Rust vs humidity vs storage
 

I would like information regarding potential rust problems for a car that will be stored for several months.

What is the minimum temp level that should be maintained in the garage?

What is the maximum humidity level that should be maintained in the garage?

I have been told that concrete from the floor releases humidity and could cause the underside of the car to rust. Is this true?

If it is, I would appreciate if someone could describe (in simple terms) why this happens. Is there more humidity under the car than in the rest of the garage?

concrete breathes..

I don't know where you live? I don't know what the conditions in your garage is? I don't know if this is a garage queen of a commuter 911? I don't know if you want to spend $$ ?

For long term storage, like over the winter, some folks recommend parking on a plastic sheet to keep the out gassing from the concrete away from the car.

A few months won't do any harm.

If it's real damp in there. I would try to keep the temp above the dew point so water doesn't condense on and in the car. In the small cabin of a boat, they leave a 100 watt bulb on for this purpose. To park on a barrier would be good if the ground moisture is high. The vapor barrier should be well sealed at the edges to prevent moisture from escaping into your storage area. And there should be vents to promote air circulation.

So in other words, ideal would be on raised floor construction with insulated walls, heat and widows. That's your bedroom. Put a garage door on your bedroom wall. Do you know where your Porsche is tonight?:D

My garage is pretty well like my bedroom except for the floor. It has insulated walls, heat and windows but with a cement floor.

If required I am willing to buy a dehumidifier. Problem is that I do not know what is the maximum humidity % I should target.

Will there be substantially more humidity under the car than in the rest of the garage?

you can buy a concrete sealer.. otherwise I'd follow Zeke's thread

I wouldn't worry about the conditions you describe. Some tips include raising the pressure of your tires to 40 lbs and leaving each tire on a decent thickness of corrugated cardboard.

A search on the term storage may help. And don't forget about mice....plenty of archived discussion on that as well.

Not if there is air circulation. Also, IMHO, there is no target humidity % as long as it's not 90 %. Once again, it's mostly in the dew point. If it makes any difference, I think museums shoot for about 60%. Can't be too dry or your soft goods will dry up and crack.

If it's a fairly new garage, you probably already have a vapor barrier under the slab preventing moisture from damp ground from entering the garage.

I highly suggest buying a chemical type dryer to absorb moisture from the interior your car. Humidity+leather=mold. Ask me how I know.

If hanging a chemical dryer (usually in a cloth sack) I'd suggest putting a bowl under it to catch any excess moisture. When these become saturated they will drip.

Consult your local museum curator for dehumidifier/humidifiers.
Buy your self a garage lift or a set of jack stands and a nice cotton car cover.

"I think museums shoot for about 60%. Can't be too dry or your soft goods will dry up and crack."

Exactly. The dew pt. is the temperature at which water vapor begins to condense out of the air. You can calculate it from the rel. humidity and current temp. Make sure the min. temp at night is far above the dew pt. i.e. keep the gaage warm.

Use of jack stands can expose parts of the shock rods so be careful there (depsite Harlan's emphatic repitition).

desicants need to be heated periodically to remove the accumulated moisture. Also, they are only useful in small spaces (say the inside of a tool box). I'm not sure how many pounds you'd need for the interior of the 911.

Screen off all air intakes with heavy screen to prevent mice nesting activities.

Has anybody got the formula to calculate dew point?

Yes, but it's complicated - about a 50 line BASIC or FORTRAN program. You want a chart -- Google for one. It will have diagonal lines on it and diverging "parabolic looking" curves on it.

Also, be sure to distinguish absolute and relative humidities.

Or just crank up the garage heater so it stays 20 oF warmer than the ext. environment. That's almost certain to be fine. Insulation is cheap -- get some and put it in the garage walls.

Why don't you see if a museum has a spare room for your car? :D

Seriously,
Regarding the Earth based "dehumidifier" a 2 pound bag is more than enough for the interior of your car...usually available at hardware stores. I think the brand name is sta-dri or something along those lines.

The silica based desiccants are the small bags usually found in jars or for smaller applications.

Randy, I normally heat my garage to 50 F. Does that mean I should be fine?