I agree with Lisa and Ken that the instrument needs to adapt to it's environment. In fact I think that more humidtiy is better than the opposite: One of my gutiars lost it's bridge due to the winter room climate (coming from the gas oven). The luthier, who fixed it explained to me, that wood continues to "work" (means changing depending on it's environment). He even gave me a handout exlaining how different degrees of humidity influencing the wood: Although it's for guitars I guess it's not much different for dulcimers, it's still a plucked wood instrument too :)
I will try to give the most important things from it (Text is from Tobias Ahlke Luthier at Essence Guitars, Oberwinter-Remagen, Germany, translated with DeepL by me). If somebody here understands German I can also post a scan from it.
Although it might sound a bit scary I also agree with Ken that you are overthinking it. Even if the dulcimer might suffer a bit from the climate at your sisters place, it should be fixable and in the worst case she can always get a new instrument. But now to the wise words of Mr. Ahlke:
"Wood is hygroscopic. This means that even after decades of storage and in the installed state, it can still absorb moisture from the air or release it into the air. If it absorbs moisture, it increases in dimensions; if it releases moisture, it shrinks.
The ideal humidity for guitars made of solid wood is about 50% relative humidity at a normal room temperature of about 21%. The woods of your guitar are stored and processed in my workshop at a controlled humidity between 45% and 50%. In this range your instrument can be played great and sounds best. If the humidity deviates, typical symptoms quickly appear and massive irrversible damage can occur:
- 60% relative humidity and above: The string action may increase, the curvatures of the top and back are exaggerated, glue bindings may break, frets and tuners are dull and tarnished, uncoated strings oxidize excessively fast.
- 40% relative humidity and below: The fingerboard shrinks, which is noticeable protruding fret edges. If necessary, the string action decreases. The wood is under tension
- 35% relative humidity and below: The grain of the wood is clearly formed through the varnish (e.g. the ceiling looks streaky). Cracks may appear in the ceiling and floor
- 30% relative humidity and below: Glue joints can break, cracks become larger and larger, the statics of the instrument are in danger!
- 25% relative humidity: glue joints can break, frets become loose, the instrument unplayable, maybe forever!
Humidity can be easily controlled with commercially available electric hygrometers, which are either stored in the instrument's case or placed at the instrument's location. If you do not want to or cannot regulate the humidity in the whole room, it is recommended to place a humidifier or dehumidifier near the instrument in the closed head. From our point of view, the "Humidipak" from D'Addario/Planet Waves has proven itself here, as this can keep the relative humidity constant at 48%, regardless of whether humidification or dehumidification is required.
Climate and temperature
A guitar feels at home where the player also feels at home: Neither does one like to sit in the blazing sunlight, in the middle of a draft, nor in front of the turned-up heater or in a damp cellar.
Temperatures between 0 and 40 degrees Celsius (32 - 104 ° F) are usually no problem for your guitar, although you should make sure that the guitar can slowly get used to the new temperature. Otherwise, fine cracks may appear in the wood and varnish and glue joints may be damaged.
40 degrees Celsisus (104 °F) and above: the woods begin to bend with the string tension, deformation occurs. The varnish may soften, feel sticky, or develop pressure marks under light loads.
0 degrees Celsius (32 ° F) and below: wood, varnish and glue joints are cold, hard and brittle, cracks and breaks are likely to occur with stress."
So I think a high humidity might less a problem than the other extrems (lower humidity and to much heat): At least the potential results sounds fixable. If the climate is really to much for the dulcimers storing them in a case with a Humidipak or a simmiliar system ( https://www.daddario.com/products/accessories/humidification/automatic-humidipak/humidipak-maintain/ ) should help. Note: As said above this tool and the hints from Mr Ahlke are for guitars, but since dulcimers are made of wood too, I thought, they might be helpful :)
updated by @jost: 05/24/21 01:32:10PM
If somebody asks for it I can scan Mr Ahlkes handout.