Humidity issue
Madison4159
| Posted in Expert Exchange Q&A on
I recently completed a full renovation of the second and third floors of my home, including closed cell spray foam insulation and all new windows. The space is now extremely airtight. I have radiant heat for the winter and Mitsubishi ductless mini-splits for air conditioning.
Over the past month, the indoor humidity has consistently remained in the mid to high 80% range. The mini-splits quickly satisfy the temperature setting, but they do not run long enough to remove enough moisture. Dry mode has also failed to bring the humidity under control. Oddly, I have the opposite problem during the winter, when the indoor air becomes too dry.
I am looking for advice on identifying the cause and finding a permanent solution, such as a dedicated dehumidifier or ventilation system.
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Replies
Madison,
Once you tighten up a house, you need mechanical ventilation. To control humidity, but also for IAQ. Depending on there you are the ventilation may well be enough to control moisture levels without de-humidification.
https://www.greenbuildingadvisor.com/article/pros-and-cons-of-three-types-of-ventilation-systems
https://www.greenbuildingadvisor.com/article/revisiting-ventilation
Seems to me you have two different problems that require different solutions.
If your mini is not running continuously with the compressor at or close to its max speed on the warmest days the system is oversized for your load. The classic complaint is that you find it necessary to lower the set point to remove humidity and the house is now “cold and clammy”
The winter low humidity is almost certainly that you house is still very leaky allowing to much moist air to escape and it is be replaced with cold dry air. A blower door test will tell you how tight your house is.
I hate to disagree with Malcolm but if I am correct that correct that your mini split is cycling on and off on hot days and your ACH50 number is over 3, adding more ventilation will only make your symptoms worse
Walta
+1 on Malcolm's advice. High summer and low winter humidity are often signs of air leakage from outside. Your envelope may not be as airtight as you think. Did you perform a blower door test?
Also, the minisplits may be oversized for the load. If they don't run long enough to dehumidify, that's generally an indication that they are oversized. All A/C systems will be oversized when running at cooler than design conditions. Has your summer weather been cooler than average, and does the system dehumidify better when it is very hot outside? Do you have a single multisplit for both floors, or separate systems? Individual minisplits often have better turndown ratios than multisplits, so they can dehumidify better at lower outdoor temperatures. If you have separate system, you might try running just the one on the third floor - this effectively increases the load on that system allowing it to run longer and dehumidify better. This won't work with a multisplit.
And finally, increasing ventilation as Malcolm recommends might not help at all with indoor humidity if the outdoor air is very humid. The ventilation is necessary for interior IAQ regardless. Some people with very tight houses have found that a supplemental dehumidifier is necessary.