How Humidity Affects Pipe Organ Materials and Reliability

Wood, leather, glue and paint take up and give off water as the relative humidity around them changes, and their size changes with their moisture content. The Canadian Conservation Institute describes this as the basic mechanism behind humidity damage to organic materials: the material is not harmed by swelling or shrinking in itself, but by being restrained while it tries to, whether by neighboring parts or by its own bulk during a fast change, so that the expanding portion is crushed and the shrinking portion can fracture. The Institute states this for collection objects generally rather than for organs, so reading it onto a windchest is an inference rather than something the Institute itself says.

The American Institute of Organbuilders describes the same behavior in the instrument itself: the organ’s wooden components expand, contract and may twist as humidity changes, and several days of continuous winter heating with extreme dryness can cause serious cracking. Tobit Curteis and William McVicker, writing in the 2025 Building Conservation Directory, describe long-term and large fluctuations causing organic elements to warp or split, elevated humidity causing timber soundboards and stop mechanisms to swell, and fluctuation leaving leather stiffened or cracked.

Humidity acts on metal pipes through corrosion rather than through the swelling and shrinking that affects wood and leather. Curteis and McVicker describe elevated relative humidity causing condensation in lead pipes and leading to lead acetate corrosion, sometimes called sugar of lead. How strongly corrosion depends on humidity is disputed in the published research, and that disagreement is set out with the damp-season symptoms below.

The three organ-building bodies whose guidance is in general circulation give three different envelopes. The American Institute of Organbuilders says a humidity gauge inside the organ should generally stay above 30 percent in winter and below 80 percent in summer, and that a humidifier should be considered below 30 percent and a dehumidifier above 80 percent. Those are outer limits, not a target. The Institute of British Organ Building, in its leaflet on church heating and the organ, gives a safe range of 55 to 75 percent and says readings below 50 percent are potentially dangerous. The British Institute of Organ Studies advises owners of historic organs to keep relative humidity at 55 percent or above, while acknowledging that this is often not achievable when the building is continuously heated.

The American winter floor is markedly lower than the British figures, which is consistent with different climates and heating regimes rather than with one body being wrong, and it is the first reason a number cannot be lifted from one leaflet and applied to any building.

Builders and service firms publish their own bands, and those differ from each other as well. Orgues Létourneau describes roughly 35 to 75 percent as tolerable for its instruments, with about 45 to 55 percent preferable for tuning stability. Bogue Services recommends 40 to 60 percent, with conditions closer to 50 percent preferred. Dodington & Dodington Pipe Organ Services gives a target of 50 to 60 percent, never above 80 and never below 30. David E. Wallace, in a 2014 American Guild of Organists convention handout, advises 35 percent or better through the winter heating season.

Church Music Dublin, in diocesan guidance approved in 2019, calls for a steady 60 percent while acknowledging that this is rarely achieved in churches. Bingley, Knight and Xing, in a 2025 paper on church organ tuning records, cite a recommended conservation range of 45 to 55 percent for church organ environments. The Canadian Conservation Institute recommends 45 to 55 percent for leather objects in collections, warning of embrittlement below 30 percent and mold above 65 percent; it is a collections recommendation, not an organ specification.

A single reading cannot by itself show that an organ’s environment is out of specification, because the published targets run from a 30 percent winter floor to a 55 to 75 percent safe range depending on who issued them and for what climate. The comparison that matters for a particular instrument is its own historical range and its builder’s stated expectation.

Time matters as much as the number. The American Institute of Organbuilders ties serious cracking to several days of continuous winter heating, not to a momentary low, so a fault report is more useful when it records how long the building has been heated as well as the reading at the time of the complaint.

Damage tracks the size and speed of the humidity swing more closely than it tracks any single level. For a medium-sensitivity object under uniform restraint, the Canadian Conservation Institute puts a fluctuation of plus or minus 10 percent at no damage to tiny damage, plus or minus 20 percent at no damage to small damage, and plus or minus 40 percent at small to severe damage. Those classes are stated for collection objects, not for windchests, but the shape of the relationship is what an owner needs: reducing the swing is what reduces the damage.

The Institute also describes what it calls a proofed fluctuation: an object that has already been through a very low humidity, around 10 percent, or a very high one, around 80 percent, is not susceptible to further mechanical damage from one more event of the same size. That offers an older instrument some reassurance about swings it has already survived, but the Institute limits the principle to objects not weakened by other deterioration, and it is not a license to allow extremes.

Warming the air in a building lowers its relative humidity unless water is added to it. In an unhumidified space, the Canadian Conservation Institute states, a temperature change of 1 degree Celsius moves relative humidity by about 3 percent; the relationship changes where the space is humidified or ventilated with outside air. This is why the American Institute of Organbuilders speaks of winter heating and extreme dryness in the same sentence, and why the British Institute of Organ Studies ties its humidity target to how continuously the building is heated. How the building’s heating and cooling system should be run is a temperature question, with its own page: Temperature, HVAC, and Pipe-Organ Tuning Stability.

When the building atmosphere becomes too dry, leather dries and splits, and the Institute of British Organ Building says the result is ciphers or silent notes in organs with pneumatic mechanisms, or leaking bellows. One cause can therefore produce two opposite symptoms in the same instrument, so neither symptom on its own points to leather; the distinguishing evidence is the physical condition of the leather, which has to be seen.

The Canadian Conservation Institute puts leather embrittlement below 30 percent relative humidity, and Stephen Adams, an organbuilder in Ireland, reported bellows splitting in a building reading 40.9 percent. Low humidity is one cause of leather failure; the aging of the leather itself is a separate matter.

