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Looking at a Relative humidity table for Baltimore, Maryland, it shows the average daily high for August as 85% (measured at 4am.) and the average daily low (measured at 4 pm.) as 55%. In an air conditioned environment would the humidity swing be even greater on a daily basis? ( Afternoon temperatures requiring air conditioning (lowering the R.H. even more) and early morning cool temperatures not requiring air conditioning (when the R.H. is the highest))
I have seen thin guitar woods react to changes in humidity in a short amount of time, but I never thought about the - hourly- changes the wood goes through on a daily basis.

At my condo which is on AC 24/7 in the summer even in the cooler mornings when the RH is high outside because the air in the condo has been dehumidified and cooled it stays that way with a much lower RH than outside. Today for example this morning it was 75% outside and 55% in my condo even though the air had not clicked on for about 12 hours. Of course when it's on it takes the RH lower. I would guess that I see a swing here of around 45 - 55% in a 24 hour period with 24/7 AC this time of year.

In our shop which is much less sealed and on the third floor with hot roof surfaces as some of our walls the swing is more pronounced but still never what we see outside so long as we keep the place closed up and the AC ready to come on when it gets hot enough.

Or, in other words the swings that I see outside I never see in the shop or my condo to the same extent.

When I was still building and back then Lance and I would try to out do each other with the sanding of the inside of the top I took a top on my back deck to sand. It was an RH of about 65% IIRC out there and 45% in my house. The top flattened out in only 20 minutes which really surprised me. Returning it to my shop and it recovered overnight. Didn't think it would change that fast.

Regarding the speed of the effect of RH changes on tonewoods:

I can offer this anecdote. I do a pretty decent job of controlling RH in my shop. It swings a little, but not a lot. And that level of control takes some effort here in WV. Anyway, I took a jointed, but not glued, spruce top to my local woodworking club, along with a glued East Indian Rosewood back. No bracing on the back. Both are decent quality and quarter sawn. The club’s shop is air conditioned, but no intentional RH control. Both the top and the back experienced significant warping in just 2 hours. They stabilized OK once back in my shop.

Very well observed and said. In particular bracing top and back, and assembling the box. Generally beyond that humidity matters far less (I use a strongback to glue fretboard to neck so that one is not on my list). When you think about it, this is not a terribly large part of the build lifecycle.

I'm lost on this comment. Spend money on what? Used mechanical hygrometers are not that expensive. All over eBay.

I'm curious - how many people here have built a guitar under incorrect humidity, and ended up with a cracked top or back? I have. It sucks. Oh it gets better, you cannot repair the crack effectively. Whenever the humidity drops it will flatten the top or back, then open up the crack. I doubt anyone here - hobbyist or otherwise - thinks they are building cheap, garbage guitars. Anyone here wouldn't be disappointed in their investment in time if this was the case with their instrument? I suppose until it has happened to you, it may not seem like a big deal. Humidity matters, as Clay pointed out, and as I suggested in several operations. It's not necessarily expensive to have decent humidity measurement and control strategy.

We see small builder guitars that someone built in their garage with zero humidity control and then charged the unknowing... big bucks for. One that I recall well was three when the owner cursed out the builder and threw it in a bon fire after we let them know that the many cracks that it's already sustained would continue. I'll add a guitar should not need a neck reset at three years old... sheesh....

Another one was ten years old and had nearly 10 top cracks and some back cracks too as well as a lifting bridge and loose braces. This builder had built over 150 when they excreted... this one. It's been retired to the trash too.

These days when a small builder guitar comes in and we have never heard of the builder we use extra scrutiny during triage and may decline completely if the fix is simply a bandaid on a poorly built time bomb.

One of the benefits for us of not being builders (currently) or selling anything is that we are agnostic when it comes to any bias on the product that we are asked to comment on. On the other hand it's very, very sad for me, shaking my head every time I see someone who's hopes and dreams, the first ever custom guitar for them are destroyed by someone who went ugly early and had no business selling anything.

It's very sad and something that we mince no words in our efforts to advocate for clients.

I have. It is easy for it to happen here in West Virginia. We have super humid summers, and about 1-2 months of very cold weather in the winter. It is predictable now, but at the time, I didn't pay too much attention to the part of the Cumpiano/Natelson book that talked about relative humidity, built the box of the guitar in the summer, and yep, winter came along and kicked me in the groin like a jealous lover. I now wear the athletic cup of RH control throughout the year.

This seems like a not inconsiderable expense to keep humidity under control, and this assumes no issues with closing and sealing the work and storage areas:

- Humidifier for winter ($100-$150 plus $40 per season for consumables)

- Dehumidifiers for summer ($150-$250 every 3-4 years)

- 2-3 hygrometers ($60 - $200)

It seems like that $300 - $600 or so spent on getting humidity under control competes with all those other things needed for building, which got me thinking about where to cut corners versus where cutting corners seems like false economy...and that is grist for another mill (and a horrible mix of metaphors).

