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High humidity in between floor space in 2-story home in Zone 3A

momndad | Posted in General Questions on

Our home has a “crawl” space in between the first floor and second floor, all of our AC duct which is insulated is in this area. It appear to be well insulated, and is the same temperature as the house most of this time, within 1 degree or 2.

The issue is the humidity in this area is 30% higher than inside the house most of the time. The humidity in the house can be as high 58% in this humid season right now, but in the crawl space it is as high 89-90%. It seems like it should not be this different?

There are times in late afternoon that is does go down closer to 10% higher, but not always. we have had recent high rainfall, and it seems to be at it’s worst now, but it can be just as high, even when it was drier outside.

We have checked for roof leaks, and don’t see any signs of water at last where we can tell. Not finding a good consistent reason, so wonder what else to look at and consider, or is it really even an issue, 90% humidity just seems like it should not be that high!

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  1. NormanWB | | #1

    Is this area insulated from the outside environment and/or has a moisture barrier of some sort from the outside? I am wondering if you are getting moisture from the ground or from the outside air which is condensing in this space due to the duct work. Perhaps there is a good way to insulate this crawl space.

    Sorry if I am off base, but I am having a hard time visualizing this crawlspace between floors.

  2. momndad | | #2

    This area is insulated just like the rest of the home, just no Sheetrock on walls, open insulation.

  3. momndad | | #3

    There are ducts that are wrapped with insulation.

  4. Jon_Harrod | | #4

    Questions: Are there ducts from the return grilles to the AC, or is the cavity itself used as a plenum? And what type of foundation do you have (i.e. slab, vented crawlspace, etc.)?

  5. user-2310254 | | #5

    Can you post a couple of photos?

  6. Jon_Harrod | | #6

    Hmmm... If I were investigating this, I'd start by listing every possible moisture source and ruling them out. Some of these will sound unlikely or trivial.

    --Roof leaks (already ruled out)
    --Plumbing leaks
    --Improper condensate disposal: Make sure the condensate from the AC is piped into a drain.
    --Unducted/disconnected bath fans
    --Improperly vented gas/propane water heater
    --Moisture-laden air coming up from a basement or crawlspace foundation.
    --Outside air being pulled in through leaks in the building shell

    The last two were identified by Norman and are the most likely in my opinion. If the return ducts in the crawlspace are leakier than the supply ducts, the crawlspace may be under negative pressure relative the outdoors. This sucks warm, humid air in through leaks in the siding/sheathing/band joist (and possibly up from the foundation through interior walls and chaseways). As the air comes in contact with interior building surfaces, it cools, and the humidity goes up. If this is the case, it can be resolved by some combination of duct sealing and exterior air sealing.

    I think you're right to be concerned. 90% relative humidity means that you're almost certainly getting condensation on exposed ductwork. My advice would be to find an energy auditor or home energy rater who can do some pressure diagnostics (blower door test with zonal pressures in the crawlspace, dominant duct leakage test for starters) to try to get to the cause of the problem.

  7. Jon_R | | #7

    Would be helpful to know if the dew point of this area tracks the outdoor dew point.

  8. GBA Editor
    Martin Holladay | | #8

    It's very hard to visualize this area that you describe as a "crawl space." Do you have oversized floor trusses between your first and second floor?

    In any cases, if the walls of this "crawl space" have exposed insulation, with no drywall, then the exterior walls are probably leaky. If return air ducts in this crawl space are leaky, the crawl space may be depressurized. The effect of that type of depressurization would be to pull humid outdoor air into the crawl space through cracks in the walls.

  9. Expert Member
    ARMANDO COBO | | #9

    Just another thought...The fact that your house has 58% humidity, it tells me that your HVAC system is not designed to remove humidity or working properly. Indoor humidity should always be ≤50%.

  10. momndad | | #10

    Attached a few pictures in a word doc, note, only some of the wrapped ducts have so much moisture appearance, others are clean white.

  11. NormanWB | | #11

    The ducts are insulated, but given the connection to the plenum, which is not sealed, I wonder if the ducts are taped and sealed or are they leaking. Also, are these supply or return ducts? I suspect supply, but want to be sure.

  12. momndad | | #12

    Supply ducts, and the metal edge of one of them not well sealed is dripping water, we just climbed up and looked closer, it's a tough area to work in.

  13. GBA Editor
    Martin Holladay | | #13

    We're still waiting for a description of this area. Is this a dirt-floored crawl space? If so, is there any polyethylene over the dirt?

    What are the horizontal boundaries of this crawl space? For example, is the floor made of dirt, concrete, plywood, OSB, or gypsum wallboard? Similarly, what is the ceiling?

  14. momndad | | #14

    the floors are wood, this area is between 1st and 2nd floor and reaches all the way to external walls and sloped roof line this is a steel framed home, with wood also. I am having a difficult time getting photos small enough to upload, but here is one more I hope will work. I also added some in a previous word doc. That did upload succesfully.