Dry wood shrinks, and in an organ that shows up as parts that stick, leak or crack. The Institute of British Organ Building lists trackers drying out and cracking, causing silent notes; sliders sticking so that stops will not pull out; and soundboards cracking internally, causing runnings or murmurs. Bingley, Knight and Xing report that when relative humidity fell below 30 percent, wooden organ components experienced drying-induced stress in the form of fractures and cracks, and Orgues Létourneau reports humidity change as a contributing factor in splitting at the base of an older wooden pipe.

A silent note has many non-environmental causes, a stop that will not move may be obstructed at the drawstop action, at the trace or in an electric stop action, and a running or murmur may equally be a leaking pallet, a leaking conveyance or a valve fault. Internal soundboard cracking is not visible from outside and can only be confirmed by a technician with access. What points toward the environment is timing: faults of this kind appearing after several days of continuous heating, which is worth recording before anyone opens the instrument.

Damp air reverses the dry-season picture: wood swells and leather softens. Condair, a humidifier manufacturer, states that excessive humidity causes wooden parts to swell and misalign moving parts or cause a cipher, and that leather components such as bellows, gaskets and valve pouches become soft and susceptible to mold. Curteis and McVicker describe elevated relative humidity swelling sensitive organic elements such as timber soundboards and stop mechanisms.

Mold growth begins at about 65 percent relative humidity and becomes strongly active above 85 percent, according to the Canadian Conservation Institute, and molds attack damp cellulosic and proteinaceous materials, leaving both staining and digestive damage. The Institute states separately that relative humidity above 65 percent is favorable for mold on leather, skins and parchment, which sets an upper boundary for organ leather independent of the embrittlement risk at the dry end. These are general thresholds across species and substrates; particular molds differ, and surface conditions matter as much as the room reading.

Damp air can corrode lead-based pipe metal, but the published research disagrees about how much humidity by itself drives the process. A University of Wisconsin-Madison chemistry department review reports that at 95 percent relative humidity corrosion products accumulate substantially faster than at moderate humidity, and that pipes holed by advanced corrosion can lose the ability to sound. Annika Niklasson’s 2007 doctoral thesis at Chalmers University of Technology, by contrast, found that lead corrosion showed a relatively weak dependence on relative humidity across the 50 to 95 percent range.

Both results come from laboratory and documentary work on lead-tin pipe metal rather than from measurement inside working organs, and they are presented here as two positions rather than one. What the organ-side sources agree on is the event to avoid: condensation forming on lead pipes in damp conditions. Water that has actually reached organ parts, from a leak or a flood, requires assessment for water damage.

Humidity affects tuning far less than temperature does. Bogue Services, a service firm, states this directly and adds that humidity acts on tuning mainly through the expansion and contraction of wooden pipes; Church of England ChurchCare guidance says that air that is too dry or too damp damages the instrument and that both conditions also affect tuning stability, without giving figures. Neither statement makes humidity the lever to pull when the complaint is that the organ sounds out of tune, and buying humidification to fix tuning misreads the problem. How room temperature moves pitch, the published figures for it, and the building-temperature questions that follow are the subject of Temperature, HVAC, and Pipe-Organ Tuning Stability.

What humidity influence there is on tuning falls mostly on wooden pipes, so an instrument’s wooden ranks are where any humidity-related drift would show. A stop that will not draw or a note that will not sound during a dry spell is a materials fault, not a tuning fault, even when it arrives in the same season as the tuning complaints. When an organ sounds wrong shortly after a tuning visit, the temperature-difference explanation is covered on Why an Organ Sounds Out of Tune After the Technician Leaves, and the wider map of tuning questions is on Pipe Organ Tuning Services.

Any change to humidity should be made gradually so the organ’s wooden components can adjust; both the American Institute of Organbuilders and Orgues Létourneau give this advice. A sudden correction is itself a fluctuation of the kind described above.

When an organ misbehaves during a swing in humidity, John Bishop’s advice to owners is to let the organ be for several days, because when conditions return to normal things will often start working again. That is advice about humidity-related misbehavior specifically; it is not advice to ignore mechanical or electrical symptoms, and Bishop himself lists separate observations that do warrant a call.

A humidifier must never be placed near a blower intake, and water must never be allowed to drip on any organ part; the American Institute of Organbuilders states both, without specifying a distance. A remedy meant to protect leather can damage the instrument if it is badly sited. The same guidance says a humidifier should be considered when the gauge inside the organ falls below 30 percent and a dehumidifier when it rises above 80 percent; those are its own thresholds and not those of the British bodies.

Published positions differ on whether to humidify the room or the organ. Dodington & Dodington Pipe Organ Services recommends a system that humidifies the air of the whole room as the most effective winter remedy and describes carefully placed local humidifiers as providing only marginal help. Talbot and colleagues, reviewing hygrothermal criteria for historic churches in 2022, note that an organ, given its complex structure and the many materials in it, may require localized humidity control rather than whole-building conditioning.

David Yearsley reported in 2013 that a 1746 Neapolitan organ at Cornell University, in a leaking nineteenth-century chapel where relative humidity runs from about 90 to about 20 percent across the year, had been fitted with interior humidifying bars of the kind used for piano soundboards. That is one instrument as described in 2013; it shows that the local approach is possible, not that it is common or still in place. The positions are not reconciled here; which is workable depends on whether the building can be conditioned at all, and whole-building conditioning of a historic church is often unattainable.

A humidity complaint is most useful to whoever inspects the organ when it comes with a record: the reading, where the gauge sits, how long the building has been heated or how long the damp spell has lasted, and which symptoms appeared when. Readings with dates set the environment against the instrument’s own history, which is the comparison a published band cannot make for you, and they give whoever examines the leather and the wood something to check against. Preparing the building for a technician’s visit, including what to record beforehand, is covered on How to Prepare for a Pipe Organ Tuning Visit.