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So it's now summer in RI and impossible to maintain proper humidity levels, at least for me. I can't afford to run a dehumidifier 24 7. I ran an air conditioner yesterday in a spare room and the RH didn't get below 68%. That surprised me.I'm considering either making tools and parts until winter returns or building some kind of box around the go-bar deck. I'm ready to cut and fit a rosette and I don't really want to under these conditions. Am I worrying too much? The guitar will live in these conditions anyway and being my first, I'll most likely keep it for myself.

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Hutch

Get the heck off the couch and go build a guitar!!!!

I think you will probably be OK installing the rosette in higher RH. The more dangerous things to do are brace the top and back, and close the box, in higher RH. On the rosette, will it be lines or a hardwood circle? If it will be a hardwood circle, maybe you ought to do that in a lower RH environment, too, just to be safe.

Keep in mind that the “living in these conditions” comfort you mentioned should not be any comfort at all. The guitar you brace and glue together in high RH today will have to survive the winter in Rhode Island, when you heat your home and dramatically lower the RH by doing so. I’m pretty sure your RH inside your home in January of 2020 will be a lot less than 68%. What happens when a guitar that is glued up at 68% RH in the summer has to live in a house with 28% RH in the dead of winter? The top cracks. This is a predictable (and therefore preventable) problem. If you glue the braces on and close the box closer to 40% RH, the guitar is much less likely to crack in the winter or swell too badly in the summer. Hitting that middle ground, so that the guitar can weather the extremes on both ends of the RH spectrum, is the whole point of building around 40% RH.

Can a dry box help? Some folks get some benefit from it. I prefer to control the whole working environment. If you can’t do that, then yes, try the dry box. You can buy some really nice (and safe) drying rods at Lee Valley. Or you can use a light bulb. I worry about fire risk, so use your head in designing the box and how to keep a heat source from being a fire risk. Good luck!

A drying box can work well. If you connect the light bulb /heater through a humidistat/dehumidistat (find them on eBay) it will maintain the humidity at a set level of humidity. I bought a humidistat and rewired it to be a dehumidistat, but the units I see now appear to do both.
As Don mentioned the indoor winter humidity level is the killer. During real cold weather my house has had R.H. down in the teens. I try to keep instruments in an unheated room where the R.H. isn't so drastic.
What I have settled on for a humidity controlled area is a plastic lined closet, with either a dehumidifier or an oil filled space heater (depending on the time of year). It is big enough to keep several projects in, but small enough to be economical.
Not all steps of guitar building require humidity control, but for certain elements it is critical to insure a stabile construction.

So what I gather with a dehumidistat, all I need is a cut out for a fan connected to a dehumidistat. That's pretty easy. Now, how long can I keep it open to work on the guitar safely?

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Hutch

Get the heck off the couch and go build a guitar!!!!

How are you measuring your RH? I ask because 68% seems awfully high for a small room with an air conditioner running in it. What was the RH in there before you ran the AC? 68% could indeed be the accurate number but it wouldn't hurt to calibrate or validate your measuring technique to be sure.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

To use a drying box effectively, you precondition the work pieces in it overnight or for a few hours depending on the thickness of the wood. You remove the work pieces for the few minutes required to apply the glue and then replace them in the box immediately after gluing and clamping and allow the glue to cure before removing the parts. Plates with cross grain glued braces I leave in the "box" (closet) until they are glued to the rims.
If you are working in an air conditioned room and it has brought the humidity down to acceptable levels you can leave the pieces out of the box for a longer length of time. Having a hygrometer in the room can give you some indication of how humid the space is and how long you can keep things in it before they absorb a lot of ambient moisture.

I bought a digital hygrometer but I never calibrated it. I find it hard to believe that a brand new hygrometer could be that much off. I got 45% readings in my basement with it during the winter.

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Hutch

Get the heck off the couch and go build a guitar!!!!

It may be hard to believe, but how do you know? Again, it may be perfectly accurate but it is a good practice to try to verify. Get a descent thermometer and take a dry and wet bulb reading then calculate the RH. Compare that to the hygrometer and see where you stand.

I only bring it up because I assume that the AC unit was having an effect on the RH and if it was still 68% then the rest of the house would be pretty high and feel uncomfortable to me.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

The rest of the house was very uncomfortable because we weren't home during the test.

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Hutch

Get the heck off the couch and go build a guitar!!!!

