Hello all,
I have a 600 sq ft garage addition I have just finished. I am using this area as my woodworking shop and have a deck built above the shop space. It is slab on grade with only one wall 3-4 feet below grade tapering to grade level within 20-25 feet. Construction is CMU and brick veneer. The one wall section below grade has a waterproof barrier as well as a french drain. I have painted the interior block with Drylok and plan to leave the wall uncovered. I have I-joists overhead with R-30 insulation between. I have 3 windows, one garage door and a standard door in the shop.
My problem is the humidity is hovering around 80% when the shop is all closed up ( which is all day when I am at work). When I open the area up the humidity drops to the outdoor level quickly. I am in North Alabama. I plan to place a plywood floor on sleepers on the slab with a 6 mil vapor barrier over the slab. I have taped a piece of plastic on the floor for 36 hours and it appears that the slab is not letting much moisture through.
What advice or suggestions would you give to control the humidity when I can’t have the shop open? I have too many tools to feel comfortable leaving windows open when I’m not home.
Thanks,
Dale
Replies
Have you tried the plastic sheet test on the below-grade wall? Assuming that the shop isn't appreciably cooler than the outdoors during the day, 80% relative humidity is quite a lot of water. That water has to be coming in somewhere.
If the shop does stay a lot cooler than the outdoors, then the increased humidity is likely to be from air moving in and out. The warm, moisture-laden air diffuses in and cools, which raises its relative humidity. You can try improving the sealing, but you'll probably end up having to use a dehumidifier regardless.
-Steve
I haven't tried the plastic sheet test on the wall. The room is at least 10 degrees cooler on the inside as compared to the outside temp. The slab didn't show any sign of moisture which surprised me as I thought this was where most moisture would be coming from.
If I go with a dehumidifier would you recommend a light commercial unit or some other type. I would want to drain it out through a wall to avoid emptying a container. Thanks,
Dale
It would seem to me that in Alabama that you would need to dehumidify the air at outside temperatures. When you cool the air the RH goes up drastically. I dehumidify almost all the time when the heat is not on. Try to keep it down to 50%.
One thing that may be better at keeping moisture out than poly is foundation film. It is quite expensive but very effective. But any time you open a door into a cooler space, a gush of moisture will rush in.
Unfortunately, that 10°F difference is probably enough to explain most, if not all, of the high humidity. I just checked the weather for Huntsville AL, where the current temperature is 71°F, and the dew point is 54°F. This gives a relative humidity of 55%. Dropping the temperature by 10°F raises the relative humidity to 78%.
I don't have a good recommendation for you regarding what kind of dehumidifier to get. I've always gotten by with just a small household model (Whirlpool) in the past, but that was in a smaller area and somewhat drier climate.
-Steve
I keep a fan running 24/7.
Do you mean an exhaust fan that is pulling fresh air through the building?
Thanks,
Dale
No, just keeping the air moving in the shop. And I try to keep outside air out.
I'm originally from North-Central Alabama (Birmingham), and I can assure you that you will absolutely require some active dehumidification of some sort. I'm thinking July, where the nighttime conditions are 80 degrees and near 100% relative humidity.
From the standpoint of what to get, that depends. If you're going to be working in the shop in the summer, you might require an AC window unit to keep the temp reasonable (like less than 95 degrees!), and of course an AC unit will also have the effect of dehumidification.
However, that might not be enough - you may also need a dehumidifier unit for the fall and spring conditions where the temperature is low enough that air conditioning doesn't make sense and the RH is quite high, leading to condensation on your tools.
I've about a 1000 sq. ft. basement shop, and have a Whirlpool consumer dehumidifier. This is a low-temp model tha was one size up from the smallest available at the Home Despot. It was about $200. Well worth not having to de-rustify the innards of my table saw, and especially my hand tools!
