Damp in the Sanctuary: Moisture and Mold in Churches and Historic Public Buildings

By John Calhoun, owner of Mold Remediation Gettysburg PA · Updated October 29, 2026

Some of Gettysburg’s oldest public buildings have already been through more than any leak will ever throw at them. Main Street Gettysburg’s markers record that Christ Lutheran Church on Chambersburg Street threw open its doors as a hospital during the battle, that St. Francis Xavier served as a field hospital, and that the German Reformed church, now Trinity United Church of Christ, hung a red banner from its cupola to mark itself as one. None of this is about any particular building; it’s a reminder that buildings like these deserve careful moisture work. The short answer for any church, hall or older public building: find the source before you treat anything, keep the indoor climate steady instead of swinging it, and plan the work around the people who use the building.

Why big old buildings get damp

The National Park Service’s Preservation Brief 39 on controlling moisture in historic buildings lists five common sources, and every church committee should know them:

1. Rain and surface water on the outside: roofs, gutters, downspouts, flashing, splash-back against the walls.
2. Ground moisture below grade: damp soil against foundation walls and under floors.
3. Leaking plumbing and mechanical equipment: old supply lines, boilers, radiators, condensate drains.
4. Moisture from use and climate control: people, kitchens, humidifiers, and air conditioning that cools surfaces below the dew point.
5. Water in maintenance and construction: wet cleaning, fresh plaster or mortar, pressure washing.

Its first rule is plain: avoid remedial treatments without prior careful diagnosis. That’s my rule too. A stain on a sanctuary wall can start at a gutter well away from it.

The Sunday-only heating problem

Most houses are lived in every day. Many churches are heated hard for a few hours a week and left cool the rest of the time. That’s where trouble starts.

Preservation Brief 24 puts it bluntly: ill-informed and haphazard attempts to adjust comfort levels, or to save energy over weekends and holidays, can cause great damage. The Institute of British Organ Building’s guide on church heating says heaters run only when needed may cause condensation, and that surveys found much less dry rot in continuously heated buildings than in intermittently heated ones. Warm, damp air from a full Sunday service hits cold stone walls, cold windows and a cold organ chamber, and the water ends up on those surfaces.

The organ is a moisture gauge

If your building has a pipe organ, the organ technician is already thinking about humidity. The British guide recommends keeping relative humidity between 55% and 75% around organs during the heating season and calls readings below 50% potentially dangerous to them. A U.S. organ builder suggests staying above 30% in winter and below 80% in summer, and warming the building gradually over at least 24 hours after a cold week.

Those ranges are written for wood and leather in an organ, not for mold prevention, and they sit higher than the EPA’s household advice of below 60%. That’s exactly why a church needs a plan rather than a single number. I’d get the organ technician and the person who runs the heat in the same conversation.

Don’t seal the walls

One costly mistake on old masonry is a coat of waterproof paint on a damp basement wall. Preservation Brief 39 says moisture should not be sealed into the wall with a waterproof coating. Old stone and brick need to dry. Trap the water behind a coating and it finds another way out, often into the plaster or the wood framing next to it. If the wall shows white crust rather than growth, read efflorescence vs. mold first, and if it’s a stone foundation, mold in fieldstone and rubble foundations covers how those walls behave.

Working around services and people

The EPA’s guide for schools and commercial buildings, which fits churches and halls well, says it’s important to communicate with occupants when mold problems are identified and that remediation should be scheduled during off-hours. It sorts jobs by size: under 10 square feet, 10 to 100, and over 100, with more containment as the area grows. It also says to leave a suspect HVAC system off until it’s checked.

In practice that means a short note to the congregation or the user groups, work scheduled midweek, containment around the area, and a clear finish line with readings before the room is used again. Containment, negative air and HEPA scrubbers explains how the work area stays separate.

The Gettysburg angle on old public buildings

In the borough’s historic district, a seven-member Historic Architecture Review Board reviews changes visible from a public street, alley or space, and most building permits there wait on a certificate of appropriateness. Interior mold cleanup rarely triggers that. The moisture fix often does: gutters, regrading, windows, a vent through a wall. Talk to the borough early. The Gettysburg housing data page has the age of the buildings around them. Old buildings everywhere carry deferred maintenance; a 2017 Pew study of Philadelphia’s 839 historic sacred places noted many had survived decades of it. A good moisture survey is how a committee decides what to fix first.

This is John Calhoun at Mold Remediation Gettysburg PA. If your building smells damp or a wall keeps staining, I’ll survey it with a meter and a thermal camera and give the committee a plain report. The list of mold services and the overview of mold remediation in Gettysburg, PA cover how I work.