plastic sheathing
· 3/4” plywood on subfloor
· Walls are gypsum plaster board
· New double pane Anderson windows throughout
. Goodman 3.5 Ton Condenser and Coil/ Gas Furnace

Unfortunately, this issue is causing major delays in the project and we would like to get humidity levels under control as soon as possible.

Additionally Insulation was installed to the complete rafter. I have attached a photo as well.

Thanks again and look forward to speaking with someone regarding the matter soon.

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  1. GBA Editor
    Martin Holladay | | #1

    Condensation occurs when humid air contacts cold surfaces.

    We need more information:

    1. Is this a vented unconditioned attic or an unvented conditioned attic?

    2. Are the ducts insulated?

  2. charlie_sullivan | | #2

    One problem is the humidity control on the main space. The other is condensation and mold in the attic.

    The weak humidity control could be beacuse of an oversized a/c unit that doesn't run enough to control humidity well. Or it could be that excessive infiltration, perhaps driven by duct leakage, is leading to a high latent load that the a/c unit has to deal ith.

    The ducts in an unconditioned attic (if I understand correctly) can be expected to have condensation on them. And it's no surprise that that leads to mold. Sealing the ducts and insulating them could mitigate the problem, but it would be better to change them to another location, or change to a ductless minisplit system that avoids the need for ducts, or fully seal and insulate the attic.

  3. emsent04 | | #3


    Only the return ducts are in the attic. I'm not use to seeing return ducts sweating like this.

  4. emsent04 | | #4

    Yes the ducts are insulated and were wrapped pretty good.

    The house originally was vented soffit. They covered the original wood soffit with vinyl and I pulled it back to see what was going on underneath. I can send pictures if you like.

    Also it has a metal roof that doesn't seem to have the ridge caps cut into rafters.

  5. GBA Editor
    Martin Holladay | | #5

    If I understand you correctly, the attic is unconditioned. That means that humid outdoor air can get into the attic. (The attic isn't air conditioned in the summer or heated in the winter.)

    In Florida, that means that your attic is filled with hot, humid air. The humidity in that air will condense on any cold surfaces, including air conditioning ducts.

    The best solution to this problem is to seal up all of the ventilation openings into the attic and to install insulation along the roofline. This creates an unvented, conditioned attic.

    That's expensive, however. If you don't want to do that, the next best approach is to install thicker insulation around your ductwork. Use special duct insulation with a vinyl jacket.

    For more information, see Creating a Conditioned Attic.

  6. emsent04 | | #6

    I have added a picture of the unconditioned space.

  7. emsent04 | | #7

    Hello Martin,

    I'm a bit confused. I read Creating a Conditioned Attic and appears I have done mostly everything in the article. I have done the following:

    1. Installed R-19 in between the rafter bays
    2. Installed rigid foam board on top of the roof sheathing.
    3. Return duct is warped with Type 75 Fiber Glass Duct Wrap
    4. We have a metal roof and it's suppose to be 30% cooler than shingles

    I can seal off the he sofitt vents from outside, if necessary. I only other thing I haven't done is install a register grill in the attic.

    Am I missing something?

  8. GBA Editor
    Martin Holladay | | #8

    Thanks for providing more details. So you have an unvented conditioned attic.

    Assuming that you have an air conditioner that is functioning properly -- that is actually cooling the indoor air to the temperature on the thermostat -- the key to your problem is identifying the source of the moisture that is causing your problems. The two main contenders are (1) outdoor air which is leaking into the house, or (2) moisture of construction (curing concrete, drying lumber, and drying drywall compound).

    It's quite possible that outdoor air is leaking into your attic through the soffit vents. It's also possible that you have other air leaks elsewhere in the building. The best way to test for these leaks is with a blower door.

    If your problem turns out to be moisture of construction, then you should operate a stand-alone dehumidifier for as long as necessary to bring the indoor relative humidity under control.

    And you should definitely install a supply air register and a return air grille in your unvented conditioned attic -- as soon as you determine whether you have air leaks through the soffit.

  9. Dana1 | | #9

    Jimmy Miller writes:

    "I can seal off the he sofitt vents from outside, if necessary."

