Helping dehumidifier run more efficiently

[Augusto Orosco]

I have a dehumidifier running in my basement shop. The shop is small, about 17x15 but there is about a 2'x3' opening to a crawlspace, right in the middle of one of the shop side walls. The crawlspace is not insulated, completely below grade and about 1,000 sf. Water doesn't get in the crawlspace very easily, but it's not completely water proof: I was able to spot some tiny puddles here and there in the crawlspace after hurricane Sandy.

The shop walls are concrete block, as is the crawling space. The concrete walls in the shop have been painted with drylock or something similar (it was done before I got the house).

I can keep the shop at about 50-53 RH, but with the dehumidifier running almost constantly. When I measure RH right at the opening to the crawlspace, it's about 60-64. So, obviously, some of the dry air is escaping through the opening. My question is: How should I block the air from escaping? Would simply covering it with plastic and tape work? Or should I work on something more hard-core, like a panel with insulation on the back. I want to be able to easily remove the cover and access the crawlspace if necessary, so I don't want a permanent cover. Also, I am not concerned about heat/cool loss; my shop is actually quite comfortable on that front without having to do anything (the furnace/central AC being in there helps a lot).

Any input is appreciated!


P.S. For what is worth, my shop is a hand-tool shop; nothing tailed that creates fine dust; just the occasional run of the power planer and a bandsaw for re-sawing will be eventually added.

[Mel Fulks]

I use a piece of foam plastic insulation just cut to snug fit with a couple of duct tape handles. Very attractive!

[Charlie Velasquez]

Just thinking out loud....
The air in the basement is 50-53% RH; right at the opening is 60-64%, some puddles in the crawl space so maybe about 90%. Mold grows at about 70%.

I would be thinking the opposite, how do I make sure I get the water vapor out of the crawl space. Maybe a small fan?

[Augusto Orosco]

I use a piece of foam plastic insulation just cut to snug fit with a couple of duct tape handles. Very attractive!

Very shabby-chick. Like the idea!


Just thinking out loud....
The air in the basement is 50-53% RH; right at the opening is 60-64%, some puddles in the crawl space so maybe about 90%. Mold grows at about 70%.

I would be thinking the opposite, how do I make sure I get the water vapor out of the crawl space. Maybe a small fan?

You party pooper. But good point. This is the first time I see water; probably given the amount of rain we got. It wasn't even a puddle, just dampness. But maybe I should heed your advice and make sure the space gets good air circulation.

[Jason Roehl]

What is the floor of the crawlspace? Dirt? Is it covered with plastic?

[Carl Beckett]

I'm another that would be concerned with mold. 70% sounds high even....One trick I have seen that might help both causes is to exhaust your crawl space to the outside. This pulls clean dry air into it from the living space, which is usually at an acceptable rH. ( you can even give it a spo to pull from if you like

[Jason Roehl]

I'm another that would be concerned with mold. 70% sounds high even....One trick I have seen that might help both causes is to exhaust your crawl space to the outside. This pulls clean dry air into it from the living space, which is usually at an acceptable rH. ( you can even give it a spo to pull from if you like

This is a horribly inefficient approach and is pretty much never recommended anymore by professionals or building codes. In the last 10 years or so, all the construction trade journals I've read have universally promoted sealing off crawl spaces to the outside and to the ground below, bringing them into the conditioned envelope of the structure. This helps control heating and cooling loads as well as moisture transfer. You can't pull in "clean, dry air" when it is raining outside.

[Carl Beckett]

This is a horribly inefficient approach and is pretty much never recommended anymore by professionals or building codes. In the last 10 years or so, all the construction trade journals I've read have universally promoted sealing off crawl spaces to the outside and to the ground below, bringing them into the conditioned envelope of the structure. This helps control heating and cooling loads as well as moisture transfer. You can't pull in "clean, dry air" when it is raining outside.

Hi jason

I agree that pulling in outside air is a bad idea ( and posted a doe report on this a while back)

Pulling previously conditioned inside air into a crawl space. Via slight negative pressure, does work. And it's generally more efficient to dehumidify with central hvac than a small stand alone dehumidifier

[Augusto Orosco]

What is the floor of the crawlspace? Dirt? Is it covered with plastic?

The floor is not dirt. Looks like rough foundation material (concrete but not smooth? I am describing the best I can, I am not familiar with construction materials). The water that I found didn't seep from the floor; I came down from one of the walls that is not fully sealed (I used to have an oil tank in there, and that's where the piping came into the house).

[Don Jarvie]

Might want to cover and seal the space with plastic and cover with some wood and see what happens. Definitely close up the old tank holes.