# Opinons...what's The Optimal Shop Humidity?



## coolidge

Stepped out into the shop this evening and the humidity level was 80%. I thought before I go buy a dehumidifier I'd check opinions on what the optimal shop humidity level should be. I don't live in the desert I live in the wet Pacific NW, shop is in my garage. I'm thinking less than 80%. I left the door to the house open for a couple hours and it dropped to 70%. Its cool and its been raining outside most of the day.


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## ELHEAD

Don't know about the NW but I do live in the South and humidity is a problem here. I have to deal with it as awood flooring contractor at least 9 months a year .  I try to keep my shop in the 45 - 60% range, 50% being about right. Still though dew point is more of an issue. A matter of temperature and humidity. Just my thoughts.


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## RJSakowski

In southern Wisconsin, humidity can be a problem.  I run a dehumidifier in the basement shop from June into November.  With the central air, the relative humidity stays around 50% and I do not have a rusting problem or mold/mildew problem.  We do not humidify the air in the winter and the humidity van drop considerably lower then.   If we get a lasting cold snap, the r.h. can drop into the teens.

Before running the dehumidifier, rusting was a constant problem.  During the summer, I will pull about 2 gallons of water out of the air each day plus whatever the central air takes out.  I should point out that the house is over a hundred years old and the is no vapor barrier in the walls and the foundation is  sandstone/limestone with lime mortar so in in the past the walls would wick moisture in from the surrounding soil.  The shop is maintained at a fairly constant temperature year around; aqout 65 - 70 degrees F.

IMO, as far as metalworking machinery is concerned, the less humidity the better.

Bbo


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## Rick Berk

I'm in central Indiana where the humidity is always high, I have an external humidistat that comes on at 50% and off at 40%. Also if the outside humidity is above 85% I never open the outside doors as sweating can cause a problem here. The real problem today is finding a good dehumidifier, since the powers to be outlawed R12, the dehumidifiers today put out more heat than water removed, also most today run the fan continually rather than cycling on and off with the compressor which drives me nuts so I installed an external switch to turn the entire dehumidifier on and off. Whirlpool no longer makes one and the ones being sold today only last 2 years and you need a new one, the old whirlpools I had lasted 22-25 years, what has happened to American PRIDE and QUALITY.


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## dave2176

Down here in the desert 30% is pretty common. No rust issue but it can cause problems due to static electricity and electronics. They add humidity in our computer room to get it to 60%. I think if you can get it between 50 and 60 you'll be in good shape.
Dave


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## planeflyer21

Living in the desert, I'd have to say 4-5% relative humidity is best.   Seriously though, I never gave humidity levels much thought.  As Dave said above, the QC departments are always controlled but I've no idea to what level the humidity is maintained.


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## Cheeseking

Lower is always better but I think at any given humidity level its wide temperature swings that cause the most trouble.  Eg condensation.


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## Doubleeboy

28 years of living in wet PNW I have found to keep rust at bay when humidity is high, run a fan to keep air moving and keep dust off little used tools so it does not trap moisture.   Wild temperature swings are more dangerous than the humidity in my opinion.  

cheers
michael


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## Treetop

I'm planning on building a 15' x 30' room in my 30' x 50' shop bldg. to house my lathes and Bridgeport.  Until I read this thread, I was planning on buying a  good quality dehumidifier to go along with my A/C.  I plan to use vapor barriers in the walls and ceiling and I am going to seal the concrete floor with an epoxy paint.  Anything else I can do to minimize humidity in my "clean room"?

Do I understand correctly, that no one builds a quality dehumidifier now?


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## coolidge

I purchased a de-humidifier this evening. Fact is I had issues with surface rust on my lathe last winter. My 3 car garage gets almost no sun, a couple hours late in the afternoon at most. I already have moss issues on the garage roof and its only 3 years old. That's also the wet side of the property. The mill was $6k plus so I'm going to do what I can to protect that investment. This should also help with my shop air moisture issue. The question is once I get the moisture level down how long will it last before the unit has to turn back on. The garage is sealed pretty well for a garage I guess, and its insulated.