    It's pretty necessary to seal them off, from either the interior or the outside.

    Martin writes: "So you have an unvented conditioned attic."

    It reads more like a VENTED but conditioned attic, which IS the problem. With soffit vents into the conditioned space it's a big uncontrolled air leak. It's like leaving a couple of windows open and then wondering why the AC isn't drying the place out.

    It may be possible to seal them off from the inside with a couple of FrothPak kits or something- depends on just how big the gaps are.

  10. charlie_sullivan | | #10

    As a temporary measure, you could also put a dehumidifier in the attic. [Edit: after closing off the soffit vents] If the attic is well sealed, it will run a lot to dry things out up there, and then run much less. If the vents or other places leak, it will continue running and use a lot of energy on an ongoing basis. So it's not a good long term solution, but it could be a useful way to get the situation under control until you solve it for real using Martin's suggestions.

  11. emsent04 | | #11

    Thanks to all for your responses. The information has been helpful and provides much needed insight.

    I believe it will be much easier to close all sofitt vents from the outside because it's completely accessible. I will remove the vinyl and seal wood sofitt. I will also foam seal all interior light fixtures and etc. throughout the house. I might also install a temporary dehumidifier to dry the area out.

    Do you have any recommendations for sealing the wood sofitt?

  12. GBA Editor
    Martin Holladay | | #12

    You need (a) an air barrier at your exterior walls, and (b) an air barrier at your roof plane, and (c) a plan to seal all air leaks between the wall air barrier and the roof air barrier. That can be tricky.

    What is your wall air barrier? Perhaps taped OSB sheathing or taped plywood?

    What is your roof air barrier? Perhaps taped sheathing? Or perhaps taped rigid foam?

    Think about all of the transitions between these two regions, and seal them completely. In many cases, the best approach is to use a two-component spray foam kit.

    As I wrote earlier, the best way to track down air leaks is with a blower door. If you try to guess at the air leaks, you may miss some really big ones.

  13. emsent04 | | #13


    The exterior walls of the home has the original wall board as well as 3/4 plywood and Tyvek. Smooth Hardie board is the wall siding.

    The roof is a little more tricky. The home originally had a shingle roof but was upgraded to a Metal Roof by the previous homeowner. The roof has rigid foam board on top of the roof sheathing but it appears that the venting for the ridge cap was never cut out.

    We will perform a Blower Door test as soon the finished flooring is installed. The hickory hardwood flooring can't be installed until the humidity in the home is contained.

    What does the installation of a two-component spray foam kit entail?

  14. GBA Editor
    Martin Holladay | | #14

    You would want to use the spray foam to seal all of the gaps between the top plate of your exterior walls and the underside of your roof sheathing. In some cases, it may be useful to cut rectangular pieces of rigid foam to fit between your rafters before spraying the foam.

    The difficult issue is access. Sometimes it's easier to do this work from the exterior, after removing the soffits. Sometimes you can do this work from the interior.

  15. emsent04 | | #15

    Hello Martin and all,

    I have been evaluating my options and talking to friends about the proposed remedies based on the circumstances. It looks like properly sealing all the openings with closed cell foam will be the last resort at best (risky and expensive). It will be very difficult to make sure everything's airtight at this home. We did locate some return boxes that were sweating badily in the attic that have not been insulated and two 4" dry wall penetrations that were allowing conditioned air in the attic. This may be the source of at least half the problem. We are going to get some duct liner to insulate the return boxes, patch/foam the holes and see if that solves the problem before we try to totally unvent the attic. It's worth a shot because it's extremely more cost effective and seems easier to identify the problem.

    My friend believes that the crawlspace has remained dry because it was been properly insulated. He pointed to the fact that the crawl space is unvented but has 20 mil plastic sheathing on the ground but dry. He believes the home is old enough to prove the attic can function and stay dry if I insulate the return boxes and stop the conditioned from getting in attic it will stop the dampness/moisture in the attic. What do you thiink?

    I'll try it and report my findings.

    Thanks again,


  16. GBA Editor
    Martin Holladay | | #16

    If I understand you correctly, you will work on a few measures and then "see if that solves the problem before we try to totally unvent the attic."

