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The thread about dehumidifiers has raised a question in my mind....

I build in my basement shop in Ohio. The humidity hovers around 50 with spikes to 52 and lows to 42. I typically don't run the dehumidifier in these conditions. In the crux of the summer, say early August, the humidity is closer to 58 and I'll run the dehumidifier to get it back to between 45 and 50. It's a simple home dehumidifier.

Do I need to worry about getting exactly 45 all the time? I like buying tools and shop stuff as much as the next guy, but a dehumidifier is not doing it for me.

Brad

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I have a dehumidifier always running and my relative humidity usually ranges from about 40 to 47. I'm perfectly happy with that.

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Steve Smith
"Music is what feelings sound like"

42% - 48% or close is a great range for builders in the midwestern US. Nailing and maintaining 45% is not necessary and I'll bet ya too that what ever hygrometer that you are using is not even that accurate.

What I always did was religiously learn to calibrate and maintain my hygrometers and then maintain that 42% - 48% range. My preference was to brace at the low end of that range but it's not necessary, just something that I believe in and others do too. Even Steinway is into bracing spruce at low RH as well.

Step one Brad would be to be sure that what ever I'm using to measure RH is accurate and calibrated. Wet bulb testing is a great way to go and does not have to cost much with a couple of lab thermometers and some creative repurposing of stuff in the shop. Comparing wet bulb tests to what your hygrometers indicate will let you know what you really have.

Bracing is the only task where humidity is truly critical. Other tasks that involve cross grain gluing like the rosette and box closing need some degree of humidity control, but generally don't cause any trouble. For tasks that don't involve cross grain gluing at all, you can generally ignore the humidity. Except for filing fret ends to the edge of the board. The dryer the better for that, or else they'll poke out in low humidity later.

One thing most builders never mention when discussing humidity is the hysteresis effect. Simply bracing in 45% doesn't tell the whole story. If the wood was wetter before acclimating down to 45%, it will still be more swollen than if you dry it below 45% and acclimate up. I'd guesstimate that wet-down 45% is equivalent to dry-up 55%. This is why newbies often have the humidity drop for a few days and turn their braced soundboard inside out, but even after returning to the bracing RH% it remains a potato chip. If they'd acclimated up before bracing, then it would return to normal. So whatever control level you use, make sure you always acclimate the same direction or you'll get mixed results.

Brad--

I think it is fine to shoot for something in the 40s and be happy with it. If I were you, though, I would first focus on making sure the RH measurements you are getting are as accurate as they can be. Hearing that you are in Ohio, part of me suspects the RH goes higher in the summer than what you are measuring. Maybe your home AC does a better job of pulling moisture from the air than mine does, maybe you are further north than I am, etc. But it gets super wet in the summer here in Charleston, WV, and I know it does over in Kentucky, too. Ohio is only a state away. Just another thing to obsess about, right?

Good point about the measuring device. I'm just using a unit I got at the big box store for 20 bucks. I'm looking into getting something a bit nicer to confirm the readings. The basement is air conditioned with no outdoor exits, just the stairs up to the rest of the house. I'll get a better unit and check again.

I'm in Cincinnati, so as south as you can get and still be in the state. In the coming weeks it will get wet like you're describing. Usually I see a spike at that time to the upper 50's. That is when I run the dehumidifier.

Thanks!
Brad


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I love Cincinnati! But knowing you live there makes me doubt even more that those RH readings are accurate. Your climate and mine are very similar; you are basically just 4 hours west, and the tiniest bit north. The temp today here in Charleston will get up to 95F, and the outside humidity will be 98%. I bet yours will be about the same. My inside RH, with AC but without a dehumidifier, would be in the mid 60s. With a decent dehumidifier, I am now maintaining about 45-47%.

The thing that worries me most is building when the RH is too wet, and then getting a top crack when the RH drops in the winter. I've had it happen. So, I am pretty focused on preventing it in the future. I have a whole house humidifier for the winter, and now a decent dehumidifier for the summer. Top cracks bug me.

Been said but I'll say it again --- 45% (like the factories) is my goal for pre-assembly storage and glue up areas. We have several moderately priced electronic measuring devices -- I don't get carried away and trust that the average readings are close enough. My S4 cell phone app is pretty accurate as well. Our de-humidifier runs night and day during our lower Michigan summers.

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Ken Cierp

http://www.kennethmichaelguitars.com/

One thing I've never seen anyone address is that I've bought a number of humidistats trying to get one that controls in a tight band.

They advertise the accuracy or resolution.

They don't advertise the hysteresis. The best I've gotten has 6 percent hysteresis. Typical is in the 8 percent range. And that is 6 percent RH not 6 percent of the value being measured.

So I doubt anyone is truly controlling to -a- number. Their hysteresis of the humdifier/dehumidifier is floating the RH a minimum of 6 percent.

Which is why I'm happy with a reasonable range (it floats around my setpoint). If you want something better you will have to come up with some $$.

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Steve Smith
"Music is what feelings sound like"

"Compression shrinkage" (Hoadley's "understanding Wood" Page #114 image 2 ) is the real enemy of braced and/or built guitars.
That's my story and I'm sticking to it.

42...

Hitchhiker's Guide?

It’s interesting... I’ve been taking steps to better control the RH situation in my shop. I contacted a few installers of whole home humidifiers and looked at some standalone units. Given my setup, and how long I plan to be in this house, I’m leaning toward just getting RH under control in the room where I store the wood stash and then setting up a go bar in there to brace with. That way I only need to control one room, making things easier.

I started this thread, Ken had already given me the answer.


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I aim for 40 - 43. I think Ken's wisdom and practical experience will live on here a long time!

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Pat

A perfect 45 RF would be outstanding, but for use with limited climate control budgets, I'm with Dennis, I wouldn't shape or glue braces on, or close a box when the humidity is above 50. Lower humidity is not a problem for doing those operations. Other than finishing, have at it on those upper range RF days..

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Joe Beaver
Maker of Sawdust

And what about bridge gluing? At least on a classical you get an over 18 cm cross gluing operation.

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member of the guild of professional dilettantes

Classical bridge probably does need lower humidity than steel string.

Humidity is still important when closing the box, gluing the bridge, gluing ebony fingerboard extension to soundboard, leveling fret ends to edge of board, gluing rosette, reaming friction peg holes, and probably some other operations I'm forgetting. But you can generally get away with higher humidity on all of those than you can on the internal bracing.

And if an inexperienced builder does get impatient and glue the bridge on even though the humidity is high, they can pop it off later, fix the soundboard cracks, and glue it back on in proper RH. If the bracing is bad, the guitar will require humidity babysitting forever, and will probably be scrapped instead.

Remember that RH is relative to the temperature. You can have the same amount of water vapor in the air and a 5-10 change in temp can change the RH to a point. It can't make an appreciable difference with very dry or very damp air. If the RH is a little too high, heat the air. If a little too low, cool the air.

Before trying to deal with the humidity with a dehumidifier, I would look at how moisture is getting in. Have you sealed the floor? Epoxy paint will go a long way towards limiting the infiltration of water though the concrete. Also look at moisture barriers for the walls. Once you deal with these it will be much easier to control relative humidity with a household size dehumidifier.