Bigmikey65

I have a 2 car, detached garage (separate structure from my house). I recently poured a new 6 inch slab with a 10 inch vapor barrier, and put a commercial grade Polyurea coating on top. The garage is in a shaded area and the surrounding area outside does get and stay damp. I sheetrocked it with 3/4” boards and sealed the surrounding seams and cracks the best I could. I have not been able to get a humidity reading yet. I use the garage to store my car - it recently went through a frame-off restoration. Is a garage dehumidifier a good idea and are they effective ? Any particular system or unit that you recommend ? My main concern is keeping the garage as dry as possible for the long term protection of my car. Thank you.

Magna86

A dehumidifier is worth it. The bigger the unit the less you have to empty it. Ultimately getting a/c in there will be your best solution. You add fans too to keep air moving to help dry things out. If you have windows small a/c units can help as well but those will use more power. I had one in a single car garage and you would have to empty it everyday. Mine was around $100 from home Depot and had the ability to add the drain hose I just couldn't use it.

scrappie

I’d say its worth it. I have one in my garage 28x38. Garage is Insulated with sheetrock. It definitely helps with moisture and bare metal not flash rusting as fast as it may without. It would fill up everyday and then I piped a hose to drain outside and it just runs constantly. Noticeably dryer/less humid than outside these days. I’d recommend getting one.

Bigmikey65

Thanks guys. Just to add - I live in the Northeast, close to the coast so we usually have high humidity during all 4 seasons, peaking in Spring and Summer.

BangScreech4-4-2

When I drain a couple of gallons a week out of my dehumidifiers, I figure it's worth it because that's moisture that won't be absorbed by my cars.

Bigmikey65

So when you hook the drainage hose up to the Dehumidifier (seems like some come with 16 feet of hose) what’s the best way to run the hose to the outside of the garage so that it can drain properly ? I would assume drilling a hole through the garage wall for the hose to drain ? Do you need to attach the hose to the garage wall with a gradual step-down/decline so the water drains properly ? How far off the ground should the hose exit be ?

briane

I run a dehumidifier in both the garage and basement. I'd say they make a big difference. I've been using Frigidaire, their larger models, something like 60 to 70 pints, which I recall is a measure of how much moisture they can pull in 24 hours. I have to empty them daily. The Frigidaires seem to perform well when they're working, however the last two stopped pulling moisture after about 2 1/2 years use. Considering the season here is only 4 to 5 months that's not very good. They had lost their refrigerant charge. The warranty is 5 years on the closed system. They sent new units to my door without any questions. I'm hoping they've solved the seal issue.

Weezer

This is easy enough for you to google, but since I'm here...

Dehumidifiers are best set up with a hose to a floor drain using gravity -- 1/4 inch of drop per 10 feet. Since it doesn't sound like you have one, you've got a couple of options. You could set up the humidifier on a shelf (or anything a bit off the ground) so that you have a decline that will allow it to drain properly. Or, you set it up with a condensate pump. These are fairly cheap and easy to install.

https://www.sylvane.com/how-to-drain...umidifier.html

69HO43

The dehumidifiers sold on the market like the Frigidaires, etc., are a crapshoot with their tubes and solder joints. THIN metal and poor workmanship tend to not hold up well in the long run. You will likely have to go through several warranty replacements until you find one that won't just "freeze up" due to low refrigerant. You can maybe fix them better and recharge them, but the workmanship is generally crap on most units. The more you spend the better they are I believe, but none of them are stupendous. They do work well and are worth the chance, IMO. I'm on my 4th one over a 4 year span, though.

All you have to do is ensure you have a gravity drain to the outside for the larger units whether a fitting through the wall or just a piece of garden hose long enough to reach outside the door. Just get it to outside or run it to a floor drain if yours is plumbed for drainage. The water collected is condensate taken out of the air, and it is cooled. Meaning it has a high chance for the drain hose to get the "mung monsters" in it. So periodically undo the line from the unit and blow some air through it or flush it out to keep it from clogging. If it backs up, it usually does it right into the catch bucket and fills up and will turn off when full.

Advice is to get one with a programmable humidity level so you can choose which percentage level of humidity you want to control. I got mine set for %50 and it keeps everything pretty dry to my liking. I also have a humidity meter that tracks highest and lowest humidity. So far it's been keeping humidity in check. I would advise against running it 24/7 because it will eat electricity and it also tells you that your unit isn't working up to snuff. Not a huge electricity draw, but you'll notice it.