    I'd just like to point out that you are the one who decided to move the insulation plane from the attic floor to the roofline. That means that you have already gone down the road to creating an unvented conditioned attic. Your decision to do that commits you to "totally unvent the attic," whether you want to do that work or not.

    All of these air sealing decisions should have been made before you did the work described in Comment #6. ("I have done the following: 1. Installed R-19 in between the rafter bays. 2. Installed rigid foam board on top of the roof sheathing.")

    Good luck with your work. Here's some advice, though: while you are doing those other things -- if you happen to notice any vent openings that are allowing exterior air to enter your attic -- do your best to seal those vents up tight.

  17. Dana1 | | #17

    "It looks like properly sealing all the openings with closed cell foam will be the last resort at best (risky and expensive)."

    Air sealing the attic with OPEN CELL foam would be cheaper, greener and easier/more-effective. The high expansion rate while curing makes it less likely for there to be gaps in the harder to get to locations, using far less polymer and without using high global warming blowing agents.

    IIRC a contractor in FL ( Curt Kinder ) always uses a theatrical smoke machine in the attic to verify air tightness prior to breaking down the foam installation equipment (almost always open cell). If there's a path left unplugged it becomes dead obvious, then it's a matter of deciding whether it's easier to seal those locations from the interior vs, the exterior.

    Until you seal the attic, you will have copious amounts of outdoor humidity getting into the house during the cooling season, whether you band-aid the hell out of the colder condensation spots or not. It's really the right solution.

  18. emsent04 | | #18

    Ok, guys you have me sold. I was hesitant because some of the guys in this area believe that trying to seal the attic would be risky because of the possibility of us not being able to reach all the unsealed areas. They also believe it will make the attic hotter, causing the roof to buckle. He said the application won't work without conditioning the attic after it's unvented. I'm going to move forward next week. I'll keep you guys posted. If you guys have any suggestions along the please feel free to post. Your help has been appreciated!

    Thanks again,


  19. GBA Editor
    Martin Holladay | | #19

    They guys in your area who urge you to vent your attic because sealing the vents "will make the attic hotter" are giving you bad advice.

    Look, lots of people think that venting an attic is a good idea. The system (installing a vented attic) has been around for decades, and it (sort of) works. (Although vented attics work a lot better in Massachusetts than they do in Florida.) But you have deliberately decided to take a different fork in the road. You chose to build a conditioned attic -- so you can't look back and listen to the guys at the lumberyard who tell you that your attic needs to be vented. After all, you installed rigid foam above your roof, and fiberglass batts between your rafters. That must mean that you wanted a conditioned attic -- right?

    So -- no vents!

  20. watercop | | #20

    I'm the smoke and blow guy Dana refers to above.

    North Florida has had the Mother of All Humid summers this year...10+ inches of rain in August alone. We have worked a bunch of condensation / mold / mildew issues.

    Nothing sheds light on the problem more than a blower door rig. I am suprised that return ducts are sweating - that's uncommon.

    What are your typical AC cooling setpoints? Some folks try to compensate for an oversized or otherwise poor humidity control situation by reducing setpoint, particularly at night in order to sleep better. Setpoints below 72 are risky in our climate, especially in cases of leaky building envelopes.

    The 3.5 ton Goodman is probably oversized as to sensible load and may lack controls (humidistat, variable air handler) to address latent load. Consider performing a Man J load calc informed by the result of a blower door test.

    Address point sources as well - ensure bath fans and range hood (if any) vent outdoors, not into the attic.

    I'm definitely with the unvented attic crowd - get that right!

  21. emsent04 | | #21

    Hello all,

    I've been away because of the early birth of my first child (baby boy):). I will be back to work shortly but I have a few questions to ask you guys before we start the unventing process. Thanks again for everyone's feedback.

    As a point of information I want to discuss the original design of this house to provide a little background information. (see floor plan and photos)

    The house was designed with a cathedral style ceiling, 4/12 pitch, I believe. The majority of the house (living area,office and bedrooms) has plasterboard screwed to the rafters/ trusses. This area has R-19 insulation batts as well. We also installed LED recess cans to j boxes to eliminate heat that is created by standard recess cans. The ceiling drops down to create an attic in hallway that leads to the office and bedrooms and bathrooms (see floor plan). The hallway and bathrooms have attic access.

