Temp & humidity concerns

[Dave Hasson]

I have decided that I want to build an acoustic guitar. I was looking over the Stew-Mac website and was reading the instructions for their kit guitar. It says "The ideal building environment temperature is 70-80F, with controlled relative humidity of 45-50%". My shop is in the basement of an 87 year old house. I have a dehumidifier, but there is no way my basement will ever see 70-80 degrees. Am I setting myself up for disappointment? Will the guitar look nice in the basement but twist into a wooden pretzel when it moves upstairs? Thanks!

[Chris Fournier]

You are asking a couple of questions that are somewhat related but not entirely.

The temperature issue is pretty much all about gluing. By rights the materials and the ambient temperature should be about 70 degrees F for best results given the glues that we use for acoustic guitars. If you can't get a room up to this temp I would have concerns.

The relative humidity issue is the tricky one. I find that most dehumidifiers do best when the temperature is in the high 60s minimum otherwise they tend to ice up and 40% to 50% is tough to achieve. The RH that you should be building in has everything to do with the typical RH in your geographic location. Where you are I'd say 40% is reasonable if you humidify your house during the heating season. I can maintain 40% during the winter no problem. If you heated with wood and could see 20% RH in the winter then a 40% build environment is not going to cut it.

To build in your basement, I would try to create one "assembly" room where you could do your work. In this room you would have to keep the temp up around 70 degrees with some heat source. Don't forget that a dehumidifier pumps out warm dry air so it will help warm the room up - uncomfortably at times. Get a hygrometer/thermometer to keep track of the RH and temp. You likely won't have to use the DH in the winter at all but it will be critcial spring/summer/fall. During your build shoot for 70 degrees 40% RH and your guitar will be just fine.

[Andy Birko]

Chris gives very good advice. Unfortunately, the only way you'll really know what the humidity in your shop is if you get a real hygrometer of some sort like a sling psychrometer. I have a Caliber III hygrometer and it happens to read spot on with my sling psychrometer but a buddy of mine did some testing of several caliber iii's vs 3 different sling psychrometers and while the psychrometers were very close, the caliber iiis were kind of all over the map but, within 10% though as I recall. If you go that rout, try to find someone you can check against before you trust the readings.

Basically build dry is what you want to do. When you do your top, don't dilly dally. Rough your braces, glue them, do your final shaping and get the top on the rim in short order while it's still dry and before the top has a chance to move. Same with the back. People were building guitars long before they knew much about RH and they came out fine, they just knew when they could do certain steps like bracing.

BTW, if you're considering the Stew-Mac kit, I can personally recommend it. I normally scratch build another instrument but I wanted to build a guitar without all the hassle of making a bender etc so I bought the 000 kit and it came out great. I got a lot of complements on sound from a lot of experienced luthiers and guitarists. Follow the instructions and you won't go wrong!

[John Coloccia]

re: hygrometers
The cheapest one I've found that consistently reads accurately is this Extech one.

edit: because Graingers website is lousy, I can't just put a link here. It's the Extech 445815, available through Amazon for $50.


Don't even bother with anything from Radio Shack. I had to go through three before I found one that was reasonable.
The Extech can be calibrated with a salt calibration if you wish. +/- 4% is actually pretty darn good without getting into expensive meters.

I have three hygrometers in my shop. An Extech near my workbench, a junky RadioShack one (that reads well, though) in a different part of my shop, and a dial hygrometer from an old humidor I don't use anymore. They all match, and the Extech's been calibrated with a salt calibration. I really like the Extech.

[Andy Birko]

BTW, if you search for "Psychro Dyne" on e-bay you can find lab grade psychrometers for pretty darn cheap. I picked one up for under $30 if I recall.

[Chris Fournier]

While it may be nice to know exactly what the RH is it is not necessary in this instance. The builder doesn't need super accuracy when it comes to the RH. If one uses the same less than accurate hygrometer for all measurements then there are no worries.

[Dave Hasson]

Thank you for the replies. I was planning on buying a temp and humidity gauge just to know exactly what's going on in the shop. I will follow the suggestions where to look and go from there. I may even start with the kit and build it upstairs. Thanks again!

[Dale Probst]

Plus one on the Extech brand of digital hygrometer. Digital data loggers are also an option and are becoming way more affordable if you want to monitor the space over time. We use them to monitor spaces where there are issues with pianos being stable.
DP

[John Coloccia]

While it may be nice to know exactly what the RH is it is not necessary in this instance. The builder doesn't need super accuracy when it comes to the RH. If one uses the same less than accurate hygrometer for all measurements then there are no worries.

The problem I had with the cheap ones wasn't the calibration (i.e. the accuracy). The precision was atrocious. One of them was so bad that I could stick it in the bathroom, steam it up and it still read only 50% or so. The driest I could get it to read was 45%. I drove myself nuts for a week trying to figure out why I couldn't dry my space out. The space was bone dry, around 30% when I brought a real meter in.

I guess what I'm saying is even if you buy a cheap one, at a minimum just at least make sure it actually reads something before trusting it! :D

[Dale Probst]

Agreed, we've gone through at least a dozen of the cheaper ones. It's not just that they are not accurate, it's the tolerances are so wide as to be of little use.
DP