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Community and Q&A

High humidity with high efficiency?

bren77 | Posted in General Questions on

We received some excellent help from the members on this site one year ago when we updated the exterior of our home (June 2015). We ended up replacing our doors and adding these layers:

  • Existing – Owens Corning “rigid polyisocyanurate foil faced foam board” with R3.6. This was added when the home was built in 1986. We kept it installed and put these layers on top…
  • New 1 – INSULATION: 1″ Thermal 3Ht, applied over existing foam insulation (1/2″) to provide sealing of all exterior walls and gaps. Seams joined by manufacturer recommended tape.
  • New 2 – TYVEK: water barrier. Seams joined by manufacturer recommended tape.

We also replaced our older gas furnace with a high efficiency Trane XV95. Our house is 1800 sq ft with a finished basement. We live in Columbus, Ohio.

So far, we’ve enjoyed the change. Our home is quieter and it seemed much more efficient during the winter.

However, this spring/summer we’re having an issue… high humidity in our home that we did not have before. We are running our A/C (it is 10 years old) but the humidity is staying high in our house (65%+) and it doesn’t seem to be as comfortable as it used to.

We have had the company that installed our furnace come out and make some “adjustments” to the furnace, but the issue is still there.

Is this a common issue with tight homes? Any suggestions?

FYI: We have used up our “exterior budget” and can’t afford to put in an expensive home ventilation system. Are there relatively inexpensive changes we can make?

Thanks!
Chad

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Replies

  1. davidmeiland | | #1

    Does the A/C run shorter cycles now that the house has better insulation?

  2. GBA Editor
    Martin Holladay | | #2

    Chad,
    Any tight home should include a mechanical ventilation system. Here is a link to an article that will explain more: Designing a Good Ventilation System.

    Operating a mechanical ventilation system will help lower indoor humidity levels during the winter. In Ohio, you don't want to run your ventilation system more than minimally necessary during the summer, however, since summertime ventilation can raise rather than lower indoor humidity levels.

    As you apparently understand, the way to lower indoor humidity levels during the summer is to keep your windows closed and operate your air conditioner. If your air conditioner (even when adjusted by your HVAC contractor) can't lower your indoor humidity enough, you may want to purchase and use a stand-alone dehumidifier. For more information on this topic, see All About Dehumidifiers.

    Needless to say, if you have high indoor humidity levels, the first step is to be sure that there is no source of moisture entry into your home. Possibilities include a damp basement, damp crawl space, plumbing leak, too many houseplants, firewood stored indoors, or too many tropical fish tanks. For more information on this issue, see Preventing Water Entry Into a Home.

  3. Expert Member
    Dana Dorsett | | #3

    Did they test and adjust the furnace, or were they tweaking the AC?

    Does the HVAC system take in ventilation air when the air handler is running, or is it isolated from the outdoors? Introducing too much outdoor air during sticky weather can overwhelm the latent cooling capacity of the system.

    If the sensible cooling loads are now much lower than previously the duty cycle of the AC might have gone down, but that is not likely unless you changed out the windows when you did the siding. It's probably related to higher volumes of air from the new air handler.

    If the rate of air going through the cooling coils are high the amount of humidity removed is lower, but if the volumes are too low the coil ices up, so it has to be carefully optimized. Assuming the refrigerant charge and the condenser unit are all up to spec, it could be a matter of adjusting the speed on the air handler to move less air in cooling mode, but not so slowly that ice forms on the coils.

    If it can't be resolved with air handler adjustments, there's nothing wrong with using an Energy Star dehumidifier in the basement to manage the whole house humidity load.

  4. chrisjri | | #4

    Be sure the fan is not running continuously. The wet air coil will reintroduce humidity back into the house after the compressor shuts off.

  5. poiesis | | #5

    Chad, what is the size of your AC unit?

  6. bren77 | | #6

    Thank you all for your help! That AC installer attempted "air handler adjustments" like was recommended. That helped some, but we still found the house to be sticky on hot humid days. So, we purchased a dehumidifier and put it in the basement. That seems to really be helping. Thanks again!

  7. curtkinder | | #7

    Hold your HVAC maintenance company's feet to the fire as to whether the existing system is performing optimally. They should confirm that superheat or subcooling are within range. They should also document the system's indoor temperature split...at least 18*F, ideally 20.

    Also have someone confirm that any ductwork outside conditioned envelope is sealed so as to be substantially airtight. A bit of mastic goes a long way.

    Find out if the air handler, thermostat, or, ideally, both working together, support enhanced dehumidification. If the present AC contractor can't clearly state the system's specific humidity control features (or lack thereof) find another.

  8. GBA Editor
    Martin Holladay | | #8

    Chad,
    Many forced-air systems have "outdoor air ducts" that introduce outdoor ventilation air to the return-air plenum. If these ducts aren't equipped with a motorized damper connected to an Air Cycler control, the ducts can introduce excessive amounts of outdoor air during the summer, raising indoor humidity levels.

    I'm not saying that's what's happening at your house, but it's a possibility.

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