    As understood, we need to seal up any points of outside air entry. Then we should add conditioned air to the attic to keep the area cool. Does the area with the plasterboard screwed to the rafters (4/12 pitch) need conditioned air after all the soffit vents are sealed? If so, how do I accomplish that? We currently have no point of entry to that area.

    Thanks again.

  22. Expert Member
    Dana Dorsett | | #22

    You needn't provide so much conditioned air to the attic space that it stays cool like the rest of the house. Your goal with the conditioned air exchange is to guarantee that the air is as DRY as the rest of the house, which will have a dew point well below the temperature of the supply air during the cooling season.

    Assuming you have the foam above the cathedralized ceiling section, and no interior side vapor barrier, you're fine just sealing up the soffit vents to keep the humid outdoor air out.

  23. GBA Editor
    Martin Holladay | | #23


  24. emsent04 | | #24

    Thanks Martin!

    I believe I understand what your assuming but for clarification purposes we added more photos of the interior. We have 3/4 rigid foam on top of the roof sheathing and R-19 faced batts in the rafter bays on the interior side underneath the blue board.
    Does the R-19 faced batts represent the vapor barrier you wanted me to avoid on the interior side?


  25. GBA Editor
    Martin Holladay | | #25

    I was assuming that you are a brand-new father.

  26. GBA Editor
    Martin Holladay | | #26

    Your original question included the information that you have installed "R-15 to R-20 Styrofoam" under your metal roofing. Now you are calling the foam layer "3/4 inch rigid foam," which sounds more like R-3 to R-3.75 of rigid foam, not R-15 to R-20. Big difference.

    In your climate zone (Zone 2), you need a minimum of R-5 of rigid foam above your roof sheathing for this type of roof assembly. This was explained in my article, How to Build an Insulated Cathedral Ceiling.

    At this point, you're kind of stuck with your bad decision, unless you are willing to remove the metal roofing to add more rigid foam, and then re-install the roofing. Hopefully, you won't get in trouble with your local code official for this technical violation of the building code.

    My guess is that the too-thin rigid foam won't result in a disaster. Cross your fingers. (Note to GBA readers: Don't make the same mistake that Jimmy made if you are installing rigid foam above your roof sheathing.)

    The kraft facing on the fiberglass batts is OK -- at least it's not polyethylene.

    You don't need to try to condition the air above your fiberglass batts in your cathedral ceilings. All you want to do is seal up your soffit and ridge vents so that no air leaks into your cathedral ceilings.

    Evidently the small attic above your bathrooms is the area where you have been experiencing most of your condensation problems. Once you have sealed all of the vents and air leaks that you can possibly seal, you can monitor the situation (that is, poke your head up and look around occasionally) to see if the condensation problems have been solved. If the problems haven't been solved, and it is still humid and dripping up there, then the small attic would benefit from a supply-air register and a return-air grille.

  27. Dana1 | | #27

    The kraft facers on the batts are "smart" vapor retarders, and will mitigate the already small risk presented by a slightly under-rated exterior R to cavity R ratio.

    However, you have a more optimal ratio than code requires, even if it's only R3. The R5 prescriptive is based on an R38 total R code minimum, a ratio of 5/38 or 13% of the total R. You have R19 in the rafters, and assuming R3 above the roof deck a total of ~R22. R3/R22 is over 13.5%, so the roof deck temp will be ever so slighly warmer (=drier) than if it were all at code prescribed levels. If those rafters are 5.5" deep 2x6, the R19s are really performing at R18, and the total R is R21, which boosts the ratio to R3/R21= 14%, even better.

    And, you have a smart vapor retarder (= kraft facers) on the interior side.

    So the cathedralized ceiling roof deck is sufficiently protected with what you have in place, and it needs no venting, but a higher performing ceiling would probably still be worthwhile. If it fits, an inch of unfaced EPS or XPS between the gypsum & rafters would make a noticeable improvement without making the interior side too vapor-tight, (though it would still be under code-min.) The vapor retardency of the interior foam layer would be enough to counter the too-thin exterior foam, even if it moves the ratio to under 13%. With unfaced EPS you could even take it do 1.5" or even 2", but XPS would start to be a bit too vapor tight at 1.5" or thicker. But if the gypsum is already in, it's not worth the do-over. The cost to rip it down and add bit of interior foam would be better spent on other efficiency measures.

