Thinking about storing your classic car over-winter?

I know of some people who put de-humidifiers in their garage over-winter to help keep things dry. At first sight this appears to be a good idea. However, they have the potential to cause far greater harm than dampness will.

When trying to keep your car dry, the critical value is the Dew Point, the temperature at which water condenses out of the air. The Dew Point is dependent on both temperature and relative humidity. For example, at a 60% relative humidity:

Ambient Temperature Dew Point Difference
80° F (26° C) 63° F (17° C) 17° F (9° C)
50° F (10° C) 38° F (3° C) 12° F (7° C)

At 80% relative humidity:

Ambient Temperature Dew Point Difference
80° F (26° C) 73° F (22° C) 7° F (4° C)
50° F (10° C) 45° F (7° C) 5° F (3° C)

The key to preventing condensation is to keep the temperature of your car above the Dew Point, the greater the temperature difference, the lesser the chance of condensation.

The foregoing table shows that at 60% relative humidity, there is approximately 15° F (9° C) difference between the ambient temperature and Due Point. In other words, at 60% relative humidity, the temperature would need to suddenly drop by around 15° F (9° C) before condensation would form on your classic car. Unfortunately, at 80% relative humidity, more typical of a UK winter, the temperature only needs to fall by 6° F (3° C), making condensation more likely. This is why we often get damp winter mornings in the UK.

One option is to heat your garage rather than use a dehumidifier. Unfortunately, installing a heater to increase the temperature, also increases the Dew Point. Lowering the Dew point (i.e. reducing the Relative humidity from 80% to 60% at 50° F (10° C) is over twice as effective as raising the temperature to 80° F (26° C)).  Even with their lower efficiencies below 50° F (10° C), dehumidifiers are probably the most cost-effective solution.

When I rebuilt my TC nearly 15 years ago, I considered a dehumidifier to keep it dry. Unfortunately, just installing a dehumidifier in a garage will serve little purpose, unless the garage is sealed. Nature abhors inconsistencies, so as the air in the garage becomes dryer, water vapour will increasingly seek to “fill the space”. This could come from the air outside the garage or drawn in through the floor or walls.

A dehumidifier will need to run 24 hours/day 7 days per week in its unsuccessful attempt to remove this moisture. There is one case known where the dehumidifier burned out and caught fire! Fortunately, the classic car was not totally destroyed.

After considering the options, I choose the Permabag storage solution. https://www.autopyjama.com/permabag-engl/cars/

As the website says, this is basically a large bag, sealed with a zip that you put your car into, along with two large desiccant cylinders. This serves the same purpose as a dehumidifier, the main difference is the car is stored in its own sealed bag. The desiccant cylinders only need to dry the air in the bag.

As a test, I took a piece of mild steel, aggressively cleaned it with a wire brush, cut it into two parts and degreased each half. I placed one half on the floor outside the bag and the second piece about 6 inches away, but inside the bag. The Permabag is a relatively expensive to buy, but has almost zero running costs. The desiccant cylinders can be regenerated any number of times by baking them in the oven. Mine require one regeneration for the whole winter period. It also comes with a temperature gauge/humidity meter that is fitted inside the bag, which in combination with a supplied graph that plots humidity against temperature, tells you when the cylinders need regenerating. The big advantage is that you do not need to leave electrical equipment powered on inside your garage over winter.

The disadvantage, although the bag will unzip all the way around, getting the car into it can be awkward, especially if you do not have much room in your garage.

The only addition I would recommend is a soft car cover to protect the paint from the plastic bag.  The photographs show my TC “bagged up” for the winter. Also note (bottom left first photo) the barrel of Sunoco Optima 98 – ethanol free storage petrol used to protect the fuel system components.

So, what about the pieces of mild steel? Before you look at the photograph, I should tell you that I live about 10 miles North East of Britain’s official semi-desert. Hard to believe, but one tiny corner of the South East of England has so little rainfall it is classified as a semi-desert.

So, for those of you in the wet North East, imagine how much worse the piece on the right would have been had it been exposed to a damper climate. (The two silver “dots” are the heads of aluminium rivets I put into the steel to “enhance” the effect of corrosion.) Needless to say, the piece on the left is the one that over-wintered inside the Permabag.

Certainly, for me the evidence is clear, the Permabag was a wise investment.     

Paul Ireland

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