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High humidity after a dehumidifier was installed

We have always had Humidity issues in our home. It was built in the 1930s, completed a complete removal 5 years ago. Roof is metal with spray foam insulation. The home contains an attic. Home is only 1,500 square feet. After installing new AC unit with standalone dehumidifier, Humidity still remains 55%+. Never has reached and stayed at 50% and has not gone lower. the dehumidifier just keeps running and running. The Cooling/heating company who did the install is very well known and does. Great work in the area. They thought it was the dehumidifier not working properly but reinstalled a new unit today. Still same issue with high Humidity. Forgot to mention this home is in Florida. I did read the post on Humidity and saw with the foam insulation maybe all the moisture is getting trapped in the attic and can't get out. The dehumidifier can't keep up with the amount of moisture removed from home. The dehumidifier is bigger than we need. Supports 3,000 sq ft home. The heating/cooling company can't figure out why and where the moisture is coming from. Does anyone have any suggestions on the next steps/tests that should be done to figure out why our homes humidity is so high where a dehumidfier can't keep up with the amount of moisture? Thanks

Erica Sayers

Asked by Erica Sayers
Posted Mar 9, 2016 4:53 AM ET


5 Answers

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There are three possible explanations for your problem:

1. Your house has a high moisture load. (In other words, lots of water is either generated inside your home or is entering your home.)

2. Your dehumidifier is broken.

3. Your hygrometer (the meter that tells you what the indoor relative humidity is) is defective or unreliable.

It's also possible that a combination of these factors is responsible for your problem.

If your main problem is (1), causes might include a damp crawl space or air leaks in your home's floor, walls, or ceilings. If you have air leaks, humid outdoor air might be entering your home. For more information on this issue, see Preventing Water Entry Into a Home.

It seems that you have done what you could to eliminate problem (2), but it's possible that the contractor who installed your air conditioning system and dehumidifier is incompetent.

Problem (3) is fairly common. It wouldn't hurt to buy two or three more hygrometers to get some more readings.

Answered by Martin Holladay
Posted Mar 9, 2016 5:04 AM ET


Thank you for your response!! Appreciate it! Would it make sense to test each rooms Hummidity including the attic? If the attic contains very high Hummidity does it make sense to ventilate the attic to release the moisture?
Where can you buy hygrometer? Hardware store ?

Answered by Erica Sayers
Posted Mar 9, 2016 8:17 AM ET


Testing the RH in the attic is useless if it's above the insulation, since it will be at a dramatically different temperature than the rest of the house. (Temperature is what the humidity is "relative" to. The absolute humidity is measured as dew point, or wet-bulb temperature.)

In a FL climate ventilating the attic of an air conditioned house usually raises rather than lowering the humidity levels in the house.

It's possible that duct imbalances could be driving excessive outdoor infiltration, which raises the indoor RH during the cooling season. This is especially true if the ducts and air handler are in the attic, above the insulation.

Answered by D Dorsett
Posted Mar 9, 2016 10:08 AM ET


Q. "Where can you buy a hygrometer?"

A. If your local hardware store doesn't sell hygrometers, you can buy one online.

Answered by Martin Holladay
Posted Mar 9, 2016 11:19 AM ET


Thank you!

Answered by Erica Sayers
Posted Mar 9, 2016 12:52 PM ET

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