  28. emsent04 | | #28

    "Thanks Martin" I am a brand new father :) Martin! First-time!

    I'm sorry Martin, I'm probably guilty of using the wrong terminology as it relates the rigid foam. I wasn't aware of the differences in what could've been used above the sheathing. The metal roof and the foam board was installed by the previous homeowner. Unfortunately, they didn't pull a permit when they installed the roof. We got historic data from contacting metal roof vendors that provided the materials to the previous owner. I will see if I can confirm the exact type of foam used above roof sheathing Martin. Always better to know!

  29. Dana1 | | #29

    As long as it's 3/4", it's not a problem, independent of foam type, given that you only R18-R19 in the rafters.

    If it were R38 in the rafters there would be some risk, but that risk would still be low.

    You write pretty well for somebody who isn't getting any sleep! :-) (The fun is just beginning- enjoy!)

  30. kaysmith | | #30

    I am not a pro by any means- but we also had very high humidity in our house and attic after renovation - and it turned out after tightly insulating the house - the house was under negative pressure. We were exhausting more air (through bathroom fans, leaky ducts, and radon fan) than we were supplying to the house. So the house ended up sucking in hot humid air through every crack and crevice, the fireplace, windows and doors which we why we couldn't control it with the HVAC system and dehumidifiers.
    This was diagnosed with a very cheap and easy smoke test. We ended up adding a fresh air intake (adding supply air to the house), closing a HVAC supply vent in the basement, and temporarily turning off the radon fan. After doing that, our indoor humidity immediately dropped 15%-20%. Maybe find an indoor air quality/ mold/ or energy efficiency professional who understands negative pressure and have them do a quick test?

  31. emsent04 | | #31

    Hello guys,

    Thanks again for all the input. I checked the foam board above the sheathing and it is 1" foam. I will be taking down the vinyl soffit that is over the original soffit and continuous vents tomorrow. I also plan to seal the original continuous vents with Great Stuff, Big Gaps Pro. This is the only closed cell foam I could find in the area. Hopefully it will do the trick to get those vents sealed up. I'll post pictures.

  32. Expert Member
    Dana Dorsett | | #32

    The Great Stuff isn't always closed-cell, it depends on how much expansion it has, but that's fine, since open cell foam is also a very good air sealer.

    Tip: If you take a spray bottle of water and mist the surfaces you intend to seal up you get substantially more volume out of a can, and fills the voids better. See:

  33. emsent04 | | #33

    Thanks Dana! I will be sure to use that tip. :)

  34. emsent04 | | #34

    Hello Everyone:

    I know its been a minute. I have been trying to manage other projects as well as adjust to becoming a father. I have sealed the Soffit vents and the foam expanded nicely. I scheduled a blow door test for the owner to assess the leakage of the property but Energy Rater doesn't believe the home is quite ready. They advised us to foam the all the joints in the sub flooring to prevent t the vented crawl space air from leaking into the conditioned space. They stated that the air leakage from the joints in the sub flooring would be too great to be worth conducting the test. They also stated that laying roofing felt paper wont be enough to keep the outside air from entering the space without first sealing the joints. The Energy Rater believes it is apart of the problem in regards to the interior humidity problem. I told them that I wanted to conduct a blower test on the home before and after I finish the renovation. Also, they do not want us to leave the attic access door open while conducting the test. I have following questions:

    (1) Has anyone had any experience with understanding the effectiveness of blower test testing under the conditions stated above?

    (2) Should the attic access stay open during the door blower test? Isn't that the only way to tell if the soffit vents are sealed properly?

  35. Expert Member
    Dana Dorsett | | #35

    Congratulations on committing parenthood! (Good luck on catching up on your sleep this year! :-)

    A vented crawlspace is a huge latent load/humidity problem in a Florida climate. The "right" thing do to would be to convert it into a sealed conditioned crawlspace, blocking the air infiltration at the foundation and ground, not at the subfloor.

