Dew point, in one paragraph
Relative humidity tells you how close air is to saturation at its current temperature, which makes it a confusing number to reason with. Dew point tells you something simpler and more useful: the temperature at which that air must give up its moisture as liquid water. Air at a 72-degree dew point will condense on anything cooler than 72 degrees, full stop. In this region, dew points in the upper 60s and low 70s are routine from May into October.
Now list every surface in your house that runs cooler than 72 degrees in July: the basement slab, foundation walls, crawl space joists and subfloor, uninsulated ductwork, cold-water supply lines, the metal frames of large windows with the AC running, and toilet tanks. Each of those is a condensation site for four to five months of the year, and none of them requires a leak of any kind.
The attic version
Attic mold in Rockville is usually built in winter and discovered later, but summer contributes its own mechanism. Warm humid outdoor air enters a hot attic; where uninsulated or poorly insulated ductwork carrying 55-degree supply air runs through it, that air condenses on the duct surface and drips onto the insulation and framing below. Homeowners find a wet patch of insulation, assume a roof leak, and call a roofer who correctly finds nothing wrong with the roof.
The winter mechanism is the more common cause of sheathing staining: warm moist indoor air leaking upward through the ceiling plane and condensing on a cold roof deck. Look for grey staining concentrated along the lower third of the sheathing, heavier on the north slope, often worst directly above a bathroom. The three causes are almost always a bath fan venting into the attic instead of outdoors, soffit intake blocked by insulation stuffed into the eaves without baffles, and air leakage through recessed lights, the attic hatch, and open top plates. Full detail is on our attic mold page.
The crawl space version
This is the clearest example of summer condensation in the region. A vented crawl space invites 72-degree dew point air through the foundation vents onto joists and ducts sitting at 60 to 65 degrees. It condenses immediately. Wood moisture content climbs past the roughly 20 percent threshold where growth takes hold, fiberglass batts absorb and hold the water, and ground vapor rises from the clay below the entire time. Because the stack effect draws air upward from the lowest point of the house, a share of that air ends up in your living space. That is why the fix is encapsulation rather than more ventilation.
The air conditioning problem
An air conditioner dehumidifies as a side effect of cooling: warm indoor air passes over a cold evaporator coil, moisture condenses on the coil, and it drains away. That process only works while the system is running.
A system sized well above the actual load satisfies the thermostat in short bursts. The house gets cool quickly, the compressor shuts off, and very little moisture is removed. The result is a house at 72 degrees and 62 percent relative humidity that feels clammy, with growth appearing on closet walls, behind furniture on exterior walls, and in the basement. This is extremely common in the tighter homes built from the late 1990s onward in King Farm, Fallsgrove, and similar communities, and it is also common wherever an older system was replaced by rule of thumb rather than a load calculation.
Fixes, in rough order of cost: a whole-house or portable dehumidifier to handle the latent load directly; setting the fan to auto rather than on, since a fan running continuously re-evaporates moisture off the coil and back into the house; a variable-speed or two-stage system that runs longer at lower capacity; or right-sizing the equipment at replacement time based on a proper load calculation.
Your summer checklist
- Keep basement and crawl space relative humidity between 45 and 55 percent, verified with a hygrometer, not by feel.
- Keep basement windows closed. Ventilating with 72-degree dew point air makes it worse, not better.
- Run bath fans 20 to 30 minutes after showers, ideally on a timer switch, and confirm they vent outdoors.
- Set the HVAC fan to auto rather than continuous during humid months.
- Insulate any accessible cold-water lines and ductwork that sweat.
- Keep furniture a couple of inches off exterior and below-grade walls so air can move behind it.
- Shut off any whole-house humidifier before spring; it belongs to winter only.