The paint is bubbling and the doors stick: humidity in a plaster-and-lath house

The symptoms, in the order they appear

A plaster-and-lath house reports its humidity before any instrument does. In Lincolnville, Nelmar Terrace, Fullerwood Park, the Abbott Tract and the Model Land district, owners describe the same sequence.

May. Interior doors begin to stick. The front door swells in its jamb. A window sash that slid in March needs a shove.

June. A hairline bubble appears in the paint on an exterior wall, often under a window or near the ceiling line. The bubble grows. If it is popped, the plaster underneath is faintly damp.

July. The closet under the stairs smells. Leather in the hall closet has a bloom on it. Books on the lowest shelf feel soft.

August. A dark line appears along the baseboard of a north room. Someone buys a hygrometer and it reads in the high sixties with the air conditioner running and the thermostat satisfied at 74.

That last reading is the whole problem in one number. The house is cool and wet.

Why plaster houses are different

Plaster over wood lath is a thick, porous, hygroscopic wall. It takes on moisture from the room air and gives it back, slowly, which is part of why these houses feel comfortable in spring and fall. Behind the plaster is an uninsulated wall cavity open at the top to the attic and at the bottom to the crawlspace, with no vapor barrier anywhere. Around every original window weight pocket, every baseboard gap and every floorboard seam, outside air moves in. On a St. Augustine summer day that outside air carries a dew point in the mid-seventies.

A drywall house built in 2005 has a vapor-retarding paint, insulated cavities, sealed sheathing and tighter windows. The plaster house has none of that. The same air conditioner, in the same square footage, has far more moisture to remove in the older house, and it is usually the wrong air conditioner for the job.

The mistakes, which are also the causes

Oversizing. The original central system, or its replacement, was sized by square footage or by whoever guessed biggest. An oversized unit satisfies the thermostat in a few minutes and shuts off. Moisture removal happens only after the coil has been cold for a while; a short cycle never gets there. The house is cold and damp, and the owner turns the thermostat down further, which makes it colder and no drier.

Fan on “On.” Continuous fan sounds like better circulation. In humid climates it re-evaporates the water sitting on the coil between cycles and sends it back into the rooms.

Window units left in. They dump condensate outside but seal poorly, and the gaps around them are open windows.

Sealing the wrong things. Owners caulk the visible gaps and leave the attic and crawlspace open to the wall cavities, which is where most of the moist air enters.

Portable dehumidifiers everywhere. They work, loudly and expensively, and they add heat. Three of them running in July can add noticeably to the power bill while the central unit’s cycle problem goes untouched.

Ask for a humidity assessment

The fixes, cheapest first

1. Thermostat fan to Auto. Free. Immediate improvement in many houses.
2. Measure. A load calculation for the house as built, a static pressure and airflow check on the existing system, and a week of hygrometer readings in two or three rooms. We want to know whether the equipment is oversized, whether the airflow is wrong, and how bad the number actually gets.
3. Seal the top and bottom. Attic-side air sealing at the top of the wall cavities and around chases, and a vapor barrier or encapsulation in the crawlspace, cut the moist-air entry at its sources. This is building work rather than HVAC work, and we’ll say when it should come first.
4. Fix the drain and the coil. A dirty indoor coil removes less moisture; a slow drain keeps water in the pan evaporating back into the airstream. Both are maintenance items.
5. Add dedicated dehumidification. A whole-house dehumidifier ducted into the return handles the moisture independently of the cooling, so the air conditioner can be sized for heat and the house can sit at 76 and feel dry. In most plaster houses this is the fix that ends the sticking doors.
6. Right-size at replacement. When the oversized unit reaches the end of its life, replace it with a correctly sized two-stage or variable-speed system, or with ductless heads that run long at low output. Smaller, running longer, is the whole idea.
7. Address the windows and walls as part of a restoration, with weatherstripping that respects the original sash. This is the long project, and the other six steps do not have to wait for it.

What the numbers should look like afterward

Indoor relative humidity in the mid-forties to mid-fifties in summer, with the thermostat at 75 or 76 and the house feeling cooler than that number suggests. Doors that open in July. Paint that stays put. A closet under the stairs that smells like a closet. Those are the targets, and a plaster house in St. Augustine can hit them without losing a single piece of original material.