# Garage humidity



## Olivier Gariépy (Apr 3, 2018)

My reservation was done in april 2016, still waiting! English is not my main language sorry in advance for any errors.

I want to know if i should put a charging station inside the garage or outside. Of course i would prefer to put it inside and be clear of snow/frost on the car.

My garage is not heated, in winter time, the humidity average 60%rh (if i park inside) while temp can go down at min -5°C. Outside temp average in Quebec is about -15°C and very low %rh.

I found that temperature and humidity are determining the amount of water contained in a kg of air. This is SPECIFIC HUMIDITY. In my understanding, "real humidity".

Hot temperature and moderate %rh will get a higher specific humidity value then cold temperature and high %rh.

For exemple :
60%rh - 0°C - 2.25g/kg
60%rh - 5°C - 3.22g/kg
60%rh - 10°C - 4.53g/kg
The g/kg value is grams of H2O per kg of air or SPECIFIC HUMIDITY

https://www.rotronic.com/en/humidit...umidite/humidity-calculator-feuchterechner-mr

What is the value of SPECIFIC HUMIDITY that would becomes damaging for the chassis of a car (rust)?
Or maybe the real question should be "is 60%rh and temp of -5°C damageable for a car?"
Feel free to comment.


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## Michael Russo (Oct 15, 2016)

@Olivier Gariépy , bienvenue et ton anglais est excellent!! @TrevP installed his wall connector outside in Toronto plus should be to shed some light on some of your questions!!

Take care, 
Mike


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## Olivier Gariépy (Apr 3, 2018)

Thank's @Michael Russo!

Hopefully i will get some help because it's a question that have been bugging me for some time now. A friend of mine told me i have to do something for my garage humidity otherwise it would damage a car in the long run.


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## PNWmisty (Aug 19, 2017)

Olivier Gariépy said:


> What is the value of SPECIFIC HUMIDITY that would becomes damaging for the chassis of a car (rust)?
> Or maybe the real question should be "is 60%rh and temp of -5°C damageable for a car?"
> Feel free to comment.


Relative humidity is the most common way to measure humidity because it's the most relevant to things we care about (firewood drying, metal corroding, dew forming). The absolute humidity is more of an interesting scientific principle. Modern cars (both EV and ICE) depend heavily upon electronics, corrosion and storage environment are valid concerns.
For keeping corrosion at bay, colder temperatures are better than warmer temperatures at the same relative humidity. The warm air will have more moisture per unit volume and warmer temperatures accelerate oxidation (corrosion). 60% relative humidity and -5C is not a troubling storage environment. It is important to avoid large swings in temperature because you want to avoid the air approaching the dew point. If the warmer, more humid air from the living area is allowed to migrate into your cold garage the relative humidity in the garage will increase. But most homes have a slight negative pressure that prevents this due to exhaust fans, vented appliances and the natural chimney effect created when the house is warmer than the outside air so this is generally not a problem.
Generally, the lower the relative humidity, the better. If your garage is slab on grade, moisture from the ground can wick through the concrete and increase the humidity in the air. That's why it's important to make sure the downspouts from your gutters are plumbed away from the structure.
Many homes and garages have gutters that deposit the rainwater all around the perimeter of the building which is not good for humidity levels in the crawlspace or garage. You can further reduce garage humidity by painting the concrete floor with a vapor barrier. 
​


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## Olivier Gariépy (Apr 3, 2018)

Thanks a lot @PNWmisty for this detailed answer. I'm sure it will help me and a lot of people.

For this winter humidity concern, someone recommended me to open the garage windows a little to get some air flow and let the cold air in. Also, i'm thinking of putting a AutoFloorGuard Containment Mat above the floor drain. Do you think those are good ideas?


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## PNWmisty (Aug 19, 2017)

Olivier Gariépy said:


> Thanks a lot @PNWmisty for this detailed answer. I'm sure it will help me and a lot of people.
> 
> For this winter humidity concern, someone recommended me to open the garage windows a little to get some air flow and let the cold air in. Also, i'm thinking of putting a AutoFloorGuard Containment Mat above the floor drain. Do you think those are good ideas?


If your garage has a floor drain I wouldn't cover it with a containment mat. You want melting snow/ice and dripping water to exit the garage via the drain, not be contained where it will evaporate into the room. Most garages have plenty of airflow due to the garage door as well as windows and additional doors but if the humidity is lower outside, this could help but it might be better to identify where the humidity is coming from to begin with. If it's coming up through the concrete slab it's really worth it to paint it with a high quality garage floor paint.


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## Olivier Gariépy (Apr 3, 2018)

@PNWmisty The mat is a bad idea in my case. I don't know how to spot the origin of this humidity, do you have any simple trick?

I'm not sure about painting the slab (and i feel like the paint will last only two years or so).

What about a through-the-wall bathroom like fan?

Are opening garage windows in winter a bad idea?

Thank you!


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## garsh (Apr 4, 2016)

Olivier Gariépy said:


> @PNWmisty The mat is a bad idea in my case. I don't know how to spot the origin of this humidity, do you have any simple trick?


If you have exposed concrete block that's below grade, then that's probably your biggest origin. Also, if you pull a wet car into the garage, that's a large source as well.


> I'm not sure about painting the slab (and i feel like the paint will last only two years or so).


They make paint specifically designed for garage floors. I would imagine that it's made to withstand the abuse of car tires and such, but I have no experience with it personally. It might not be bad to just touch it up every year if it does wear out.

Plus there are even more durable coatings you can apply to a garage floor.


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## PNWmisty (Aug 19, 2017)

Olivier Gariépy said:


> @PNWmisty The mat is a bad idea in my case. I don't know how to spot the origin of this humidity, do you have any simple trick?


Yes, look at it to determine where the humidity is coming from. I have no idea what you have going on there because I haven't seen it. But, per my previous recommendation, check roof downspouts too. And remember that concrete is porous and functions like a wick. That's why I recommended an impervious coating if that was the source of the humidity.



> I'm not sure about painting the slab (and i feel like the paint will last only two years or so).


Huh? What are you basing that on? Yes, using the wrong product or using a product improperly can result in failure. A good product, properly applied should last many years.



> What about a through-the-wall bathroom like fan?


That shouldn't be necessary (assuming you have taken care of the SOURCE of the humidity).



> Are opening garage windows in winter a bad idea?
> 
> Thank you!


Not necessarily a bad idea, but it shouldn't be necessary if you have taken care of the SOURCE of the humidity.


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## Olivier Gariépy (Apr 3, 2018)

garsh said:


> If you have exposed concrete block that's below grade, then that's probably your biggest origin. Also, if you pull a wet car into the garage, that's a large source as well.


Thank you for that info.

There are a few things to know here; This is my first house, i did not build the house, the roof have been changed about 5 years ago, The concrete slab was made about 20 years ago. When we are talking about the source of humidity, note that the floor drain always have water in it and when to water rise above a certain level, water level goes down a couple inches, but a lot of water remains in the trap. This has to be a big source of humidity, right?

I'll just keep garage windows open in winter for now, until i finish the inside walls (for look only) and probably paint the slab.


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## garsh (Apr 4, 2016)

Olivier Gariépy said:


> ...but a lot of water remains in the trap.


Water is supposed to always remain in the trap. You *never* want the trap to be dry. The water in the trap prevents (potentially dangerous, and always smelly) sewage gas from escaping into the house. The trap is not your primary source of humidity. The surface area is too small.

https://en.wikipedia.org/wiki/Trap_(plumbing)


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