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Community and Q&A

How low is too low for indoor humidity?

John Sexton | Posted in Energy Efficiency and Durability on

So you guys have answered numerous questions about humidity for me during the summer. I have a 20 year old two story house that we bought this year and promptly bought two new HVAC 5 ton units and put open cell foam in the attic and encapsulated and foamed the crawlspace. The attic has a couple of supply vents and the crawl has four supply vents and a return. My question is about the low humidity I am now getting now that it is finally getting cold in Nashville. My hygrometers tell me that I am now dipping into the high 20s on the main level of my house, into the high 30s in the attic, and low to mid 40s for the crawl. The crawl is nice, but high 20s for my first floor living area is worrisome. I don’t want to increase my likelihood of sickness or wood splitting. Any suggestions?thanks!

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  1. D Dorsett | | #1

    Indoor humidity that low is a symptom of either high ventilation rates or high air infiltration rates.

    The low end of human comfort & health is about 30% RH @ 68F, though most people are still fine at 25%. Below 25% chapped lips, split nails and dry throat irritation becomes a comfort factor.

    The solution is to dial back the infiltration & ventilation.

    Ten tons of cooling load implies a pretty gia-normous house, is this a 12-15,000' mansion suitable for a C & W star's extended family or something?

  2. Steve Knapp CZ 3A Georgia | | #2

    Allison Bailey just posted an article on this topic ( Based on his guidance, your numbers sound pretty close to optimal (although I wonder if you should close one or both of the attic supplies). Perhaps other members will chime in.

    Couple of more thoughts...

    This would be a good time to see if you can improve your air sealing. See and the multiple links in the sidebar. Air infiltration certainly adds to the challenge of controlling indoor humidity.

    If you are actively ventilating, you also might need to reduce the volume of incoming air to keep humidity levels where you want them. Of course, that's a balancing act since you need a certain amount of ventilation to maintain a healthy indoor environment.

  3. GBA Editor
    Martin Holladay | | #3

    There are a couple of red flags here, as Dana and Steve have pointed out. Two 5-ton units is the first red flag. The most likely explanation for these enormous cooling units is that your HVAC contractor bungled the Manual J calculation (or failed to perform a Manual J calculation) -- but it's also possible, as Dana suggests, that you have an enormous mansion.

    You don't want a supply and a return in your crawl space. Assuming that it is a code-compliant sealed (unvented) crawl space, you either want a supply without a return, or you want a continuously operating exhaust fan in the rim joist of your crawl space. In either case, you need a grille in the floor above the crawl space to allow for passive air flow between the crawl space and the floor above. These details are explained in this article: Building an Unvented Crawl Space.

    Finally, your indoor humidity levels are, indeed, unusually low, indicating that either (a) your ventilation system (if you have one) is overventilating, or (b) your house has a very leaky thermal envelope. If you have the latter problem, the solution is blower-door-directed air sealing work.

  4. John Sexton | | #4

    Thanks guys. I will read those articles as soon as possible. Yes I definitely do not have a mansion but it is about 6400 square feet on the two levels with a lot of volume ceilings. I had to argue till I was blue in the face that even the five tons per level would be enough with the contractors that I had out here, including a ResMed / BPI contractor that I found from the links on energy Vanguard. But in any case that's what I have now, so I will have to make the best of it. I suspect you are right and that I need to get a blood test done to see where leaks may be coming from. How do I find a reputable local person to do that? It's funny though, you would think that would lead to very high humidity during the summer and my humidity was only about 45% at that time. Right this second it is 33% and the summer stayed almost 45 all the time in the main living area.

  5. John Sexton | | #5

    Haha that typo said blood test....meant blower test.

  6. Steve Knapp CZ 3A Georgia | | #6

    John. You should hire a new RESNET rater. Maybe the first rater was correct, and you need 10 tons for 6,400 square feet. But I would want a new Manual J to confirm that number.

  7. Jon R | | #7

    Not mentioned is that indoor humidity is a result of the balance between production (highly variable) and removal. In a low occupant density house, low indoor humidity is likely, even with reasonable air sealing and proper ventilation rates. If so, you will need to humidify.

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