Woodshop Humidity- Do I need a dehumidifier?

[Peter Aeschliman]

Hi all,

I have an unfinished basement in my new (old) house that I plan to eventually move my shop into. I need to gut it, get some structural work done to move a few posts and put in a new beam, add electrical service, insulate it, etc. But man, I'm really excited.

One of my considerations is how to deal with moisture. Long story short, I'm trying to determine whether I need a dehumidifier down there.

I bought a temp/RH meter that has a few zones. I first verified their relative accuracy by putting them next to each other in the same room, where I got the same readings. So I put one of the devices in my basement, and another in the living area of the house. My thinking is that most of my projects will be made for our house, so my target basement shop humidity whatever matches the house when measured at the same temperature. This way I will minimize swelling and shrinking when I move my projects from the basement to the house.

At 68 degrees F in both the house and the basement, I read 48% humidity in the house and about 55% in the basement. Assuming this difference stays constant throughout the year, is this enough of a difference to require a dehumidifer?

Currently the basement is uninsulated and a bit drafty. I plan to solve that with new doors, new windows, and spray foam insulation in the wooden portions of the walls. So I'm inclined to think that tightening the place up a bit will minimize the difference enough that I won't need a dehumidifier. What do you guys think?


Thanks in advance!

[Peter Aeschliman]

Ha, I just noticed another very similar thread on humidity just got started. Funny timing. I think our questions are pretty different though, so ill leave this here for now.

[roger wiegand]

Chances are good you won't need one in the winter, you need to finish your insulation and sealing to see what the humidity is like in the summer. In MA in my old basement shop I needed to add humidity in the winter and run a dehumidifier in summer, in Seattle you may well be fine year round. In my new above ground shop with good insulation it's pretty good year round with the winter temp kept at ~58 while working, 42 the rest of the time.

[Jim Andrew]

A hvac guy told me once, if you put a vent in the return of your furnace system, your heating and cooling system will make the basement humidity match that of your upstairs. To clarify, he meant to cut a vent in the sheet metal duct down low on the side of the furnace, for the return air.

[Peter Aeschliman]

A hvac guy told me once, if you put a vent in the return of your furnace system, your heating and cooling system will make the basement humidity match that of your upstairs. To clarify, he meant to cut a vent in the sheet metal duct down low on the side of the furnace, for the return air.

Interesting. Although I plan to physically isolate my shop from the furnace so that no dust or finishing fumes can blow into the house. I'm going to use electric heat for the shop. But that's an interesting idea!

[Charles Lent]

I had a basement shop in my house in NY State. The house was built in 1941. The basement was all poured concrete and very solid, with a concrete floor and no cracks anywhere. I first coated the walls with Bil-Dry, a water proofing coating. I did this partly to try to reduce moisture infiltration, but mainly it was done to make the place look better. Nice white walls brightened up the shop significantly. This basement always appeared to be very dry.

Shortly after setting up my shop in there I noticed that my tools were getting rusty. I measured the relative humidity and it was 73%. So I bought a dehumidifier and set it to run if the humidity was higher than 45%. It ran almost all the time, except in mid Winter, taking as much as 3 gallons a day out of the basement shop air. I had to hook it to a drain so it wouldn't need the container emptied 3 times per day. My tools stopped rusting and the shop was comfortable, but noise from the constant running dehumidifier was very annoying and my electric bill went up about $15/month. The house was air conditioned, but the basement level was not connected to it because I wanted to keep the smells and sawdust out of the house. This didn't work very well either. Warm air rises, and sawdust goes with it, through the cracks the wood frame construction and into the house. I was also forever getting yelled at for bringing sawdust upstairs on my clothes and shoes.

When I moved South to near Charlotte, NC I vowed that my next shop would be above ground and not connected to the house. Unfortunately, the only house that we could both agree on had no shop space at all and was on a lake front lot. I built my own above ground shop, but it's only about 4 ft above and 60 ft away from the lake, and about 100 ft from the house. I thought for sure that I would need a dehumidifier for it, but in 31 years a dehumidifier has never been necessary. The humidity year round inside the shop runs between 30 and 50 % without any humidity control. I've since added a window style heat pump high and through the North facing wall for my own comfort, but it only runs when I am there, except during the coldest months when I run it, set on it's lowest setting to keep the shop above freezing when I'm not there. It has helped reduce the humidity some in the Summer months, but my tools have never rusted before it was added or since. and with no constant running dehumidifier. My wife never seems to complain about sawdust getting into the house from the shop either. Her only complaint now is that she has to phone me or come out to the shop to get me when she wants me for something. This 100' gap has significantly helped me get longer and uninterrupted shop time. I can usually work several hours without an interruption, and then it's usually because I need the bathroom and have to make the trip to the house myself. If you ever build a detached shop, make sure to install a sink and toilet. A shower would be good too, if you can afford the space for it. I sadly, don't have the space for any kind of bathroom and can't add it, so a trip to the house is necessary.

Charley

[Curt Harms]

I live a ways north of Charley but had a similar experience with rust. We run a dehumidifier during the summer months. That helps with rust, 'basement smell' and lumber stays at around 8% in the summer, down to around 6% during peak heating season. Without a dehumidifier in the summer boards stored in the basement would run 12% or more, kiln dried or not.

[Peter Aeschliman]

Thanks guys, that helps.

What I'm garnering is that I have to monitor the humidity for a full seasonal cycle before I can make any conclusions, and winter is probably the least representative season of all.

There are two concerns I have, ultimately:

1) Machinery rusting
2) Humidity's effect on the wood, and specifically, the likelihood of excessive shrinking/expanding simply by moving a finished piece upstairs

For #1, my experience in my climate in Seattle has been that simply slowing down changes in temperature solves this problem. The thermal mass of my machinery is much greater than air, so it takes much longer to change temperature than the air does. When the air gets warm and my machines stay cold, I will get rust. So I thnk the key is just maintaining reasonable even temperatures. My experience in my current shop bears this out: My current shop (slab on grade 450sqft out-building) used to be uninsulated and unheated. My machines would rust over multiple times per year. Super annoying. Once I insulated and drywalled, even when the shop was left unheated and unused for a month or two at a time, I never get rust. So I expect similar effects in my basement shop.

For #2, this is the primary concern. I'm coming to find that relative humidity is actually not a very useful metric in general. Not only does it mean different things in different areas of the country due differences in the dew point, but it also fluctuates based on temperature, even when absolute volumetric moisture stays constant. So I think what I really need to know is how much absolute moisture is in the air... my research continues...

Wow. I am a huge nerd.

[Charles Lent]

You're learning.

Remember to keep the basement windows closed when the humidity outside is high and also in the warmer months. Letting warm moist air into the basement where it's cooler than outside will cause a sudden increase in basement humidity and this moisture to condense on your tools. Drying the basement air to 45% or less with a dehumidifier is the best way to keep your tools from rusting.

Charley