I'm waiting until we upgrade the house HVAC unit and stealing the old stuff and moving it to the shop and get rid of the dehumidifier. If I had a small 2 car garage, I'd probably buy a split mini-unit HVAC something like this:

67OAI

Maybe I am lucky! I bought a Keystone KSTAD50B three years ago and have been running it constantly. In high humidity conditions ti requires emptying every other day. It also gives you the option of plumbing a drain hose to eliminate the need to drain it manually. Think I got it from Amazon. If your garage space is fully insulated, a mini split or even a window unit may be advantageous.

Bigmikey65

How much would a mini-split run - ballpark figure ? Installed. Figure 500 square feet.

83hurstguy

Quote one at alpinehomeair.com - I just installed a 2.5 ton unit for 1250 sq ft with high ceilings, and have about $2500 into the project. You could probably get away with a 1 ton for 500 sq ft, which is likely $1300 or so for the full DIY kit, some of them can run on 120V power. I've been really happy with mine, keeps the garage dry and much more habitable.

Here's the sizing estimator, they have other system selection tools as well: https://www.alpinehomeair.com/sizing-estimator

VC455

Mikey,

I worked as an industrial HVAC engineer for 30 years for a world-wide engineering company and I can give you some $300/hr advice, free of charge. Here's the Cliff's Notes version...

To preserve your cars effectively, you need humidity control not temperature control.

As you have been told, an air conditioning unit is more reliable than a dehumidifier for Summer duty. You just need to select it unconventionally.

Your situation requires an undersized air conditioning unit. That way it can run constantly to eliminate humidity without over-cooling your storage area.

The smallest mini-split will work wonderfully in this duty for many years. You would control it with a humidistat not a thermostat. You don't care about the indoor temperature, you just want dry air.

If you install a "properly" sized unit (the guides you will find on line will suggest a unit size too large for your purpose) you will not have adequate humidity control and the unit will be more expensive than necessary.

The most challenging time for corrosion will be Spring. Your cars will be cool. Warm breezes will bring lots of humidity that will eventually get into your building and condense on your cool cars. A dehumidifier or air conditioner will be useless in this situation because the garage temperature is already cool.

You have a choice about Spring humidity control:

1. You can buy a heat-pump version of a mini-split. Again you would control it with a humidistat. It will automatically switch over to heat in Winter and Spring and the heat will control the relative humidity.

2. You can add a conventional natural gas or propane heater--controlled with a humidistat (again, not a thermostat). You would not need a heat pump version of the mini-split in that case. Operating cost will be low because you are not trying to heat the area--it only needs to get warm enough to reduce relative humidity when required.

Some mini-splits are made as heat pumps but they may be too large for your use and they wouldn't control humidity effectively in Summer. If you can find one in the range of 0.5 tons or so, buy it.

If you want to discuss this further, send me a PM with your phone number.

Cheers,
Gary

Hammerdrop

You are right to be concerned. A friends 05 Corvette spends winters in a unheated garage with no vapor barrier. Recently the brake lines rusted through.

83hurstguy

Or you can "right size" the mini split so it can cool when needed and run the mini split in dry mode (which they have) when you just want dehumidification - it turns the fan speed on the evap down low and also the condensing unit load. Not sure how many will be able to do humidistat control without becoming expensive.

VC455

Luke, good observation about dry mode. A larger unit will cost more but won't have a downside if you use dry mode. You'd then have the option to cool down or heat up faster when you want to work.

Humidity control isn't very expensive. If you are considering it, buy a dehumidistat (Honeywell makes one for about $50) and a power relay. Relays go for about $30 on eBay and Amazon. Wire the dehumidistat to control the relay, which will control the power to your unit.

Gary

Bigmikey65

I think that Gary has a very good point about using a Humidistat. I did some preliminary research on some Mitsubishi mini-split heat pump units. 6000 btu (1/2 ton) and 9000 btu (3/4 ton) units. There is a Dry Mode on these units. But, according to Mitsubishi’s User Manual for these units : “Dry Mode : Dehumidify your room. The room may be cooled slightly. Temperature cannot be set during DRY mode.”

So with this particular unit, installed as is, you can control the humidity in Dry Mode but apparently not the temperature. Am I missing something ? How would this work in a garage application where the main concern is humidity - not temperature ? Obviously electric use and efficiency is also a concern.

