Question about controlling humidity levels in an outbuilding

A friend of the family asked me to stop by and take a look at a humidity problem they’re having with an outbuilding at their home, I’m interested if anyone else has some insight into a solution to the problem that we haven’t considered.

The outbuilding is a framed building on a concrete slab with an attached carport. Attic space runs over the outbuilding and carport. The building has gable vents and soffit vents. The interior of the outbuilding was finished by a previous owner. We live in Georgia where are summers are incredibly humid. It seems like ambient humidity is essentially just being trapped inside the outbuilding as there’s no AC or ventilation systems to circulate air.

We’ve talked about adding an exhaust or attic fan to help ventilate the space. We were also considering installing a whole house dehumidifier unit, but that might be overkill for a 20’x20’ space that’s used every once in a while.

Again, curious if anyone has any expert insight or a solution that we’ve totally overlooked. Thank you in advance for any help!!


Fyi… Humidity doesn’t care how often a structure is ‘used’. It will do it’s damage anyway! Install a dehumidifier anyway. A fan will need to draw replacement air from somewhere, and guess what that makeup air will contain? Yup… HUMIDITY!

@jjonas has the answer. I live in GA as well. Anything in my out buildings is going to mold. Circulating air will help, but it will have to run 24/7. You could use a AC and benefit from its cooling and its dehumidification properties. We used a PTAC in my enclosed carport with no issues.

If you’re not conditioning the space and no one lives in it, there’s not much point to dehumidifying it. You’d just be throwing money in a hole in the ground. And it will grow mold, since the relative humidity inside of the space will never get lower than the exterior ambient RH.

You might want to think about a neutral air system or DOAS setup. Either way, it’s considered space conditioning.

The problem you’re going to run into is that since the space is so small, you’ll reach cooling setpoint way before you remove any of the humidity. I ran into this issue with a boutique hotel in the Caribbean. All of their beachfront villas were 24’x24’ single story jobs with vaulted ceilings and mini splits to provide cooling. The mini splits were reaching temperature setpoint long before they could remove the humidity from the showers and infiltration from opening the doors. The interiors just kept getting wetter and wetter. The floor boards were curling. Mold on the ceilings, condensation on the windows. It was ugly. As a stop gap, I recommended installing little HRV units to try and drive the humidity down. Had to fly them in from Florida. It ended up working like a charm.

You also see HRV’s in the basements of large residential mansions for the same reason. I had one client move in to her new mansion that had been vacant for 4 years, and as her family started living in it, the upstairs millwork started drying out and shrinking (leaving wood splits everywhere), and the basement spaces were getting wetter. The home had six furnaces with central air coils, and 4 HRV’s. Turns out the humidification systems were all shut off, and the HRV’s were all inoperable.

I do not understand this. Encapsulated crawlspaces or basements reach levels below 55% all the time, even when exterior RH is high. That salon is much different than this room because of the added humidity to the space (showers). Many small spaces do just fine with AC’s or dehumidifiers when sized properly.


When the humid air from inside the outbuilding has been expelled. drawing from the crawlspace, where will the make up air come from, Brian?

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With a dehumidifier, the air is just recirculated. The humidity condensates and is drained away thru a condensate pan which is piped to the exterior.


Gotcha…for some reason I had my head wrapped around an AC. :man_shrugging:

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I window AC or PTAC unit has a similar process. But, it removes less.


Just go to your nearest big box store and buy a dehumidifier. You can pipe it to the outdoors or empty the tub in it periodically. I use one in my shop in the summer. Makes a big difference.

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@bmarney - I know this is late, but was on vacation for a bit…I would start with having them clean the roof on a regular basis. Those leaves will trap a ton of moisture in the underlayment for the roof.
Secondly - Obvious air flow will help with air exchanges.
Something that may help and be less invasive and costly if it is on the interior finish side and be a good starting point would be to put in a Panasonic Vent Fan with a built in humidity sensor. I personally like theirs for noise reasoning and they just do it right. Make sure it is vented positively through to the exterior.
If that is not a fix, then other more expensive systems may be needed as recommended by folks from that area. This would be a cheaper starting point.