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I have a small, 40 year old garage in the backyard of the block of apartments where I live, which I've turned into a workshop. I replaced the roof, and repaired and strengthened the back wall, filled the missing bricks in the side walls, and cast a new concrete floor. It was a makeshift repair - the best I could do under the circumstances.

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Now, the problem I have with this garage is the insane amount of moisture that accumulates in it. I inspected the roof during heavy rains, and there are no leaks whatsoever. There is plenty of ventilation (gaps in the walls, steel door slits, between the back wall and the roof, etc.). Despite that, after each winter, I come to find all of my steel tools with a thick layer of rust on them (even though I clean and oil them before winter), and wooden surfaces showing signs of mildew and rot. Even woodscrews in their boxes gather rust. I had some rags laying on the table in the garage, and some of them were so damp, that I managed to squeeze out a few drops of water... I live in a cold and humid climate zone, where the ground itself is radiating moisture all the time. Every time there's rain, moist air finds it's way into my garage through the hundreds of gaps in the walls, and settles on every surface. Every morning, there's heavy layer of dew as well. I measured humidity inside my garage yesterday - it was 75%. Outside the garage it was just 30%. Almost an unbelievable difference. Again, as I said, there are no leaks from the roof - all this humidity is just from the moist air.

I need to find a way to dehumidify this garage. Heating is not an option, because plugging every gap is just impractical, and without insulation, the electricity bill would be insane. I sometimes joke that there are more gaps than walls in this garage.

I considered another option - hanging bags of calcium chloride to collect all that moisture, and dry them from time to time in the oven back in the apartment. However, since the winds are howling through the garage through every gap, these bags would saturate after a few days.

With all of this in mind, I would like to ask for advice. What would be the most practical and economical way to keep this garage dry? The essential limitation here is that it is impossible to insulate all these gaps. There's just no way to prevent all that moist air from entering the garage, but maybe there is a way to collect it without any significant impact on the electricity bill.

Really looking forward to your suggestions. Thank you in advance.

  • No passive moisture absorber will keep up with the airflow through the gaps. You need to seal the garage up somehow. Poly sheeting, rigid foam, paneling. Whatever. It's either that or an electric dehumidifier, which will be expensive to operate, and even that will struggle. – isherwood Apr 23 '18 at 21:02
  • Adding any kind of sheeting or paneling will reduce the usable space in an already cramped garage, not to mention the cost. The best I can do, given the circumstances, is spread some expanding polyurethane foam. But I really doubt I can seal the garage enough, there are just way too many cracks everywhere... – J R Apr 23 '18 at 21:16
  • Poly is 4 mils thick. You could fasten it to 1x2 strips along the top and bottom. You'd lose less than an inch. – isherwood Apr 23 '18 at 21:34
  • But I would still have to cover up that poly with something. Having plastic sheeting hanging down the walls in a heavy duty workshop is just asking to accidentally tear it... Not to mention the installation of shelves and other stuff. – J R Apr 23 '18 at 21:38
  • Hello, and welcome to Stack Exchange. @isherwood has given you some good advice, and he's right: if you have outside air freely blowing through the garage then there's nothing you can do about the humidity. – Daniel Griscom Apr 23 '18 at 23:52
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If the humidity outside the garage is so much lower than inside, you could go the other way and use a fan to increase ventilation to prevent humidity buildup inside.

We made a poly 'clean room' inside an old garage before and had to tear it all out after the outside of the poly started to mildew against the walls...

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