A brand new hygrometer may never have been calibrated. Some are "calibrated" by placing a wet towel over the opening used to allow humidity into the unit, waiting a prescribed period of time, and then setting the needle to read 100%.
I bought a cheap unit whose instruction sheet listed how far off it could be at various humidity levels. In the range we need it was fairly accurate, and others it could be as much as 10%.
I recently checked a thermometer I use to measure the water bath temperature in my glue pot. I live within 100 feet of sea level and there were no hurricanes passing overhead. The thermometer indicated that water was boiling at 200 degrees fahrenheit. Now I need to figure out how to calibrate it to let me know when it is reading 140 degrees.

As hard to believe as it might be, you should accept it as a possibility. Lots of people on the OLF have bought new hygrometers and found out they were utterly worthless in terms of accurately measuring RH in the range that matters to us. It's a real thing.

To deal with RH issues, you first have to be capable of accurately measuring RH in your space. That is harder than it should be, if life were fair. Read up a bit here on the OLF regarding RH issues and you will benefit from the knowledge you obtain. But it will also probably convince you that you have to put more work into getting accurate RH measurements.

Once you can accurately measure RH in your space, then you can work out a strategy for how to control it. Controlling it in workspaces along the East Coast usually means dehumidifying your space in the summer and humidifying it in the winter. Or, you can keep the workpieces in RH control boxes and hustle to get the work done while the workpieces are outside the boxes. That does not appeal to me, so I do what I can to control the RH in my workshop. But, each of us has to figure out the best way to get the work done, within the constraints that apply to us individually.

I found a 32% test kit https://www.amazon.com/Boveda-Low-RH-One-Step-Hygrometer-Calibration/dp/B00O929V0U. Should this do the trick?

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

The best way is to calibrate your hygrometer in the range you would like to keep the RH. If it is calibrated to accurately read at 75%, it might not be that accurate at 35%. I'm sure there are kits similar to that one that would calibrate to 43%. A dry/wet bulb test will give you a good idea of where you are no mater what the level is and you can periodically take measurements to see how close your hygrometer is.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

First of all, no, you're not worrying too much. Bracing in high humidity = doomed guitar.

As others have said, importance of humidity varies by task. I wouldn't glue a rosette in 68%, but I would join a soundboard or glue a headstock scarf joint or headplate. I wouldn't glue an ebony fingerboard to the upper bout of a soundboard, because even though there's no cross grain, ebony's dimensional change with humidity is so much more than spruce that it can cause cracks along the edges of the fingerboard extension in low humidity. Quartersawn rosewood fingerboard should be ok to glue in high humidity, though.

I've recently built a hot box (about 2x2x4ft), but so far haven't had much luck with it. It's a thin wood frame with foam board walls, which I've double layered with an air gap inbetween in the areas where it felt warm (i.e. was losing a significant amount of heat). Inside is a masonite "floor", with a gap around the edge for hot air to circulate up from the lamp underneath (also some aluminum foil between the lamp and masonite to help spread the heat, although the floor temperature is still not as even as I'd like it to be). The whole thing is very lightweight, so I can take it up and down from the ceiling easily.

A 60 Watt light bulb raises the temperature by around 15-20F, but seems to have very little effect on the humidity inside. It seems like the RH is equalizing to the outside level regardless of the temperature, because if I open the door and fan the air out of the box, it drops by 10-15% for a while and gradually climbs back up. So I think the fresh air gets heated quickly by the residual heat of the box structure, which drops its RH, but then moisture starts coming in the cracks around the door and lamp until the RH is equalized again (everywhere else is fully sealed with box tape and hot glue). Darn physics I thought the moisture content is what equalized, and then RH would depend on temperature. But I guess it's RH that equalizes, so the box needs to be airtight in order to maintain a lower level than outside.

I also tried putting a small peltier dehumidifier in there, but it doesn't seem to have much effect aside from adding another 20 Watts of heat. The most I've gotten in the water tray was a few drops.

So I think the next step is to try and come up with a way to seal the gaps around the lamp, and maybe improve the door seal as well although it's already pretty good (except for that dehumidifier cord wedging it open at the moment).

This is interesting. I have not given much thought to the extension before. Thanks.

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Hi Dennis,
Keeping the box sealed up wouldn't be a bad thing, but a small amount of air exchange shouldn't be a problem. Your original thinking is right - heating the air lowers the R.H. Your house is a bigger "hot box" when you heat it in the winter. It is generally not air tight. The relative humidity falls dramatically when the temperatures drop. As long as the outside air stays cold and the house is heated but not humidified the indoor relative humidity will remain lower than the outdoor R.H..
How are you measuring the R.H. in the box and the R.H. in the house and how do they compare?

