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.
My questions:
Will the vapor barrier over the floor make a big difference on humidity level? I planned to put the plywood floor down primarily to make it easier on the feet.
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.
Replies
Brand new construction always creates high humidity for up to a year while materials off gas the moisture and stabilize overall.
Do you have a way to let it vent at an upper window?
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I do have 3 2'x2' windows that open individually. My problem would be rain coming in that window if I wasn't home to close it. With such a small section below grade it surprised me that the humidity is so high. I have considered a dehumidifier that could drain outside but was hoping there may be a cheaper solution.
Thanks,
Dale
A heat recovery ventilator may be overkill, but it would work. A properly sized exhaust fan would also work, but you'd need to engineer intakes.
If the garage humidity drops significantly when outdoor air is introduced, your solution could be fairly simple. Install an exhaust fan together with opening(s) that will allow outside air to enter continuously. The exhaust could be high and the intake low. A louvered, screened intake opening would keep out wind-driven rain and bugs.
The exhaust could be controlled by a humidistat, or a timer, or be manually switched. A fan with about 200 CFM capacity should do the job. If your 600 sq. ft. garage has a ceiling height averaging 10 ft., a 200 CFM fan would exhaust the 6000 cu. ft. volume of air every 30 minutes. An intake of 12" x 12" would easily accommodate 200 CFM without creating undue resitance or high face-velocity.
You can do a simple test to see how much moisture the below grade concrete is contributing: tape down a square (about 3' x 3') of 6-mil poly sheeting to the concrete floor in the area where you suspect moisture transfer. Leave it overnight. If there's a lot of beaded-up moisture on the underside of the poly in the morning, you'll know the source of your high humidity.
Edited 4/23/2009 9:54 am ET by rdesigns
Thanks for the info! The humidity drops very quickly to the outdoor level once I open up windows and the door. The exhaust fan sounds better than a dehumidifier which I think I would have more maintanence issues with. I haven't yet performed the floor test for humidity but will do so this evening. If the slab is the major culprit I would hope a vapor barrier under my plywood floor (yet to be installed) may help.
Thanks again,
Dale