From follower Bryan G,

Is it necessary to use distilled water when filling the guitar soundhole humidification devices such as the dampit® and why or why not?

Tap water will work just fine in an acoustic guitar humidifier. The issue however is that the calcium, magnesium and iron found in the tap water will form scale over time and block the humidification activity sites on the device. This will cause the unit to eventually lose its ability to humidify.

With reusable guitar humidifiers costing in the $10 – $20 range and distilled water costing $1 – $2 per gallon, a cost comparison of using distilled versus using tap and replacing the unit more often really is moot. However, there are a few other things to consider when using one of these humidification units.

• Because acoustic instruments are not environmentally closed off to the outside world, air freely moves in and out of an instrument. If the relative humidity of your room is 60% or greater, that humidity will overcome the device and your instrument will be subject to higher humidity. A portable device has no ability to stop the surrounding environmental osmotic pressure.
• If your environment is typically low in humidity, such as in the desert (10% – 30%) then the portable units will work to keep the instrument at the desired range.
• Remember that the water in your humidification unit will be devoid of chlorine and sunlight. These are both key contributors to the growth of algae, mold and fungi.
• Wood is unlikely to be damaged by low humidity over short periods of time. Generally the wood cracks referred to in the product are largely a result excessive heat which unnaturally forces humidity out of wood. In addition, harmful radiation from long term exposure to sunlight can cause cracking in an instrument.

Wood has the ability to change its physical properties by taking in and shedding off humidity. Dry woods are easily reconditioned with humidity application. Wood is also highly unlikely to have humidity problems in the average American household. So the takeaway here is, regardless of the water you use, or if you don’t use a humidifier, your instrument really could care less. Just ask the buyers of aged Stradivarius violins that people brought into pawn shops over the years.

I have played guitar for 50 years and have never experienced a humidity issue.