Thanks for the reply. After watching the humidity level the last few days I definately will have to get a dehumidifier. I am OK working in the summer with doors and windows open (with a fan running). When I went in my shop last night the humidity was in the upper 80's. As usual when I opened the windows and door it returned to the outdoor level. I am trying to research whether I should get a small commercial dehumidifier or if I could get by with a consumer model over the long term. I would like to plumb it through the wall so I wouldn't have to worry about emptying a bucket all the time.
Anyone's input on their dehumidifiers would be welcome.
Thanks,
Dale
Dale - I suspect that you will find that you can purchase three or more consumer units for what a commercial one costs. Mine's lasted 5 years so far, and shows no signs of problems. Plumbing a drain is as easy as drilling a hole through the wall. Because what's collected by the dehumidifier is pure (absolutely pure) water, there is no sewage requirements.
In fact, you could mount the dehumidifier high enough on the wall to collect the water outside in a rain barrel - it's excellent water for plants.
You comment about the humidity in your garage being very high with the doors closed could be caused by a couple of different reasons, or a combination. You may indeed have a sub-grade vapor transmission problem, or you may just be seeing the physics of heating up the air - the 80% humidity air will go down by about half if the air in the garage is heated up by 10 degrees. In fact, you might be seeing a situation where 60% R.H. outside air is filling your garage with the door open, and when that's closed up and left for a couple of hours, it cools down sufficiently to get to 80%.
If you do think you've a moisture transmission problem through the concrete floor and/or a sub-grade masonry wall, you might consider epoxy for the floor (comes in cheap kits at the big-box home stores) and Dry-Lock paint for the wall. It's inexpensive, more permanent than a plastic vapor barrier, and not as prone to damage.
Good info. Thanks.
I have drylok on the walls and have tested my slab with a taped on piece of plastic for 2 days. The slab didn't show any sign of moisture when I removed the plastic. I think my situation must be from the warming of the air once the shop is closed up. The portion of the wall that is below grade has a waterproof membrane on the outside and also a french drain- it doesn't show any signs of moisture on the inside. That is one of the reasons I was so surprised at the high humidity level since I couldn't understand where the moisture was coming from. Thanks for all of your input.
Dale
Dale - perhaps a better test of your slab might be to cut a thin sheet of scrap pine (say, 1/8"), 8-10" wide, with face grain on the faces (i.e., not quarter sawn), and place it on the floor for a day or so. If there's substantial amounts of moisture coming up from the floor, the board will cup, with the convex side the one that was on the floor. That won't tell you the mass rate of moisture, but it might suggest whether it's a concern at all.
The plastic test is a good quick and dirty type of thing, but if the air in the shop is warmer than the slab, water won't condense on the plastic, even if a substantial amount is coming up from the floor.
One final thought - you can get mini electronic "weather stations" for dirt cheap these days, and a lot of them have recording functions in them. That might be very diagnostic, and is a useful thing to have in conjunction with a dehumidifier.
Any possibility to installing a whole house fan? I should think this would provide a lot of air movement/exchange.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob - As a native-born of the South, I can assure you that a big fan doesn't help much in Northern Alabama during the summer - the daytime temps are above 95 degrees, with near 85% humidity, and that drops down to a balmy 85 degrees or so (at 3 a.m. in the morning), with saturation humidity (100%). North Carolina (where I now live) is similar, it just doesn't last quite as many weeks as in Alabama. It's murder on metal tools - there aren't many antiques found in barns around here that aren't piles of rust.
David, I guess I didn't realize how hot it gets down there AND the humiditty too. Crimus Peter it must hurt your eyes to blink in that weather.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
For me i use LPS Heavy duty rust inhibitor on my tools. On all tables, on all my tools i apply a thin coat every 2 months and my tools have stopped to rust in the summer. My work shop is in a basement and the temperature goes low around 15-20 in the summer everytimes i open my garage door when it was 30 C my tools rusted but now with LPS no problem, they never rust.
http://www.lpslabs.com/product_pg/corrosion_pg/LPS3.html
They also sell a 4 liters, that what i use, we have close a paper mill and they have used it to prevent rust on all equipment. It's a very good product to prevent rust.
