Summertime Shop Dehumidification?

[Marty Tippin]

My shop is a 40x40x12 ft pole barn, insulated and drywalled. I have 3 ton A/C (also a heat pump) that I run in the summer to keep the temperature reasonable, around 78 to 80. The pad that the pole barn is built on, unfortunately, doesn't drain as well or as fast as I'd like after a rain and it seems as if I get moisture coming up through the concrete - I haven't verified that's what's going on, but it's a hunch. I also park my daily driver in the barn, so the 10x16 garage door is open twice a day for a few minutes.

Anyway, with the shop insulated like it is, the A/C seldom kicks on for very long and so humidity (and the resulting rust) is a problem. I tried last summer to run the A/C down to 72 or so, which does a good job drying the air - but once it gets that cool, it takes a long time to warm up again and when the thermostat goes back to 78 degrees, it sits for a day or so and the humidity comes right back.

So, my plan for this year is to install a de-humidifier, plumb it to the condensate drain for the A/C and let it run 24x7. I'm looking for any suggestions as to brands / models to consider or avoid, as well as experiences of those who've used dehumidifiers in their shop. What say you, Creekers?

[Don Jarvie]

Wood floor with a vapor barrier and insulation will solve your problem. Vapor barrier will keep the moisture out of the shop.

[Todd Mason-Darnell]

To help verify that it is moisture coming through the slab, tape a 1' x 1' piece of clear heavy duty plastic tarp to the slab and look for condensation under it.

To be honest I doubt that it is moisture coming up through the slab. if you have that much moisture coming through the slab on a regular basis, you will also have a mold problem in the walls.

What part of the country are you in?

[Jim A Walters]

I have a similar shop that is 40x64x16 and also insulated. However, the concrete pad has a vapor barrier underneath so I don't seem to get much, if any, moisture up through the concrete. I also don't have a A/C unit but I keep it closed up during the summer and only open it occasionally to drive a vehicle in or out so it stays fairly cool.

I run a dehumidifier all summer. It's just a simple home type dehumidifier that you might typically use in a basement. I think it's rated at 70 pints which means it will draw out 70 pints of water per day. I set it near my floor drain and never have to bother with it. I'm sure it's way smaller than what an area that size would call for, but it makes a huge difference. Without it the humidity builds and of course the moisture starts to condense on the cooler-than-air tools. With it, the air feels drier and is more comfortable for working. I was skeptical at first that a unit that size would ever catch up, but it does and will turn on and off to keep it at the set level. If I have it off for some reason or when I first turn it on in the summer it will take several days to catch up.

If there's moisture coming up through the concrete then a unit this size will likely not remove it fast enough. Try what Todd suggested by taping down some plastic and see what happens.

Jim

[Marty Tippin]

To help verify that it is moisture coming through the slab, tape a 1' x 1' piece of clear heavy duty plastic tarp to the slab and look for condensation under it.

To be honest I doubt that it is moisture coming up through the slab. if you have that much moisture coming through the slab on a regular basis, you will also have a mold problem in the walls.

What part of the country are you in?

I'm in Kansas City.

It may very well be that the moisture isn't coming up from the concrete; I'll check later this spring after it warms up. I know there's a vapor barrier under the slab, so it shouldn't be an issue. But as I mentioned, the slope away from the building is poor on 2 sides and we had an incredibly wet summer last year, so I believe water may have accumulated under the slab.

[James W Glenn]

My garage shop is a toltaly different enviroment with an insulated floor put down. I used Delta -FL underlayment and 3/4 foam under Advantec. In the summer I run an undersized window ac that dehumidifies as it trys to cool the space down. I am going to get a small 30 pint dehumidifier that I can mount above the shop sink and run the ac less. Im not sure if I will save on energy consumption, but I cant afford any more losses to rust.

[phil harold]

my 30x40 with 12' ceilings I use a small 1300 btu window air conditioner
runs a lot during the hot summer but keeps the shop dry and moderately cool
uses the same amount of power as the dehumidifier I was looking at, but i get the added benefit of cooling too

[Jeff Bartley]

Marty,
Dehumidifiers aren't terribly expensive so it would be worth a try.
From a purely academic angle: I wonder if the cool concrete slab is forcing the air near the floor (and all that cast iron) to condense? Would a fan help move the air around so the air at the floor wasn't as likely to condensate?
I'm really just thinking out loud here.......

[Bob Falk]

IMO, the best dehumifiers are made by Sante Fe. They are pricey, but will remove gallons a day. I got tired of replacing cheap dehumidifiers every few years. I found one used and it has been running continuously for 6 years in my basement with no problems. You can run duct from some models which helps draw air from a larger area.
I bought a second one for my shop/garage which has a heated floor (radiant). In the winters here in Wisconsin, our cars drag in a lot of snow, which melts....the dehumidifier works well to take care of that. I hang it from the ceiling (it is a model intended for crawlspaces)....will remove 70 pints a day, but it cost $1100.....I don't have any rust problems in my shop.

[Jim Andrew]

I was going to point out that you should have laid plastic under your slab, but you say you have a vapor barrier. Amazing what a difference it is living 3 hours west of you.