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## coolidge

Treetop I did some checking and there is a lot of complaining about current dehumidifiers failing after 1-2 years. As I have no real choice I purchased a $269 unit vs the cheapest I guess we'll see how long it lasts.


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## JimDawson

Getting some heat in there would be a big help to keep the temperature reasonably stable.  I think temperature swings are worse than the humidity.  Freezing weather for a week, then the next day it starts raining, the temperature goes up to 50 and 100% humidity, every cold surface sweats like crazy.

It is said that a rolling stone gathers no moss, that does not apply to the PNW.  Moss grows everywhere.


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## coolidge

So far so good. Humidity was about 72% this evening and I let it run for 4 hours, humidity dropped to 58% and it sucked about 1 gallon of water out of the air. This unit is quiet even on high, not whisper quite but maybe 65 decibels, just an air whooshing sound the compressor when it comes on is pretty quiet. This model has a pump and a 16 foot hose so I could just plum it over to the shop sink but for now I'm using the bucket.

Trivia - this model can also be set to heat the shop up to 86 degrees, I'm guessing it works like a heat pump. I may give that a try this winter.



I decided to test the moisture in my air supply, which the amount of condensation collecting when running the Accu-Lube started this whole humidity management effort. Previously I just had a 50 foot line from the compressor to my mobile air station. I swapped that out and installed 200 feet of air line to give the air time to cool and for the moisture to condensate. I also installed another filter to try to capture most of the moisture before it even gets to my mobile air station. At 58% humidity and about 56 degrees in the shop I emptied the 20 gallon compressor about 1.5 times and not a single drop of moisture collected in the filter.


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## Downunder Bob

Living in Adelaide South Australia we have  a fairly dry climate, winters are cool and damp, and summer is hot and dry. Humidity is rarely a problem Although with the last few years of global warming or climate change, whatever you prefer to call it, the humidity levels have begun to rise. We normally use evaporative coolers for air con in summer, I think you guys in the states call them  swampys. As I understand it, it's not the humidity per se, but the dew point.

 If the metal surface temperature, or any surface, drops below the dew point then you will get condensation. Dehumidifiers will certainly help control the problem, but a decent reverse cycle air con will also work, some even have a dehumidify setting. A dehumidifier is really only an air con with a slightly different setting.

The problem is that after cooling the air to get the moisture out, it is often too cold so you need to reheat it for comfort this can be arrange by running the air over the condenser coils, so there is no extra power required for the reheating.

Large commercial units are often set up that way especially in humid climates.


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## whitmore

Here in Seattle, a basement is OK for metals with a dehumidifier set for 50%.   That's partly
because, even unheated, a basement doesn't get much temperature fluctuation. 

A 20 degree temperature drop from 70 degrees F to 50 degrees F with
no change in ABSOLUTE humidity changes the RELATIVE humidity
from 50% to 100%.   To put it another way,
if your relative humidity reading isn't near 100%, you don't expect rust.   But
it isn't enough to dehumidify, if your temperature swings faster (a half day?)
than the dehumidifier cycles the total air volume in the shop.

So, insulation and air circulation are as important as dehumidification.
Heating is important, too, because few dehumidifiers (none that I've had) are
operable below some critical temperature (55 degrees?) because they freeze
instead of liquefying the moisture, and it never drains.

Dehumidifiers that fill a bucket, can do it every couple of days.   They
must be tended, and monitored.  The electric bill can be substantial.


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## Downunder Bob

coolidge said:


> I purchased a de-humidifier this evening. Fact is I had issues with surface rust on my lathe last winter. My 3 car garage gets almost no sun, a couple hours late in the afternoon at most. I already have moss issues on the garage roof and its only 3 years old. That's also the wet side of the property. The mill was $6k plus so I'm going to do what I can to protect that investment. This should also help with my shop air moisture issue. The question is once I get the moisture level down how long will it last before the unit has to turn back on. The garage is sealed pretty well for a garage I guess, and its insulated.




How long the low humidity will last once you get it down to an acceptable level? It depends on many variables, basically how does new moisture get into your shop, does your shop have a good vapor barrier, how much fresh air is circulated into the shop from outside, are there any open water containers in the shop, even a dripping tap into a hand basin will be a source of moisture as it evaporates in the now dry air. What is the ambient temperature in your shop. If the temp is below comfort level try running some heat that in itself will reduce the relative humidity. conversely if it's above comfort level run some air con that will also reduce the relative humidity.


