Why is my shop so humid?

[sandman2234]

My guess is also dew. Sounds.like your bouncing around the dew point alot at that temperature. Being unheated space attached to the house, the house may be keeping it close to the dew point causing you more grief.
David from Jax

 

[AHNC]

I'd suggest that you look at Room to Room Fan : Crawl Space Ventilation : Dryer Booster Fan : Tjernlund Products, Inc. at the new Xchanger product they are making. A few years ago they made a crawl space ventilator for southern sales and it did so well that they made the Xchanger for basements, and in your case a shop. A lot of times what you need is the air movement. It is supplied with a dehummidistat for control. some folks plug the main power into a time clock. Only draws about 40 watts compared to a LOT more for a dehummidifier. As always, I must note my bias as I represnt Tjernlund in my home market, but this might help you out.:thumbsup:

 

[dodge man]

Did you put gravel covered with plastic under the concrete floor? If not, I'd say thats your problem. If you did, like others have said, it seems like under certain conditions, your floor will atract moisture no matter what. I keep my doors closed when possible under these condtions. I run a dehumdifier in my garage, and as long as its closed up, it stays very dry.

 

[nealfris]

The concrete sealers are highly over rated. I have used it in many applications. Did you buy quality sealer from a masonry supply or box store??? Some products are no better than water. Also many coats with drying time is mandatory.
Also you could ask either the local masonry supply house or masons for advice. Local knowledge is invaluable. The wall paint, if its old could be letting in more moisture than you can imagine.
As a licensed builder I can tell you the ceiling could be your culprit. When you have a high humidity condition feel the ceiling with your dry hand. If it feels very cold and clammy, than you have moisture coming through the roof just like fog settling in a valley. With little insulation you have little resistance to climate change. In the scheme of things insulation is cheap. If you do add insulation, make sure you properly ventilate the attic. My father garaged six school buses in a big old garage with concrete walls and floor. it had trusses with the old homosote panels on the ceiling. It practically rained in there. A friend gave me over 100 bags of abandoned blow-in type cellulose insulation. I blew in about 12" over entire ceiling. Problem solved!

 

[Transit]

If you don't have a vapor barrier under the concrete floor moisture is most likely wicking up from the ground.

 

[Carl Bert]

First, as others have asked, did you use plastic between the gravel and the concrete floor?
Second, does it have a floor drain, and if so does it have a trap?
Third, is it completely above ground on all sides?
And last, you said the roof was insulated. Do you mean the actual roof joists are insulated or the ceiling (attic floor) is insulated, or both? I'm assuming this is a pitched roof and not a flat one.

The best thing you could do, since you are heating this space anyway, is to frame out and insulate the exterior walls and put a SEALED vapor barrier, taped at ALL seams. Then insulate the shop ceiling (R30), also with a vapor barrier. Do not insulate the roof joists (again assuming this is a pitched roof) and make sure the attic is well ventilated. You should have a combination of soffit vents, and a ridge or roof vents. Then seal the floor with a quality masonary sealer.

Probably more then you wanted to hear, but your just wasting money if your heating an uninsulated space.

 

[zmoz]

First, as others have asked, did you use plastic between the gravel and the concrete floor?
Second, does it have a floor drain, and if so does it have a trap?
Third, is it completely above ground on all sides?
And last, you said the roof was insulated. Do you mean the actual roof joists are insulated or the ceiling (attic floor) is insulated, or both? I'm assuming this is a pitched roof and not a flat one.

Thanks for the information everyone. I didn't build the shop, so I'm not quite sure what is under the slab, but I do know it is about 12" thick and reasonably sure there is gravel under it. No floor drains, all above ground. One side has a pole barn attached to it so it's basically protected from the weather. (not even going to attempt to dry out the barn)

The insulation is just a thin/spotty layer of blown in fiberglass on the attic floor.

I have a giant exhaust fan on the roof, which pulls air from the shop through the attic man hole. I could leave that running, but this being Oregon...the humidity outside is often just as high. I really just want to seal the whole thing up so that after the dehumidifer does it's job, it will stay sealed and dry. As it is now the humidity starts climbing the second I turn off the dehumidifier. (which is an old 5,000 BTU air conditioner) I am absolutely amazed how much water it can pull out of the air...and I don't understand how it comes back so quickly!

One thing I'm thinking about is the block walls were built, then the floor was poured inside. The floor has shrunk maybe 1/16" all around the edges, maybe I should seal up that gap?

I do have some "stuff" bubbling through the paint in one small area, but it's dry and chalky, not wet at all.

I do want to repaint the inside white...is there any special type of paint I should use? I know they make sealers for basement walls and stuff, but I think you have to put that on before any paint...

 

[nealfris]

Your walls being dry and Chalky makes me very curious. If the air is as humid as you say, the wall paint should at least be damp. What to repaint the walls with has more to do with whats on it now. I know this will sound crazy but, You could tape paper towels all around where you think there's a problem. If you find wet towel, or tape falling off, bingo. You mentioned an attic scuttle hole. If you leave that open all the time than the damp night air will pour in your garage.
Is the pole barn shadowing (on the south side) the garage completely? The 12" pad thickness makes me very suspicious. A residential garage slab is generally 4", a commercial slab usually 6" with more steel and more pounds per yard mix. Is it 12" thick because the builder knew something was not right and he was over compensating to prevent cracks? Is there any indication this garage was built in a big puddle with no place for water to drain? The 1/16" gap should be a none issue unless you have standing water under the slab.
The large exhaust fan in the attic is generally used for removing heat in the summer. Leaving it running will feel like its removing moisture, but may be sucking it in from the out side just as quickly. Although you haven't said directly, you indicated an attic so we might assume you have a gable roof with rafters. If this is the case, it would be advisable to install both soffit and ridge vent. The reason this type of venting is built into most building codes is because it is venting 24/7. It also saves you electricity.

 

[nealfris]

I was just second guessing myself. Rather than paper towel, you would be better served to use plastic. Put large squares over walls, on ceiling and laying on floor. That way you could see if moisture was on the inside or outside. Sorry for confusion.

 

[Transit]

Zmos, the more I read, the more I feel as suggested, that the slab is sitting in a swamp. Can you dig a few test holes around the foundation about a foot deeper than the footings to see if they fill with water?