Edited 5/2/2009 6:26 pm ET by lgaudre
"David, I guess I didn't realize how hot it gets down there AND the humiditty too. Crimus Peter it must hurt your eyes to blink in that weather."
It gets just as hot here in Raleigh, NC (2 summers ago there were over 25 days that were hotter than 100 degrees F. THe hottest day was 106 F, with about 85% humidity), it just doesn't stay hot as long as it does in Alabama. I went to Auburn University for my undergrad degree, which is on the plains of lower Alabama, and it gets a lot hotter than even Birmingham and points north.
But - Just about every Southerner will freeze their arses off at any geography point north of the Mason-Dixon line. It must amuse northerners greatly to see a football game come on from two SEC teams where everyone in the stands is bundled up in 40 degree weather. I think the guys at Lambo Field would probably be wearing shorts and a t-shirt if it every got up to 40 degrees during the pro season.
I'm from a completely different part of the world so this might be ridiculous but would an input and output fan keep the air in the basement shop the same as outside and eventually equalize everything? Or would this require the basement walls to heat up as well to prevent condensation build up? On reflection maybe this is completely counter productive but comments would be appreciated in the interest of education.Eoin
That would sorta kinda work until the temperature dropped below the dew point, at which point water would condense out of the air on everything. And in the South, the temperature frequently falls below the dew point at night during the warmer months.
-Steve
Steve,
Correct me if I'm wrong but it sounds to me like ones dehumidification efforts might best be done at night? Also could one conclude that keeping the moisture out in the first place might be an area to concentrate on?
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
The air is always going to be more saturated with water vapor at night, but humidifiers work by creating a temperature differential, and the achievable differential is going to be lower at night, when the ambient air is cooler. So it's probably a wash. My bet is that for a given water load (that is, a given dew point), the humidifier will work more efficiently when the ambient air is warmer. But that's just a guess.
In any case, most humidifiers have a built-in humidistat, so they turn themselves on and off as necessary.
Sealing moisture out is certainly one way to do it, but it's hard to do. In a garage shop, it's all but impossible; even the best garage door seal doesn't do much to stop air exchange. You'd essentially be converting the shop into a conditioned space, just like the inside of the house, and as with the house, you'll pay more if the space isn't sealed and insulated well.
-Steve
Dale,
Greetings from Birmingham, AL, home of the 2020 Summer Olympics! (that's an inside joke for us Alabamians) I too have a basement shop, but can't say I've experienced any significant problems with the humidity. It's about 600 square feet where I work, and although I once had a dehumidifier to control the humidity it ran all day long and had very little effect on the moisture level in the air. Typically, according to the dehumidifier, the humidity in my shop is around 70%. I gave up on the dehumidifier and since then, I just clean/wax all of my tools at least once every three months and I've never seen a spec of rust anywhere in my basement.
If you go the dehumidifier route, I have two things for you to consider.
1) If it runs all the time, like mine did, expect your power bill to increase about 30-40% even during the summer with the A/C on all day. They suck up the power.
2) If you're central cooling unit for the A/C is in your basement, as it is in mine, there should be a pump or drain of some sort for condensation. I ran the discharge hose for my dehumidifier into that condensation drain, which then gets pumped out into the back yard through a small plastic tube exiting the rear wall of my basement.
Good luck with the moisture. As you probably know, it's just a part of living in the South.
I have the same problem over in South Mississippi I have purchased a
dehumidifier and I also have a window unit that I keep on 78 degrees I also purchased some blocks of rust inhibitors from Lee Valley Tools and keep them in my box with my fine woodworking tools. I have about 7 planes and 2sets of chisels. I also collect tools and most of my collector tools have light rust on them but I don't let them rust I wipe them down with bio-shield a protectant.
Dale,
Try utilizing a small portable dehumidifier, they are readly available at Sears or Home Depot. They not only maintain humidity very well but also help maintain personnel comfort in the space. Keep it set for about 50% humidity level, they are also pretty energy friendly. Hope this helps.
Dave
Edited 5/5/2009 7:12 pm ET by walonguy
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