  15. momndad | | #15

    Trying to attach a new picture.

  16. momndad | | #16

    The insulated ducts coming out of the plenum on one side are clean no issues, the ducts coming out of 2 other sides are stained, look moldy, wet, but only about 3-4 foot near plenum, the rest of ductwork is clean from there on.

  17. GBA Editor
    Martin Holladay | | #17

    If your ducts are dripping, that means that the air in this crawl space is very humid. It also means that your ducts are poorly insulated (and are not equipped with an adequate vapor-barrier jacket).

    It's hard to be certain without a site visit, but in all likelihood, the solution is (a) to find out why the space is depressurized, (b) to seal the air leaks in the walls, and (c) to improve the duct insulation.

  18. momndad | | #18

    Even though not all ducts are dripping? Thats what is confusing.

  19. GBA Editor
    Martin Holladay | | #19

    For the ducts to drip, the ducts need to be cold. For the ducts to have cold surfaces exposed to the air, three factors are required:

    (1) They have to be supply ducts, not return ducts.

    (2) The air conditioner has to be operating (which means that the ducts won't drip during the winter).

    (3) The ducts have to be poorly insulated, or have damaged insulation, or to have a damaged polyethylene jacket that allows humid air to come in contact with a cold surface.

    If some, but not all, of your ducts are dripping, it means that the dripping ducts suffer from the three conditions listed above, while the non-dripping ducts don't suffer from one or all of the listed factors.

  20. momndad | | #20

    so first AC guy tells us, we don't really haves a problem, he has seen alot worse looking ducts, he says we could redo the ductwork, install a dehumidifier, or a vent or just let it alone. I guess I hoped something could be found to be wrong, I partly agree, but don't agree the 50-60% humidity in our home is what it sounded be, who do you talk to about resolving humidity. Or would a dehumidifier do the job. We currently have a heat pump type heater/AC. He also said the colder the output setting the more humid it will be, he likes to set his output on 40°, any thoughts?

  21. GBA Editor
    Martin Holladay | | #21

    If you have so much condensation on your ducts that the ducts are dripping, it's nuts to fix the problem by installing a dehumidifier. Step one is to repair the duct insulation and the damaged polyethylene jacket on the duct insulation.

  22. momndad | | #22

    Dagree, but question does duct insulation just get that way overtime in our climate, hot humid region, it's not Houston! But outdoor average relative humidity is 65-68. Our indoor humidity is 50-60. Can they be improved in some way, to help make it drier?

  23. Expert Member
    Dana Dorsett | | #23

    If the crawlspace were air-tight to the outdoors and the house has "reasonable" rather than high ventilation rates, there shouldn't be chronic condensation issues on the ducts & insulation. The dew point of the indoor air would pretty much track that of the air-conditioning air. There may be large air leaks to the outdoors, leaks that get multiplied by air-handler induced pressure differences.

    The outdoor & indoor relative humidity numbers are meaningless on their own- we need to know the temperatures to which they are relative. 100% relative humidity at 25F will never be an air conditioning duct condensation problem, since it's dew point is 25F, and the duct is always warmer than that. But 65% relative humidity at 85F is a potential problem since it's dew point is 72F, and the supply ducts will be much colder than that. Only absolute humidity states as "dew point" is relevant, since that defines the temperature below which a cool duct will accumulate moisture. Outdoor air with a high dew point leaking into the crawl space keeps replenishing the moisture in the space, even as condensation onto the ducts removes it.

    Indoor air that is 60% RH @ 75F has a dew point of about 60F, which is still high enough to be a condensation issue for cooling ducts in some systems, but nowhere near the problem that 70F+ dew point air would have.

    Sufficient insulation over the duct keeps the surface temperature of the insulation above the dew point, and no moisture condenses out of the air. But if the vapor barrier on the outside of the insulation is ripped up and the insulation is air permeable, some condensation will still occur. Sealing the vapor barrier to make it as air tight as possible is important. There are many existence proofs that insulated cooling ducts passing through humid spaces can go for decades without degradation or condensation.

  24. momndad | | #24

    So thankful for all the responses, but having a hard time understanding humidity, dew point, pressure, etc. So I know all this week so far dew point has been 73-75. I know our ductwork in enclosed envelope between floors is looking like it has moisture issues, doesn't feel wet, but you can see in the photos it's been getting wet. I'm hearing we may have a pressure problem where outside air is being ducked in to this area, even though it's temperature is the same as inside housr, but the humidity level is 10-30% more. So what type of person do we call to check to see what the problem might be, and how to fix it as best we can?

  25. GBA Editor
    Martin Holladay | | #25

    You need a home performance contractor -- one who is skilled at pressure diagnostics. (For more information on this topic, see An Introduction to Pressure Diagnostics.)

    I would start by interviewing professional home energy raters certified by RESNET or BPI. (Visit their web sites to search for a professional in your area.) You want someone with a blower door who has experience with pressure diagnostics.

    Good luck.

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