What Humidity Should a Violin or Cello Case Be Kept At? (And Why 62% Is the Wrong Number)

Music shops, school music departments and repair benches ask us the same thing every autumn and spring: what humidity should a violin or cello case be held at, and will a two-way pack do it? Very often the enquiry arrives having already seen a 62% pack — 62% is the number the cigar and botanical markets made famous.

The short answer is 45–55% RH, with 40–60% as the working envelope. 62% is the wrong set point for a wooden instrument, and the reason is worth understanding rather than taking on trust — so this article gives the numbers the conservation and forestry literature publishes, what goes wrong at each end, why stability matters more than the set point, and where 62% really comes from.

What the authorities actually say

Several independent bodies — museum conservation science, national conservation institutes, working luthiers and instrument retailers — have arrived at overlapping answers from completely different directions. They do not all publish the same number, but the overlap is narrow.

Stack those and the common ground is 45–53%, with the band up to 55% still comfortably inside every source's acceptable range — though one authority already calls 53–60% cautionary, so the top of the band is the part to stay away from if you have a choice. Either way it sits inside the tighter material limit — hide glue's 30–60%.

Relative humidity is only a proxy. The real variable is the wood's moisture content

An instrument does not respond to relative humidity. It responds to its own moisture content, and RH is what drives moisture content towards equilibrium. The conversion is published: the USDA Forest Products Laboratory Wood Handbook tabulates equilibrium moisture content against relative humidity at a given temperature.

Now the recommended band stops being an abstraction. The whole of 45–55% RH is only 8.5% to 10.1% moisture content — a spread of 1.6 points. Moving an instrument from 50% RH to 62% RH is a jump of more than two full points of moisture content, larger than the entire recommended band. On a cello plate, across the grain, that is a real dimensional event, not a rounding error.

Two details make it worse than a simple swelling calculation suggests. Wood moves most tangentially, about half as much radially, and only slightly along the grain — so a quartersawn top glued to constraining ribs cannot widen or narrow freely. Forced to, it splits along the grain rather than shortening. And none of this involves free water: the fibre saturation point, above which further moisture stops causing dimensional change, sits around 30% moisture content — far above anything a case will see. Everything in the table above is the regime where dimensions change.

What goes wrong at each end

Too dry

Start with the insight that explains most winter repair bills: hide glue is engineered as a fuse. It is deliberately weaker than the wood around it, so when a top shrinks against constraining ribs, the glue line is meant to let go first. An open seam is the fuse blowing and doing its job. A crack means the fuse failed to blow.

The rest of the low-RH list follows from shrinkage: cracks near the saddle or soundpost, where stress concentrates; pegs shrinking and slipping; a thinner, harsher tone and buzzing from open seams; bow hair contracting until it is over-tight or brittle.

Too wet

Above the band, glue joints weaken as the hide glue takes up moisture. The neck bends, neck projection drops and the action rises; the fingerboard can depress. Pegs swell and jam. Bow hair lengthens and goes slack however far the frog is wound out. From roughly 70% RH, mould becomes a live risk on an organic, varnished, case-lined object.

The asymmetry matters more than either list. Low RH produces fractures — open seams, cracks — and a competent luthier can repair fractures. High RH produces creep and permanent set in the wood, which frequently cannot be fully reversed. Conservation guidance notes that areas buckled above 75% RH remain permanently distorted even after relative humidity returns to normal. If you are going to be wrong, slightly dry is the more recoverable error. High humidity is not the safe side.

Stability beats set point

The most important idea in the conservation literature is not a number. The ASHRAE classes as published by the Canadian Conservation Institute grade risk by fluctuation magnitude, not set point — which is why classes sharing a 50% target carry very different risk ratings.

Michalski's proofed fluctuation sharpens this: if future conditions do not exceed the range an object has already survived, the risk of further mechanical damage is negligible. An old instrument that has been through decades of a particular seasonal swing has already been tested to that range. It is the excursion beyond it that does the damage.

One honest caveat, because it is widely violated: no rate-of-change limit — no "% RH per hour" or "per day" figure — exists in the conservation literature. Anyone quoting one invented it. The literature supports magnitude, not speed.

So a stable 42% is safer than a 45% average that swings between 35% and 58% every week. But that only holds inside the material safe zone: a rock-steady 25% or 75% is not safe at any degree of stability, because the damage there is not caused by fluctuation.

So why is 62% sold at all?

