Cool and still clammy: why a Lincolnville frame house and a Palencia stucco house get damp for opposite reasons
The thermostat says 75 in both houses. Both feel like a bathroom after a shower. One is leaking humid air in; the other cannot let its own moisture out. They need different repairs.
Two houses, one complaint
The first house is an 1890s frame two-story in Lincolnville, raised on piers over sandy ground a few blocks from the Matanzas River. Sash windows, a porch, retrofit ducts under the floor, a system that is a little too big. The second is a 2022 stucco two-story in Palencia, foam-sealed attic, casement windows, a heat pump sized by the builder’s engineer. On a mild morning in late May, both read 75 degrees and 66 percent humidity, and both owners describe it the same way: sticky.
They are sticky for opposite reasons, and that is why one fix does not serve both.
What the hygrometer should read in St. Augustine
Buy a hygrometer; they cost less than a filter and tell you more than the thermostat does. In summer the target indoors is 50 percent, and anything under 55 is comfortable. At 60 and above for days at a time, mold grows on cool surfaces, wood swells, and the house smells. Most St. Augustine homes we meter for the first time read in the 60s. Outdoors from May to October the dew point sits in the mid-70s, which means the air outside is nearly saturated; whatever gets in has to be dried by something.
The leaky house: moisture coming in
The Lincolnville house is being dampened from outside. Humid air enters through the crawlspace, around a century of window and door gaps, up through the floor, and through return ducts that leak in the crawlspace and pull outdoor air straight into the system. The air conditioner drips water all day and cannot keep up with what is coming in. On top of that it is oversized, so it runs short cycles that cool without drying.
The path for that house: seal and dehumidify the crawlspace, seal the ductwork, tighten what can be tightened in a historic house without touching the exterior, and when the system is due, size the replacement smaller with a two-stage or variable unit that runs long. A ductless system with dry mode is often the right end state for a house that was never built for ducts.
The tight house: moisture with nowhere to go
The Palencia house has the opposite problem. Almost no outdoor air gets in, which is good. But two adults, two kids, two showers, a dishwasher and a pot of pasta put a surprising amount of water into the air every day, and the only thing removing it is a heat pump that, in May and October, barely needs to run. The house is cool because the weather is mild and damp because nothing is drying it. In July the system runs long enough to do the job; in the shoulder months it does not.
The path for that house: a whole-home dehumidifier on its own humidistat, set to 50, ducted into the return, so the house is dried regardless of whether it needs cooling. Often an energy-recovery ventilator alongside it, because a house that tight also needs fresh air on purpose.
What fixes both, and in what order
Both houses benefit from the same first two steps: fan set to Auto rather than On, so water on the coil is not blown back into the house between cycles, and bath fans that actually vent outdoors and run twenty minutes after a shower. After that the paths split. The leaky house needs sealing before equipment; the tight house needs equipment before anything else. Both, when the system is next replaced, need it sized to the house rather than to the old nameplate, because in this climate the right-sized system is the drier one. We meter first, and the reading tells us which page of this plan you are on.
Humidity questions from two kinds of St. Augustine house
Our heart-pine floors cup every summer in our Lincolnville house. Is that the air conditioner's fault?
Partly. The boards take on moisture from a damp crawlspace below and dry from the conditioned room above, so they curl. The air conditioner is doing its half; the crawlspace is undoing it. A sealed, dehumidified crawlspace usually ends the cupping within a season.
Why is the new Palencia house damp in May when the heat pump barely runs?
Because the heat pump is the only thing drying it, and in May the house does not need enough cooling to run it long. A tight house on a mild, wet day has no dehumidification at all. A whole-home dehumidifier on its own control is what that house is missing.
Does a bigger air conditioner dry the house faster?
The opposite. A bigger unit cools the air to the thermostat setting quickly and shuts off before the coil has condensed much water. In St. Augustine’s dew points a slightly smaller system running longer removes more moisture, which is why right-sizing matters more here than in a dry climate.
We leave the Davis Shores house empty from May to October. What should the thermostat be set to?
Around 78 to 80 with a dehumidifier holding 55 percent, and a thermostat you can read from out of state. A house set to 85 with no dehumidification comes back in October smelling of mildew, and the coil has spent the summer wet and idle.
Cool but clammy?
We'll measure the house, tell you which kind it is, and fix the right one.