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## ewkearns

"Rust only forms when conditions are right – when there is enough water and air. *Iron will rust when the relative humidity in the air climbs above 50%*, *and steel rusts when the relative humidity reaches 80%.* If your shop is unheated, *iron and steel tools will also rust when the nights are cooler than the days*. At dawn, the temperature of the air rises faster than dense solids and moisture condenses on the cool metal surfaces. One way to prevent rust is to change the shop environment – either heat the shop to keep the tools surfaces warm or install a dehumidifier to remove the humidity from the air."

Source:
http://workshopcompanion.com/Demos/...lassroom_files/Shop_Notes_Rust_Prevention.pdf
http://workshopcompanion.com/Demos/...lassroom_files/Shop_Notes_Rust_Prevention.pdf


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## ch2co

Currently 25% in my house and I consider that sort of humid..  Rust? whats that?


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## woodchucker

An old thread, but I noticed that my metal toolbox in the garage will condense when it gets hot after being cold.  Everything is coated with condensation even inside it.
My wood tool box insulates everything from the condensation taking place. So in that respect, I understand why wood boxes can be an advantage, especially in an unconditioned env.  
I also understand why I see so many toolboxes rusted at garage and estate sales.

My wood and metal shops are in the basement. I let in a small amount of air conditioning in the summer from upstairs to keep the humidity down. I tried a de-humidifier but it gets too hot, then I would have to open windows, and that makes it more humid.. So I try to keep it shut.  Only in the spring and fall do I get a chance to open it up.  I have a full set of gauges so I know what's up with the humidity...
For very important expensive tools, I keep kitty litter in a small tin to soak up any moisture in the drawer just in case. I learned that from a Wood working magazine. It works. I also save tons of packets of desiccate. 
I keep my machines oiled, My WW machines waxed. 
That's the best I can do.


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## ewkearns

" ...my metal toolbox in the garage will condense when it gets hot after being cold."

Actually, one finds condensation with a cold toolbox (or machine) and warm humid air....  IOW, it sweats like a cold drink bottle......


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## woodchucker

Yep, but it's not so bad for a wooden toolbox. I think the wood insulates the extreme temp. change  Wood doors insulate well on a house, much better than a steel door. So I think the metal transmits the temp change to the interior too. As well open wood (not finished) will absorb the moisture and prevent the tools from rusting. The wood will swell , but it will come back to equilibrium with the environment in time.


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## ewkearns

Yep, wood is a poor conductor of heat, steel is a pretty good conductor, and aluminum is a nightmare.....


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## ch2co

119 deg. in phoenix today, 96 in Denver, 71 in my BASEMENT where the shop is. Ahhhhh. When the outdoor temperature gets less than
the upstairs temperature a large window (very heavy duty window fan) turns on and pulls the "cooler" outside air into the house through a couple of large furnace filters. 
By morning it will be in the mid to low 60's in my basement at about the time that the upstairs gets cooler than the outside air and the fan shuts down. The only humidity that I know of comes when I take a shower.

CHuck the grumpy old guy


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## Downunder Bob

ch2co said:


> 119 deg. in phoenix today, 96 in Denver, 71 in my BASEMENT where the shop is. Ahhhhh. When the outdoor temperature gets less than
> the upstairs temperature a large window (very heavy duty window fan) turns on and pulls the "cooler" outside air into the house through a couple of large furnace filters.
> By morning it will be in the mid to low 60's in my basement at about the time that the upstairs gets cooler than the outside air and the fan shuts down. The only humidity that I know of comes when I take a shower.
> 
> CHuck the grumpy old guy


Low humidity in the shop is always good. Fortunately where I live it is normally low, and only occasionally too hot, but does get a bit cold, although not as cold as you guys in Nth America, in winter. My shop is in the garage which is provided with air con air from the house, and also connected to the underfloor heating system, So winter min is about 16c and summer max about 30c. that would be on a very hot day with outside temp about 44c + usually more like 23c in summer in the shop.


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