The two hygrometers in the second photo. Various sequences of leaving them in their usual spot in the middle of the room, in the box without the heat on, and in the box with the heat on, and circulating fresh air in the box. The wood one takes a long time to acclimate, so I've mainly been going by the digital for most of these tests.

Of course the room humidity is not controlled and varies throughout the day, so that could be causing some of the confusing test results. It's also possible that the temporary drop registered by the digital one when circulating fresh air into the box is just a false reading due to the sudden temperature change. I'll try it again and see exactly how long it takes to go back up.

But regardless, I'm not getting it dry enough for guitar building. I've also tried a 100 Watt reptile heat lamp, which gets the temperature up another 5 degrees or so, but still not much if any change in RH. I'd rather not use any higher power than that, both for fire safety and energy cost. But I could add more layers of foam board insulation to try and raise the air temperature further.

I'm not seeing much promise here. I'm beginning to think my only option is to wait for winter and just make parts and tools in the summer.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Hi Dennis,
If your dehumidifier isn't catching any water then it is either set too high or is not working, or there is not much water to catch. Does it work outside of the box.
Heating the air should lower the R.H., but as the room temperature goes up, heating the box becomes less efficient at lowering the R.H. - one reason I switch to a dehumidifier in the warmer months (conversely a dehumidifier doesn't work well in a cold environment so I use a heater).

http://www.engineeringtoolbox.com/moist ... d_281.html

I've been running more tests of the hot box over the past few days, and the good news is I'm getting more than low enough for guitar work now. But the results are still very confusing.

It's been really hot outside, so I've had the air conditioner running unlike previous tests. It keeps the room at a pretty steady 80F and 49% RH.

The dehumidifier works fine as long as the heat is off inside the box. It collects up water reasonably quickly, and seems to hold the humidity at a steady 35%. If the heat is on then it usually doesn't collect anything.

When the heat is turned on, the temperature rises by about 17F in 30 minutes to an hour, but the humidity remains more or less unchanged.

Here's the full test run over the past couple days:
1. Opened the box and fanned out the air, let it equalize to the room conditions for half an hour or so (settled at 82F, 49%, a bit warmer than the mid-room reading since it's up near the ceiling).
2. Turned on the dehumidifier and closed the box.
3. Several hours later, stabilized at 85F, 35%RH. Opened the door and emptied the dehumidifier.
4. Another hour or two, no change. Turned the heat on.
5. An hour later, 100F, 34%. Shouldn't the temperature increase affect RH immediately?
6. Hour later, opened the door and turned the dehumidifier off. It hadn't collected any water since the previous emptying.
7. Couple hours later, no change to RH. IIRC the temperature dropped by a degree or two.
8. Went to bed, woke up. Temperature unchanged, but humidity had gotten down to 25%. What the heck?
9. Few hours later, no change. Turned heat off. It's now just a closed, not-quite-airtight box sitting there with nothing running in it.
10. 5 hours later, 83F, 24%. How is it still so dry? The dehumidifier never got it below 35%, so it should have returned to that level after the heat was turned off. But just as temperature rise seems to have no effect on RH, temperature drop doesn't either. Apparently the actual moisture level decreased while it was left with the heat on overnight. But I've left the heat on for days at a time and never saw anything like that previously. This is the first time with the air conditioner running, so maybe that was somehow involved.

The wood hygrometer responds more slowly, but has more or less agreed with the digital one throughout these tests.

I'll repeat this experiment and see if I get the same result again.

I haven't gotten around to repeating the long experiment yet, but after running just the dehumidifier for over a week, it seems to be a good solution. It maintains RH around 35-38%, collecting a few cc's of water each day. I do prefer drier for bracing, or at least drying below 30% to get past the hysteresis point and then acclimating up to 35%. I think the heat source in my box will be able to do it, but a regular kitchen oven can also be used.

So for a few bucks worth of 1x2's, a few bucks worth of dollar store foam board, a piece of masonite, a piece of plastic for the window (mine is from a walmart poster frame), a couple hinges, some screws, a roll of box tape, and a $25 ebay dehumidifier, you can beat the summer humidity with a monthly electricity bill of about $2

Also note that the door on my box is angled a bit, which helps when sliding long things in and out. Although it does add a couple of scarf joints to the frame, which may be tricky for beginners.