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I have a nominally dry crawl space that is around 22' square and about 4' tall with a concrete floor. It has no indication of leaks and is dry, but too moist during the hot Maryland summers to keep anything that is not durable in there. It has the air vents from the HVAC for the room above running through it. What is the best way (if possible) to keep the humidity low enough in summer to keep tools/card board boxes/ anything less durable in there during the humid Maryland summers? My initial thought was to add a vent or two down there and keep them closed in the winter and opened slightly in the summer. Any thoughts/insight would be greatly appreciated.

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    I have had luck in basement sealing the floor with 2 part epoxy paint. I did have a dehumidifier and the amount of water it pulled out of the air was about 2/3 less than prior to painting the floor. – Ed Beal Sep 7 '19 at 22:06
  • in the summer the temperature in the crawl space will be lower than ambient and exterior vents will promote condensation. – Jasen Sep 8 '19 at 7:11
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    Is it really practical to store boxes of household goods and tools in a 4 ft high space under the house? This would create harborage for rodents and roaches. It could be a fire risk. – Jim Stewart Sep 9 '19 at 16:04
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An alternate method avoiding high electricity bills and the necessary water drain for the dehumidifier: Installing a fan of an old PC at the Luv side, connected to a small power supply from an old phone or similar and a timer. If the fan runs only in the night, it pushes in statistically colder and dryer air. In 1 or 2 weeks the space will be much dryer.

  • If this would work, this would be an ideal "green" solution, but most people would need a commercially designed and manufactured fan unit to accomplish this function. – Jim Stewart Sep 8 '19 at 12:14
  • you could use a commercial ventilation fan with the plug timer instead. – Jasen Sep 9 '19 at 10:20
  • I'm unfamiliar with the term Luv? I'm gathering that the idea is to put the fan at the opening between the conditioned side and the unconditioned side? If this is the case would the fan blow into or out of the crawl space? – Dan B Sep 10 '19 at 13:49
  • Luv side is the side where the wind (if any) is blowing into the crawl space. In the northern hemisphere it is normally the most western part of a building, but could be different due to neighboring buildings, mountains etc. In Middle Europe less then 6% of all windy days are having east wind. The fan blows outside air into the space, timer settings from midnight to 5AM. Most PC fans are made for 12V, but socket power supplies with 7 or 9 Volt are also working. Electricity cost ca. 40 Euro-Cents per month @ 30 Cents/kWh. – xeeka Sep 10 '19 at 19:26
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In the summer the temperature in the crawl space will be lower than ambient and exterior vents will promote condensation.

But yeah adding outlets from your air-con system that feed dry air to the crawl-space should work.

during the winter you probably want to feed outdoor air instead.

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    If you added outlets from an a/c duct passing through the crawl space to direct conditioned air into the crawl space, this would be transporting air from the living space into the crawl space. The net loss of air in the living space would create a slight negative pressure in the living space relative to the outside and cause infiltration of outside air into the living space. I don't think it is advisable to hack into the ducting in the crawl space to obtain dryer air for the crawl space. – Jim Stewart Sep 8 '19 at 12:45
  • So if I have a return to my HVAC from this space I have a fighting chance, but not without it? Am I understanding this correctly? – Dan B Oct 7 '19 at 15:49
  • @JimStewart - Do you have a better solution or is it a moot point? – Dan B Oct 7 '19 at 15:50
  • I do not think it is a good idea to have air from the crawl space going back into the house to be breathed by the occupants. It will also be a significant load on house a/c. Any dehumidification system should expel air to the outside. I really don't think a 4 ft high space is good for storage. It would be a nightmare to get stuff in and out, and would provide harborage for roaches, rats and other vermin. I say forget about using this space for storage. – Jim Stewart Oct 8 '19 at 19:45
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You could put a dehumidifier in the space. A dehumidifier is basically a refrigeration unit which is optimized to condense water vapor from a space. It will not cool the space and in fact will be a net heat source.

There would be a problem in getting rid of the condensed water. You would want to pipe it outside and have it drip into the ground. Some dehumidifiers empty into a reservoir which has to be emptied by hand which would not be desirable. Some dehumidifiers might have a pump to allow the water to be piped away and even raised to get it out of a basement.

EDIT

If you added an outlet or outlets from an a/c duct passing through the crawl space to direct conditioned air into the crawl space, this would be transporting air from the living space into the crawl space. The net loss of air in the living space would create a slight negative pressure in the living space relative to the outside and cause infiltration of outside air into the living space.

So you would need an air return to allow the air from the crawl space to get back to the air handler. This would basically connect the crawl space to the living space. You should carefully consider this before hacking into the ducting in the crawl space.

It may be that since you have a concrete floor in the crawl space you could get just enough air flow into and out of the crawl space to keep it dry enough for storage. The register on the vent could be adjusted so the air flow would be just enough to accomplish what you want, but I think you should obtain professional opinions on whether this actually works in practice.

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