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For the times they are a changin'

- Bob Dylan

Ms. G, you make a good point about trade-offs, and list a few costs. I thought about this, and given the many hobbyists here, certainly as long as their guitars are worth less than $300-$600 you identified, rolling the dice on humidity certainly a trade-off worthy of consideration. Depending on climate, most anyone is going to need both a humidifier and dehumidifier, or just one of them if the climate is lopsided. I don't know of many Nevada luthiers that require a dehumidifier. ... that said, good points!

" I thought about this, and given the many hobbyists here, certainly as long as their guitars are worth less than $300-$600 you identified, rolling the dice on humidity certainly a trade-off worthy of consideration. "

Rolling the dice on humidity control is never a good idea when building instruments, no matter the price point. The better you understand humidity issues and when it is critical to the process the more you can determine when to monitor and control it.
Most hobbyists are trying to build better quality instruments and are more careful in the construction of them than many factories. There is a learning curve, so their first efforts may be lacking in some aspects of design and build, but they don't intentionally build inferior instruments.
Unless you don't count your time as worth anything it would be difficult to sell an instrument for the prices quoted. In some cases those prices wouldn't cover the cost of the materials used.

Agree Clay ... sarcasm likely did not convey well in my post.

AndyB - What's this about?

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George :-)

As I understand it, English honorifics are a manner of respectful address. Given Ms. G's proclivity to extend such titles, likewise, I extend her the same courtesy.

Energy efficiency upgrades to homes - both new and existing - was my profession for 32 years. What I learned was that interior humidity changes come about when your house leaks air from the outside, and that air displaces air inside and pushes it out. Every cubic foot of air that leaks in has to force a cubic foot of air out - or else your house would inflate. If the house is perfectly tight (very hard to do) you have a terrarium where you never add or take away water and have the same amount in there at all times. If you keep the thermostat at 70 in the winter and 75 in the summer, the RH would be very close at all times.

Your house dries out in the winter not because there is "dry heat" - no such thing. No form of heat, be it oil, gas, or electricity, dries the air. (wood heat can dry a house because it forces outside air in for combustion, and it can add moisture because the stored wood is still drying). What happens is this - you took a shower, cooked spaghetti, and just breathed, and that put moisture in the air. The cold air leaks in, pushes that moist air out, and as the cold air warms, its RH drops - voila, dry house. More leaks, more outside air, more moisture forced out = dryer house. This is completely independent of insulation - you can have a very tight but uninsulated house, or a very leaky well insulated house (mostly what we run across).

What CAN happen is local dryness inside the house. The air coming off of an electric baseboard heater can be 135°, and that air is extremely dry until it cools off again in the middle of the room - no moisture added or subtracted. Any guitar that is in the path of the hot hot air is toast. On the other end of the spectrum, a heat pump only puts out 95° or so air, so it is no where near as dry and it does not dry out things as badly. Oil or gas heat can be at about 120° and a wood stove can be very hot nearby.

So the first thing to do if you have a couple of valuable instruments is to make sure your house is not leaky. Once you have done that, it is much easier to control the humidity. Maybe you cook less spaghetti than your neighbor and you might have to add some moisture, or maybe your basement is a little damp and you have to take some away.

Most major utilities have a program, sometime with a state mandate, to help people improve their efficiency. Many states offer a very cheap energy audit that includes an air leakage measurement using a blower door - a large fan that clamps in a doorway, exhausts air out of the house, and actually measures and locates leakage. An energy audit without that big fan is not good enough. The audit will also look at insulation levels, equipment efficiency and safety, and grading and gutters for wetness.

Once the audit is done, many states/utilities will subsidize repairs - seal the leaks first, then insulate.

As an example, my house is 2800 foot square 2 story with a heat pump in the mid-Atlantic region. It is extremely tight and well insulated. My RH goes from a high of about 51% in the summer to a low of about 32% in the winter. And since the house is tight, it is less responsive to outside RH and it does not vary throughout the day. My basement shop is a couple of points higher than that because it is a couple of degrees cooler down there, so I run a dehumidifier this time of year, but not in the fall, winter, or spring.

We are coming up on winter so it is perfect timing to be thinking about this. So have an audit, seal the house, and things will get a little easier to control

Ed

Ah, good to know. Thanks for explaining what it is you bring to the community.

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George :-)

Like most things in life, it's less what I bring and more what you take.

"Like most things in life, it's less what I bring and more what you take."

Well Said!

"Agree Clay ... sarcasm likely did not convey well in my post."

Although I thought you were being somewhat facetious, there may be a few readers who are new to the hobby and would give it some credence.