Garage Humidity

[krisway]

I'm wondering if I have something done wrong. First here are the details:

-I live in Newfoundland, on the east coast of Canada.
-garage is 16x36, 576 sq/ft, with concrete floor
-insulated walls and ceiling
-vented eve and vented roof
-walls sheeted over with non painted 1/2" plywood
-three heaters totalling 5500watts
-Dec to May garaged heated and kept at 10*c

This time of year is damp and the humidity is high in the garage. Without a dehumidifier I've seen it reach over 80%. I bought a dehumidifier that is sufficient for the square footage and I use it periodically. But the garage still ranges between 60-70%. Is this still too high? I do see some black mould on the plywood, but not sure when this started to form....maybe last year before I had a dehumidifier. Basically I don't want to see this mould getting any worse.

-Do I have something not done properly with venting?
-Should I run the dehumidifier more often to keep the humidity at a certain level?
-What should the level of humidity be?
-Should my walls be painted?

Any suggestion? Thanks.

 

[no1steelsmith]

Hi Krisway, what is your outside humidity?

I am in the Corpus Christi area of South Texas and it is humid here most of the time! In the summer it is usually 80%+ humidity all of the time, with temps in the low 90's, stinkin' hot!
This time of year it drops back into the mid 60% range, unless a storm is coming. The Gulf of Mexico is less than a mile from where I live and summer water temps are in the mid to upper 80 degree range (fahrenheit). This has a lot to do with our high humidity!

How close to the ocean are you and what is your average water temps there? Since you are so far North compared to where I am, I'd say your concrete slab is acting as a heat sink and is adding to your high humidity issue. Heating your slab/getting it up to temperature and keeping it from getting cold will lessen the mold problem. Now the problem will be, keeping the slab warm!

 

[yeldogt]

You could be getting some moisture from the slab -- if it does not have a vapor barrier.

The only time unconditioned outside vents work is if the humidity is lower then the interior -- and you are not introducing too hot or cold air that you then must condition.

I have a Santa Fe unit in my studio and have it set to keep the humidity around 50 - 55%. I have a tight building and also use this unit to introduce some outside air.

70% is too much

 

[fatboy99]

I would set the dehumidifier to your desired level and have it on all the time when your set level is reached it will turn off. You will just have to keep up on keeping the tank empty or hook up a drain hose.

 

[kbs2244]

Are they electric heaters?
If combustion, are they vented?
Burning anythig makes a lot of moisture.

 

[Barnabas]

I have a dehumidifier in the garage that runs all the time. It is set to keep the humidity down to 50, so it runs all summer, but stops automatically when below 50, like now. I put the dehumidifier on top of an upright freezer, and ran the discharge hose about 15 feet to empty in a sink. I try to remember to clean the air filter once a year.

 

[509SC]

I'm guessing your heat source is electric, which shouldn't be adding to the problem. I wash our vehicles inside during winter, and raise the humidity level to 70% real quick. I usually just open the garage door for a couple of minutes and the humidity level drops like a rock. I then run the dehumidifier over night and it's at 50 or less by morning. Usually in winter, when the the temps are lower, so is the humidity outside.

 

[BobRae]

I live on the prairies and heat my 25' x 25' with a 5 kW electric heater. Humidity ranges from 50% to 20% in cold weather. The outside RH is usually 30% or lower in the winter. Usually if you bring fresh, cold, dry air in and vent out humid air, you can dry the air out pretty quickly. It adds to heating cost though. Of course, running a dehumidifier 24/7 costs too.

 

[jvitez]

I'm wondering if I have something done wrong.

.........I do see some black mould on the plywood, but not sure when this started to form....

-Do I have something not done properly with venting?
-Should I run the dehumidifier more often to keep the humidity at a certain level?
-What should the level of humidity be?
-Should my walls be painted?

Any suggestion? Thanks.

1. You've done nothing wrong. That's the proper way to build a structure with a ventilated attic space.
2. Humidity should be less than 50% at a minimum, ideally less than 40%.
3. Yes, paint your walls. Mould needs organic material to live off of, so plywood/OSB/dimensional lumber, natural paper face of drywall, uncoated wooden trim, etc are all food sources for mould.

