Whole house mustiness in humid weather: this is a humidity problem
No visible growth and a smell that follows the weather points to indoor humidity, not a colony. The EPA's target is below 60 percent relative humidity, ideally 30 to 50. Fix that and the smell goes.
What is probably happening
- Indoor humidity running high in summer: an oversized AC that cools without dehumidifying, a damp basement or crawl space, or a house sealed tight with no ventilation.
- Condensation on cold surfaces (windows, pipes, closet walls on exterior corners) feeding small, scattered growth you have not spotted yet.
- If the smell is much stronger when the AC runs, the system itself may be wet. That is a different leaf.
Do this in the next five minutes
- Buy a hygrometer. Read it in the main living area and the basement for three days.
- Above 60 percent: run a dehumidifier and empty it daily. Set it to 50.
- Run bathroom fans during and after showers, the kitchen hood when cooking, and vent the dryer outside.
- Check the closets on exterior walls and behind furniture on exterior walls for growth.
- If the basement or crawl space is the wet spot, that is the fix. Go back and pick that.
Do not
- Do not buy an air test to chase the smell. The EPA says no federal limits exist, and every house has spores in the air. The number will not tell you where the moisture is.
- Do not fog the house. It removes nothing and the moisture stays.
- Do not run the AC fan on 'on' all the time in humid weather. It re evaporates water off the coil and pushes it back into the house.
Who to call
An HVAC contractor, if a dehumidifier alone does not fix it
Persistent high humidity with the AC running is usually a system sizing, airflow or drainage fault. A mold company cannot change that.
BestHonestPro is a free comparison resource, not a contractor. There is nothing to sign up for and we do not sell your details. When we begin connecting callers to contractors, we will be paid a referral fee by the contractor, never by you, and this line will say so.
What it should cost
$100 to $250
HVAC service call fee, national range. Whole home dehumidifier work priced separately
Source: Angi, HVAC repair cost, 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- What is the indoor humidity you measured, and what is the system doing about it?
- Is the AC sized right for the house, and how do you know?
- Is the condensate drain clear and the coil dry?
- Would a whole home dehumidifier be cheaper than fixing the AC's dehumidification?
End the call if
- A mold company selling a whole house treatment for a smell with nothing visible.
- An air test with a 'high spore count' and no humidity reading.
- A new AC system quoted on the first visit with no diagnostic readings.