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I just bought a house that has a HUGE detached garage. The problem is that there is NO air movement at all. It ends up smelling really musty and some of the items inside seem damp. A friend suggested adding some vents, but I don't understand how that helps. Wouldn't it still get damp from the morning dew or a humid day?

Currently, I don't store my car in there, but I have plans to do so. I'm also planning on doing tasks like stripping paint and repainting, etc.

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  • One thing that might help you with understanding this is understanding how dew forms in the first place - it's not all that different from the answer to this question!
    – Joe
    Oct 26 at 15:44
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You ask how ventilation helps with reducing humidity. Here's how:

  • An enclosed space, such as a garage, holds the air inside it fairly still. At the same time, it's warmed from the outside by the sun for at least part of the day on some days.
  • This heats the air inside the garage; if you put a thermometer there, you'll find that even without any form of heating, it tends to be a bit warmer inside than outside.
  • Warm air can hold more moisture than cold air can. If you take cold air at 100% humidity, and then you warm it by a few degrees, it will no longer be at 100% humidity.

Given these facts, here's how ventilation will make the garage dryer:

  1. Cold air moves into the garage.
  2. The air heats up; this makes it capable of absorbing more moisture.
  3. The air absorbs some of the moisture from the garage.
  4. Warm moist air moves out of the garage.

This obviously works best if you have at least two openings for ventilation. Having them on opposite walls, and one low and one high will help. The opposite walls help because then any wind present will help move air, and the low/high helps because warm air rises, so even without wind warm air will exit the top ventilation hole and be replaced by colder air drawn in at the bottom ventilation-hole.

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    Thanks! That makes sense. I'm glad everyone is confirming the double ventilation system, too. I just need to figure out the walls because one wall has another storage building attached, so I have 2 walls next to each other and the door wall
    – Brandy
    Oct 26 at 13:02
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Vents at top and bottom (you need both) will help. How much? That varies a bit. But even without any mechanical ventilation, air will flow naturally, particularly as it warms up.

However, this is likely not going to be enough when for stripping paint and similar activities. The old-fashioned solution is to deal with any nasty-smelling stuff with the garage door open. But you could also install a big exhaust fan in front of a gable vent and run it when you need extra ventilation.

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  • Thanks! I did see the information about top and bottom. It's about 700 sq feet with a very high roof. I'm just not sure how it will keep it from getting damp in there.
    – Brandy
    Oct 26 at 0:13
  • It won't totally solve the dampness, but it will help. Oct 26 at 0:14
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    You have a roof keeping rain off, and the vents allow the air in the building to come to equilibrium with the air outside. Dew, like rain, will not fall in the building. Humidity may condense in the building when parts of the building are cooler than the dewpoint, but the air movement will help the building to come to a temperature similar to the outside and evaporate that condensation, unless you are sitting in a fog bank. When the building heats up in the sun, air will actively move (given upper and lower vents) and take any excess moisture inside the building out with it.
    – Ecnerwal
    Oct 26 at 0:56
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    You might also need to address grading and/or gutters/downspouts (if you don't have gutters, concentrate on the grading first, IMHO) around the building, if it's damp inside because water flows off the roof and runs back to soak the (presumed) concrete slab floor, rather than running away from the building.
    – Ecnerwal
    Oct 26 at 1:00
  • @Ecerwal - Gutters need to be added, eventually. That's a whole other thread. Thanks for the information
    – Brandy
    Oct 26 at 18:09

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