What is the relative humidity OUTSIDE during your winters? Right now, Environment Canada says it's 90%. Is this common where you are? If so, ventilation alone won't do it. You have three choices to reduce the indoor relative humidity: increase the temperature inside your shop, run a dehumidifier all winter, both.

You'll need to use a high quality stain blocking primer now that mould has started on your walls. After this you can paint with whatever paint you like, but do two full topcoats. Mould will have a much harder time starting when there is nothing organic the spores can start growing on.

 

[Radix2]

You can also improve the situation by keeping the air moving - add a fan or two to keep some movement. You are more likely to get mold with still air, which allows pockets of 90+ air that rapidly condenses and feeds the stuff.

 

[yeldogt]

Moving the air around only helps if the humidity is coming from a particular spot. You want the dryer air to pick up the humidity from those areas with more moisture -- maybe the floor.

I leave mine set all the time -- it runs when needed.

The small dehumidifiers with the internal/ removable tanks are energy hogs -- and they will not work in a cold shop -- most of them drop off lower then 50 degrees. The better units are much more efficient and have more accurate controls with stronger fans to move the air around.

 

[kbs2244]

Newfoundland, Canada is almost an island in the Atlantic.
It is always going to be damp.

 

[krisway]

You could be getting some moisture from the slab -- if it does not have a vapor barrier.

My concrete floor is never wet or sweaty, so I'm guessing it does have the proper vapour barrier.

 

[krisway]

Are they electric heaters?

Yes, electric heaters. Two baseboard heaters and one heater with built in fan.

 

[krisway]

I wash our vehicles inside during winter, and raise the humidity level to 70% real quick.

I don't ever wash my vehicles in the garage. In the winter, the only time I open the door is to take out the snowblower. (I hate snow )

 

[krisway]

...Yes, paint your walls. Mould needs organic material to live off of, so plywood/OSB/dimensional lumber, natural paper face of drywall, uncoated wooden trim, etc are all food sources for mould.

You'll need to use a high quality stain blocking primer now that mould has started on your walls. After this you can paint with whatever paint you like, but do two full topcoats. Mould will have a much harder time starting when there is nothing organic the spores can start growing on.

Thanks, I guess next summer I will have to invest in some quality paint!

 

[Stuart in MN]

10 degrees C is 50 degrees F, which is at the lower end of where regular household dehumidifiers will work. You may want to turn the heat up a little bit to see if that will help. Another thought is to set up a fan to provide some air circulation.

 

[krisway]

10 degrees C is 50 degrees F, which is at the lower end of where regular household dehumidifiers will work. You may want to turn the heat up a little bit to see if that will help. Another thought is to set up a fan to provide some air circulation.

Thanks.

 

[Peter Mc Mahon]

I see a couple of people mentioning a vapour barrier under the concrete. Why is this recommended? What is it preventing?
Thanks.

 

[Stuart in MN]

Moisture from the ground will tend to migrate up through the concrete floor, causing humidity inside the garage. a vapor barrier will prevent (or at least minimize) that.

 

[Lapkritis]

What R value did you use in the walls/ceiling?

 

[hoyt]

I have humidity issues due to to a lack of a vapor barrier under the slab and poor site drainiage. As I have improved the drainage, the humidity problem is reduced.

 

[Peter Mc Mahon]

Moisture from the ground will tend to migrate up through the concrete floor, causing humidity inside the garage. a vapor barrier will prevent (or at least minimize) that.

Will my concrete appear wet? I am reading a lot about this, and where I live, it is not recommended, and I have never seen it done. I worked in residential construction for almost 7 years so I have seen a lot of pours done, but I am on the fence about the value of it.

 

[Lapkritis]

You'll get condensation from above when the slab is cold and there is a sudden increase in ambient air temperature. The moisture is in the atmosphere and not just bleeding through the concrete. I suspect this is part of your problem. Do you have blue board around the perimeter of the slab or anything to isolate the slab from the earthen environment/frost line? You can also put down an epoxy coat that will act as a moisture barrier. My family built a large 2 car garage on a mountain top down the coast from you in New England. This was 20yrs ago now - the contractor poured directly onto bedrock several places. The floor is uncoated so it sweats/puddles every spring when temps warm... in the fall/winter it's nice and dry.

 

[Lapkritis]

Good explanation although he doesn't go into humidity/condensation concerns: