Humidity, mold and your AC: the Houston triangle

The triangle

Houston air is wet most of the year. The air conditioner is the only thing drying the inside of the house, and it only dries while it is running. When the AC is sized wrong, installed wrong, or fed by leaking ducts, the house stays cool and stays damp. Mold follows. That is the triangle: humidity, the AC and mold are one problem, and the fix usually starts in the attic, not in the wall.

Why an oversized AC causes humidity

An air conditioner removes moisture by running air across a cold coil long enough for water to condense and drain away. A unit that is too big for the house hits the thermostat setting in a few minutes and shuts off. The coil never gets cold enough for long enough. The air is cool but still wet. On a mild spring day a badly oversized system might run ten minutes an hour, and the house sits at 65 percent humidity.

Houston houses are oversized often, because installers round up to be safe, because a bigger number sounds better, and because nobody did a load calculation. A proper sizing is called a Manual J calculation and it uses the house’s square footage, insulation, windows, orientation and ductwork. Ask for it on any replacement bid.

The attic and the ducts

Most Houston ductwork runs through a vented attic that reaches 120 to 140 degrees in summer. Ducts leak, and the return side leaks matter most. A leaking return duct pulls hot, humid attic air into the system and into the house. A leaking supply duct dumps cold air into the attic, where it condenses on the ducts and drips onto the ceiling. A duct system losing 20 to 30 percent of its air is common in houses that have never been tested.

Other attic problems that show up as humidity: an air handler with a dirty or iced coil, a drain line that is clogged so the pan overflows, insulation missing from the ducts, and a plenum that was never sealed.

Signs of a humidity problem

• Hygrometer reading above 60 percent for days at a time.
• Condensation on supply registers or on the inside of windows.
• Musty smell, especially when the AC starts.
• Dark spots on ceilings near registers, in closets on exterior walls, or behind furniture against outside walls.
• Doors and drawers that swell. Books and paper that curl.
• Bathroom ceilings that stay wet after a shower.

Signs of mold

Mold looks like dark, green, white or fuzzy spots that spread. The common Houston places are the AC supply registers, the coil and the air handler cabinet, the ceiling under the attic unit, bathroom ceilings, closets on exterior walls, under sinks, and behind baseboards after any water leak. A musty smell with no visible mold often means it is inside a wall or in the ductwork.

What a competent HVAC technician checks

A technician who understands humidity measures rather than guesses. On a humidity complaint, expect:

1. Indoor temperature and relative humidity, and the humidity at the supply registers.
2. Refrigerant charge, by superheat and subcooling, not by feel.
3. Airflow across the coil, and the blower speed setting. Too fast an airflow sheds moisture back into the house.
4. Coil condition and the drain line.
5. Thermostat settings. The fan should be on AUTO, not ON.
6. Duct leakage, with a duct blaster test or at minimum a visual inspection of every connection.
7. Static pressure, which reveals undersized returns and crushed ducts.
8. A load calculation if the unit is suspected to be oversized.

If the technician’s first and only recommendation is a bigger unit, get another opinion.

The fixes, in rough order of cost

• Set the fan to AUTO, replace the filter, clear the drain line.
• Seal the ducts, especially the returns, and insulate them properly.
• Correct the blower speed and the refrigerant charge.
• Add a whole-house dehumidifier tied into the ductwork, which handles mild days when the AC does not run.
• Replace the system with one sized by Manual J, often a variable-speed unit that runs longer at lower output.
• Encapsulate the attic with spray foam, which brings the ducts into conditioned space. This is a larger project with its own trade-offs, and it needs a contractor who understands how it changes the house’s ventilation.

Texas mold licensing

Texas regulates mold work through the Texas Department of Licensing and Regulation. For mold over 25 contiguous square feet, a licensed mold assessor must inspect and write a remediation protocol, a separate licensed mold remediator does the work, and the assessor comes back to clear it. The same company cannot assess and remediate your project. This separation exists so that the person finding the mold is not the person paid to remove it. Below 25 square feet, a homeowner or a general contractor can clean it, and the usual approach is to fix the moisture source, remove the affected material and clean the surrounding area.

When to involve a mold assessor

• Visible mold larger than a closet door, or in more than one room.
• Mold that returns after cleaning.
• Any water event that soaked walls or floors for more than two days.
• Musty smell with no visible source.
• Anyone in the house with asthma, allergies or respiratory symptoms that improve when they leave.
• Before buying a house with signs of past water damage.

Start with the moisture. A remediation that does not fix the AC, the duct leak or the roof is a cleaning, and the mold comes back.