Because two-way packs were developed and scaled for other markets, each with its own correct set point. The ladder looks like this.

Typical stated accuracy across these products is ±2% RH in an airtight environment, and a second manufacturer's instrument-specific product holds 45–50% RH. Note what that means: the manufacturers themselves separate the instrument set point from the botanical one. 62% was never offered as an instrument number.

The evidence that 62% is wrong for a violin is not one argument but four, pointing the same way. The manufacturers' own segmentation puts instruments at 45–50%. 62% is the outer edge of wood's measured safe range of 30–62%. It is outside hide glue's 30–60% altogether — and hide glue is the joint holding the instrument together. And the moisture-content gap from 50% RH is more than two points, larger than the entire recommended band. The predicted symptoms are precisely the documented high-RH failure list: slack bow hair, jammed pegs, rising action, weakened joints.

One genuine nuance, because it causes confusion: 72% packs really are sold for wood instruments. They are for seasoning a new, dry, empty case, so the lining does not act as a sponge and steal moisture from the instrument placed inside. They are never for storage with the instrument in the case. If a shop sells you a 72% pack, ask which of those two jobs it is for.

What a case actually does — and what it does not

A closed case is a buffer, not a climate. It damps short-term weather events: a cold snap, an afternoon of hotel air conditioning, an hour in a car. What it does not do is change the equilibrium the instrument eventually reaches. Leave a case in a room at 30% RH for a fortnight and the instrument inside is heading for 30% RH, pack or no pack, unless the pack has capacity to keep giving.

Be sceptical of anyone who quantifies this. No published measurement of the case-to-room humidity differential, or of a case's time constant, exists. The buffering effect is real and well attested qualitatively; the number is not. If a supplier tells you a case holds "X% above ambient for Y hours", they made it up.

Measure it properly, or you cannot diagnose anything

The dial-type analogue hygrometer built into many cases is decorative. Those movements are uncalibrated and drift freely. This matters more than it sounds: if the gauge is wrong, you cannot tell whether a humidity pack is working, and the usual result is that the pack gets blamed for a reading error.

Use a factory-calibrated digital hygrometer, keep it in the case with the instrument rather than the room, and read it before changing anything. Every decision — whether to add a pack, which set point, whether to intervene at all — rests on a trustworthy number.

Buying, and stocking, two-way packs

For a wooden string instrument, specify a two-way element at an instrument set point — the ATMOSIScience instrument set point is 49% RH, in the middle of the 45–55% consensus and inside every material limit above. The platform is a plant-fibre substrate carrying a humidity-control composition: it both absorbs and releases, defending a set point rather than simply drying the air.

Shops, schools and repair benches regularly ask about stocking and reselling these. That conversation — dealer terms, volume, printing and pack formats — starts at info@atmosiscience.com.

Frequently asked questions

I already put a 62% pack in my violin case. Have I damaged it?
Probably not in a short exposure, particularly if the room was dry to begin with and the case never reached the pack's set point. Swap it for an instrument set point, put a calibrated hygrometer in the case, and look for the tell-tales: slack bow hair, stiff pegs, action creeping up.

Is 40% or 60% safe if I cannot hold 45–55%?
Both are inside the published working envelope, so yes for ordinary use. But they are the edges, not the target, and 60% is the less forgiving edge because high-RH damage is the kind that does not fully reverse.

Does a cello need something different from a violin?
The set point is the same — same wood, same glue. The difference is scale: a cello plate is far wider, so the same change in moisture content produces a much larger absolute movement across the grain, and the case is a bigger volume to buffer.

How fast is too fast a change?
No published rate-of-change limit exists — that is the honest answer. Control the magnitude of the swing and the direction of travel, and treat any "% per hour" figure you are quoted as marketing.

Do I still need a pack if my room is humidity controlled?
If the room genuinely holds 45–55% year-round, the case is already in a good environment and a pack adds little. Packs earn their place in transit, in dry winters, in heated or air-conditioned buildings, and anywhere the instrument leaves that room.

Tell us the instrument and the environment

If you are specifying humidity packs for a shop, a school inventory or a collection, tell us the instrument type, the case volume, the climate and the season you are protecting against. We will come back with the set point and the format — including when the answer is that your room control already does it.

Start on the fiber technology page, or email info@atmosiscience.com.

Related reading: Two-way humidity liner or desiccant: which one do you actually need? · Humidity control liner