    In your climate an air-conditioned house with a vented crawlspace can see the moisture content of the joists and subfloor can be extremely high during the summer months, since the temperature at the floor often dwells at or below the outdoor dew point. If you seal up the crawl space to keep outdoor air out, and have a ground vapor barrier to keep ground moisture out, the dew point of the air in the crawl space tracks that of the conditioned space (especially if you ventilate it with a tiny amount of conditioned air from the rooms above.)

    In climate zones 1 @ 2 it's not even necessary to insulate the crawlspace walls to meet IRC code minimums, though an inch of fire-rated Thermax wouldn't be a terrible idea, since that would "earth couple" the house to ground that is at a nearly ideal temperature.

    Yes, leave the attic hatch open during the test, and close it during the test to see if it makes a difference. That's (ab)using the rater's test for diagnostic purposes rather than rating purposes, but as long as you don't hold them up with 47 experiments of open vs. closed doors and hatches they probably won't mind a quick filp of the attic access once or may twice. With a large window fan you can probably pressurize the house enough to see if air flows easily into the partially cracked attic access even before then.

  36. emsent04 | | #36

    Thanks you Dana! My son is growing like a weed. :)

    I have read a number of articles including the one that was posted regarding closing the crawl space but there still seems to be some grey area. The articles had disclaimers about letting conditioned air into the crawl space air. This option seems risky if not hazardous due to the radon and pressure treated lumber gases that could migrate into the conditioned living space. The crawl space currently has a ground vapor barrier going up the wall as described in these articles but the vents are still open. According to my research the vapor barrier has been laid out for at least a year and a half. I have also inspected the wood for mold growth and I don't see any significant signs of growth. I am also concerned and nervous about creating such an air tight space with gas appliances in the home.(Living Space is sealed very well, windows doors and etc.) Even though, I will have to bring some fresh air from outside for combustion purposes, it still leaves me with tightness concerns. I would rather the homeowner worry about a little air leakage from crawl space opposed to carbon monoxide and radon gases and etc. Attached are photos of the Crawl space, Rim Joist conditon, and Foundation Piers.

    As a result, I am considering intalling 2 UnderAire Deluxe Two Fan Crawl Space Ventilators (See Photo) to pull the moisture out of the crawl space. Hopefully this will manage and keep the moisture going into the conditioned living space under control. Each 2 fan ventilator pulls 220 cfm and has a build-in dehumidistat that can be set according to the description. The ventilators provide calculations that suggest the number of ventilators needed dry out the space in 15 minutes by multiplying the length X Width X height and dividing by 15 minutes. I will spray the seams of the sub floor as well to minimize air leakage to the conditioned living space. I also plan to monitor the situation by periodically checking for mold growth and humidity temperatures by using a Fieldpiece, pocket psychrometer. Here are a few facts about the house:

    -The house currenting has 11-8x16 vents around the house. (see attached photo)
    -Crawl space height is 2.5 height
    -Supply Ducts were seals with Sure-Grip 404 mastic by CARLISLE and wrapped well with, Type 75 Microlite made by Johns Manville.

    Does anyone have any objections or suggestions regarding this strategy?

    Thank again!

  37. Dana1 | | #37

    When the temperature at the subfloor is below the dew point of the outdoor air (common during the cooling season), actively ventilating the crawl space doesn't remove moisture, it pulls moisture in.

    Sealing up the crawl space walls is not going to affect gas burners any more than sealing up the subfloor would, and air leakage into the crawl space is not a substitute for adequate make-up air.

  38. GBA Editor
    Martin Holladay | | #38

    If you want to use an exhaust fan to pull air from a conditioned crawl space, you should size the fan according to code requirements. These requirements are explained in my article, Building an Unvented Crawl Space.

    For formula calls for the fan to be rated at 1 cfm for each 50 square feet of crawl space floor area. Your proposed fans are rated at 440 cfm -- enough for a 22,000 square foot crawl space. That's a pretty huge crawl space. In fact, your crawl space measures about 1,900 square feet -- so all you need is a fan rated at 38 cfm.

    This approach also requires you to install a floor grille that allows conditioned air from upstairs to enter the crawl space through the floor grille.

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