Guitar humidifiers, and whether you need one

What dry air actually does

An acoustic guitar is thin wood under tension. Wood gives up moisture to dry air and shrinks as it does, and the top — the thinnest, widest piece, glued at its edges and braced across its grain — cannot shrink freely. Something has to move. First the top sinks and the action drops until the strings buzz on the frets; then the frets start to feel sharp at the edges as the fingerboard shrinks across its width; and in the worst case the top splits along the grain, usually beside the bridge or the fingerboard.

The timing is seasonal and predictable. The dangerous period is the heating season: indoor air that was fine in September can fall to single-digit relative humidity in January in a heated home in a cold climate. That is also the period when a guitar bought as a Christmas present spends its first weeks in exactly the wrong conditions.

How much this matters depends on the guitar. A laminated top is far less prone to cracking than a solid one — it is plywood, and plywood is dimensionally stable, which is the one genuine advantage laminate has. If your beginner guitar is laminate, the honest answer is that you probably do not need any of these products. If it has a solid top, as the better picks on best beginner acoustic guitar do, keep reading.

Measure before you buy anything

This is the part most pages skip, and it is the part that decides whether any of the rest is money well spent. Humidity varies enormously by climate, season, heating type and even which room the case is in. A hygrometer that lives inside the case tells you the one number that matters, and it costs less than most humidifiers.

The widely published target range for a solid-wood acoustic is roughly 45 to 55 percent relative humidity, and the rule of thumb most repair shops use is that below about 40 percent you should be adding moisture and above about 60 percent you should be removing it. If the case reads inside the range all winter, the correct purchase is nothing at all.

One-way or two-way

A one-way humidifier — a sponge or a gel tube in the soundhole — adds moisture and cannot take any away. It works, and it is cheap, and it depends entirely on somebody remembering it. Forget it for a month and it is an empty sponge; over-soak it and water sits next to a lacquered finish. In a basement or a humid summer it can also push a case past the top of the range, which causes its own problems: a swollen top, a high action and glue joints under stress.

A two-way system uses packs that absorb moisture as well as release it, holding the case at a set range with no judgement required. There is nothing to fill, nothing to drip and no weekly decision to forget, which is why it is the recommendation for a teenager’s guitar. The trade is cost: the packs are a consumable and need replacing a few times a year, so over two years it costs more than a sponge. It is also the only type on this page that protects against the wrong direction.

What not to do

Do not put a damp cloth, a wet sponge in a bag, or a slice of anything in a guitar case. Direct contact between water and a finish or a glue joint causes a different and worse problem than dry air does. Do not store a guitar on a stand next to a radiator or a heat vent, which is the fastest way to dry one out — the case is the safe place in winter, which is a real argument for a case over a guitar stand during the heating season.

And do not trust a reading from a hygrometer sitting in the room. The case is a different micro-climate from the room it is in, and the case reading is the one that governs the guitar.

The seasonal routine, and rescuing a dry guitar

The calendar is the easy part. Check the case reading when the heating goes on for the first time, which in most of the country is October, because that is the week the indoor humidity falls. Through the winter the job is maintenance: top up a one-way humidifier on whatever schedule it needs, or replace two-way packs when they go hard. In spring, when the heating goes off, take a one-way humidifier out — leaving a sponge in a case through a humid summer causes the opposite problem, and a swollen top with a high action is just as unpleasant to play as a sunken one.

If a guitar has already dried out, re-humidify slowly. Put it in its case with a humidifier and leave it closed, checking the reading every few days rather than every few hours; wood that is brought back fast moves fast, and movement is what cracks a top. A sunken top and a low, buzzing action will often come back on their own over a fortnight of this. What will not come back is a crack, a lifted bridge or a loose brace, and all three are a repair technician’s job rather than a home one. On a beginner guitar, get a quote before committing to the repair: a crack can cost a real fraction of what the instrument cost, which is the whole argument for a twenty-dollar hygrometer in October.