69HO43

In a nutshell, dry mode will work less, so it can save on energy bills a little, and drop humidity. But only about 25% or so. It slows the fan speed down across the evaporator to allow more moisture to collect on the evap coils and drain away. But it's not a dedicated dehumidifier so it will solve specific humidity issues, but not all. So if you live in the northeast along the coast, humidity is more of an issue than heat most of the time, and a dedicated dehumidifiers are probably the way to go, or can work in tandem with the mini-split if you need some air conditioning. If you live in Houston, where it's hot AND humid a lot, you'll likely need both a/c and extra dehumidification. If I wasn't sure, I'd get the mini-split with dry mode and see how it goes, and if it doesn't get the humidity where you want it, buy a dehumidifier to help it out. Dehumidifiers alone will not cool off your garage.

One-trick-pony dehumidifiers operate on cooling the air coming into the unit so it can condense and physically remove that moisture through the bucket or drain tube. Then on the way out, that air passes over the condenser to heat back up thus raising the now drier air's temperature slightly. This drops the relative humidity of that air going back to the space even further. When that drier air mixes with the moist air in the room, the average relative humidity throughout the room starts to go down. As long as it's keeping up with or besting the rate at which moisture is entering the space, it will become drier. This heating of the air back up before re-entering the room may not be as great as the cooldown, so it might drop the air temperature a very slight bit, but nothing like the way an air conditioner would. Basically, if it was 95 degrees in the space before you started, it might be 94 later. So does it make it cooler? Not really. It's not what it does. Drier air does feel cooler on the body because it's more readily able to evaporate moisture from the skin. Most of the pricier dehumidifiers run off of humidity setpoints and not a temperature setting because it doesn't care about temperature. You can run them continously as well, but you'd end up with the Sahara desert levels of humidity.

83hurstguy

By virtue of how they operate (no external heat sink like an A/C unit), a dehumidifer adds sensible heat to the room (from the compressor and fan doing work), so the dry bulb temperature has to go up in a closed space, but by removing moisture, the reduction in wet bulb/relative humidity makes it feel cooler.

VC455

Yes, a large unit running on dry-mode with a dehumidistat would work well to reduce humidity.

However, think about how dry mode works.
--The fan sped decreases
--Therefore the air coming out of the unit is cooler and has left more moisture behind.

Now think about energy use in the cooling process.
--An air conditioner uses energy to "pump" heat from a cooler space to a warmer space.
--If you engage dry mode, the pump has to work harder because it starts with making cooler air and has to pump the heat farther up to get it outside.

That means that dry mode increases operating cost.

But the selection process is all about balancing first cost and operating cost to get where you want.

I just wanted to give you all the facts to make an intelligent decision.

Gary

VC455

Luke, another source of sensible heat from a dehumidifier is due to condensing water from the air into the bucket.

Each gallon of water condensed releases about 8000 Btu latent heat.

Depending on the efficiency of the dehumidifier, the heat from water condensation adds about 50% on top of heat from compressor and fan work.

Gary

drop top olds

Generally, a concrete slab poured over a 10 to 25 mil vapor barrier will stop the vapor transmission through the concrete. With the polyurea top coating that should also act as a vapor inhibitor as long as the coating stays intact. You mention a 10 inch vapor barrier, where is the location for this, is it in the walls? Since the garage is already constructured much of the construction details that will benefit your concern with water vapor transmission cannot be implemented now. My suggestions would have been:

Where are you located?
A minimum 10 mill vapor barrier with taped joints under the slab would go a long way to stopping vapor transmission through the floor.
Using the "Zip system" on the walls would be beneficial. That is a treated OSB sheathing material that then has all sheathing joints taped. It is also one of the best ways to build a resilient structure. Fully sheathed walls (wood structural panels) are are way stronger than the thermoply and other thin membrane sheathing that builders are using now.
Spray foam insulation in the walls and the roof would give you a massive R value rated walls and roof. It also "seals" up joints.
Next with a super tight wall and roof you need humidity control. The way VC455 suggested would be a good approach. Just be sure to condition the entire inside of the building, including the attic Otherwise you will end up with a mold problem.
Last places to attack are the wall openings. Garage door is going to be a big hole in the wall. Any man doors are same. Make sure you have enough air conditioning to accomodate these.
Looking outside, if you graded the area around the garage so all water flows away that is a plus. If you have negative draingage towards the garage, then a french drain or some type of interceptor drain is needed to for the water flow. Of course passive is best but with french drains sometimes sump pumps go along with them.
Floor drains- my thoughts on them are if you use them to drain water fine, but be aware water can also use it as a pathway in as well. Plumbers and Mechanical Engineers know of back flow preventers that can help if you deicide you must have a drain (hole) in the floor. I would probably pass on the floor drain.