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Tight home — help with indoor humidity

user-1012653 | Posted in Mechanicals on

Some basic background: Finishing the new home build. Zone 6 (Iowa). Currently hot, humid days. About 90-95 outside with 80%+ humidity levels.

House is 3800 sqft, (half main level, rest is basement, fully ducted and conditioned however) r28 full ICF with R60 blown attic with spray foam attic seal, triple pane windows. Heating is through a ducted furnace (Lennox SLP98) and an XP16 air conditioner. HRV for the home.

The house was sized for a 2 ton ac. It is not having any issues keeping up. I have been keeping it at 73 to test things out. When it has longer run times it keeps the indoor humidity at about 42-48%. When it shuts off, the house holds 73 for several hours typically. However the humidity can climb to mid 50s, pushing 60. I have the HRV shut off as I do not want to dump additional humidity in the room.

I have been playing with the function of the stat changing between continuous circulation vs automatic. The problem is it still doesn’t kick on the ac unless it calls for cooling.

Anyone have any ideas how to keep the humidity in check without adding additional equipment?

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  1. GBA Editor
    Martin Holladay | | #1

    A stand-alone dehumidifier costs about $250.

  2. ntisdell | | #2

    With new homes it is usually said that humidity levels can be very very high for the first year due to wet concrete, wet lumber from the construction phase. Likely contributing to your low cooling load - moderate humidity issues.

    You can get plenty of humidifiers for $150-200 and really IMO is a great addition to any home. Seems like a lot of homes could benefit from one in the basement, even if the AC takes care of the humidity most of the time in most of the home.

    Depending on your thermostat you might have a 'circ' option which cycles the furnace fan for 30% of the hour (opposed to 'ON') - that mode is nice to circulate some of that dehumidified air by dehumidifier around the home. Honeywell vision Pro8000 has that feature - fairly common contractor installed stat..

    My new house has a similar issue - my dehumidifier has worked great during low load periods or to ring out humidity after long periods of having windows open (cool, high humidity) and switching over to AC when it gets to hot.

  3. Expert Member
    Dana Dorsett | | #3

    Running the HVAC in continuous fan mode only increases air-handler driven infiltration, increasing your indoor humidity issue slightly, even in a fairly tight house. There's no substitute for mechanical dehumidification in low sensible-load houses- it's a common problem. Martin's $250 solution works.

    If you didn't already have the central AC, the Daikin Quaternity mini-splits operate to independently settable temperature & %RH setpoints (in both heating & cooling mode), but that's an awfully expensive dehumidifier, if that's all you intend to use it for.

  4. wjrobinson | | #4

    Outside hot and humid... Inside... at 73... Don't expect humidity lower than you have. The other posts make good sense also...

  5. user-1012653 | | #5

    i figured running the dehumidifier was most likely the answer. I got one I can try out and see how it does.

  6. Jon_R | | #6

    Better air sealing will reduce latent load and humidity. But only a dehumidifier will dry the air when you don't want cooling.

  7. user-1012653 | | #7

    I am assuming, due to the tight home, is what is making it more humid inside. The home pre-tested out right around 1 ach @ 50 pac, but its still brand new so I assume curing materials is contributing to that to a certain degree. Since we are still finishing up construction on the inside (trimming, etc) we are in and out the doors often so that is also letting in the humid exterior air.

  8. Expert Member
    Dana Dorsett | | #8

    A tight home raises the humidity in wintertime, but at your stated....

    " 90-95 outside with 80%+ humidity levels"

    ...air tightness and lowered ventilation rates are a boon. At 90F/80% RH, cooling that air to 76F would result in interior fog and copious condensation on walls, since the dew point of that air is north of 80F.

    I'll go out on a limb and hazard that your actual outdoor RH is lower than that at 90F, since US climate zone 6 is nowhere near a tropical jungle, but even 75F dew points (which we've experienced at my house in recent weeks) are pretty torrid. Healthy comfortable 75F indoor air would have a dew point of 55F or lower, which takes active dehumidification whenever the outdoor dew points are higher than that.

  9. tommay | | #9

    you say you keep the house at 73, almost a winter time heat level. Though it may sound strange, if your house is as tight as you say it is, turn on the heat to 73 for a short time to dry up the interior. Once the hunidity is gone, switch back to ac.

  10. Expert Member
    Dana Dorsett | | #10

    Tom: Raising the temperature neither adds nor removes moisture from the house. The relative humidity goes down at higher temperatures, but that's because relative humidity is "relative" to the temperature. Dew point (and "wet bulb temperature") is a measure of the absolute humidity of the air, and it will not change by raising the temperature.

    The absolute humidity can be changed by lowering the temperature, if you take the temperature below the dew point temp, causing the water to condense, provided you remove the liquid moisture. And that is exactly what is going on with the coils of an air conditioner or dehumidifier- the moisture condenses on the coils which are maintained at a cool temperature well below the dew point of the air, delivering air to the room that is drier than the air entering the coil.

    Healthy comfortable 30-50% relative humidity 68F-75F air has dew points between 40F-60F. When outdoor dewpoints exceed 60F, some amount of mechanical dehumidification becomes useful for keeping the mold under control, independent of how tight your house is.

  11. curtkinder | | #11

    A quick scan suggests the XP16 is a two stage AC. Good. I hope the air handler is a true, ECM blower motor equipped variable speed model. If it is, a good thermostat with integrated humidity control can command the air handler to reduce CFM per ton in response to high humidity.

    Generally speaking, when outdoor dewpoints hit 60+, triggering humidity discomfort as Dana correctly notes, there is enough sensible load to trigger AC operation, which, if optimized for humidity control, will do a fairly decent job of dehu.

    I agree that high exterior door traffic coupled with drying fresh lumber and concrete (drywall mud, too) may tilt the scale for a few months in favor of supplemental dehu.

    I'm a lonely voice here suggesting that supplemental, standalone dehu should be deployed only AFTER all other means are exhausted.

  12. user-1012653 | | #12

    Yes, the furnace is a variable speed EMC motor. It has the Lennox iComfort stat with it. Since we are not finished nor fully moved into the house, I have not messed with it much beyond programming temps and times. This last week temps dropped to mid 70s, so humidty was in the high 40s inside so I have not been as concerned about it. I will get a dehumidifer and run it and see how that goes.

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