Storage humidity and dehumidifiers

Why a T-Type is not a steel car

Almost all published storage advice is written for steel monocoques, where the owner has one enemy and one direction of travel. Drier is better, up to the point where it stops mattering. A TA, TB, TC, TD or TF is a different object: a steel chassis and steel panels over an ash body frame, with leather, cloth, felt and a good deal of rubber. Wood is hygroscopic. It takes up moisture from the air when the air is damp, it gives moisture back when the air is dry, and it changes dimension as it does so.

That is why the lower bound matters here. Published guidance warns that below about 40 percent relative humidity, wood, leather and rubber dry out and crack. On a monocoque that warning is a footnote about the seats. On a T-Type it is a warning about the structure of the body, because the timber that shrinks is the timber that carries the panels, holds the door pillars square and takes the screws.

The owner of a T-Type has to worry about the lower bound as much as the upper one. That single sentence is the difference between storage advice for this car and storage advice for everything else.

None of this is an argument against a dehumidifier. Damp, still air in an unheated garage is the worse of the two failures, and the ash frame suffers from wet at least as badly as it suffers from dry, since water in timber brings rot and softened glue joints with it. The argument is against a dehumidifier left to its own devices. A machine with no humidistat, in a small tight space, does not know when to stop, and the timber cannot tell you it has had enough until something has already moved.

The structural background to all of this is set out separately in the ash frame, which is worth reading first if you have not looked at how your car is put together behind the panels.

The target, and where the sources disagree

It would be convenient to give one number. The published sources do not agree on one, and pretending otherwise would be the wrong service to an owner about to spend money on a machine. Here is what they actually say, attributed.

• Auto Classica Storage and Ole Dissing: 40 to 60 percent. The reasoning is that rust all but stops below 60 percent, while wood, leather and rubber can dry out and crack below 40 percent. That is the band that explicitly recognises both ends of the problem.
• CarCapsule: 40 to 50 percent, citing corrosion research from the US National Institute of Standards and Technology.
• Cotes: 35 to 55 percent.
• Munters: 45 to 50 percent.
• Dantherm: 55 to 60 percent at 18 to 20 degrees. Read that one for what it is, guidance written for a professional storage facility with heating and controlled air, rather than for a domestic garage that follows the weather.

The disagreement is real and this archive is not going to resolve it. What can be said is where the bands overlap. Take those five together and nearly all of them include 45 to 55 percent. That is not a compromise reached by averaging; it is the region every source except Dantherm agrees is safe, and Dantherm is describing a different kind of building.

For an ash-framed car that overlap is not a nicety, it is the point. The lower figures quoted elsewhere are aimed squarely at the steel. Cotes' 35 percent floor would be too dry for this car on the same published reasoning that warns about wood, leather and rubber below 40. Sitting in the middle of the overlap keeps the steel out of the range where corrosion runs freely and keeps the timber and trim out of the range where they shrink and crack.

To be explicit about method: the archive is reasoning from the timber, not asserting a figure of its own and not claiming any authority has published a T-Type specific target. If you want a number to set a humidistat to, the honest version is that the middle of the published overlap is a defensible place to start, and that a hygrometer in the car and a hygrometer in the garage will teach you more about your own building than any table will.

Compressor against desiccant

Two kinds of machine are sold for this job and they work on different physics, which is why the cold of a British garage decides between them.

What each one does

A compressor unit, also called a refrigerant unit, works like a fridge. It draws air across a cold surface, the moisture in the air condenses onto it, and the water runs into a tank or a drain. That depends entirely on the machine being able to make a surface colder than the air it is treating. As the air gets colder, the margin shrinks, and at some point the cold surface starts to frost rather than drip.

A desiccant unit works chemically instead. Air passes over a rotor coated with a material that adsorbs water vapour, the rotor is then warmed to drive the moisture off and expel it, and the process does not rely on the surrounding air being warm. Desiccant units are agreed across the sources to work down to near freezing, which is the whole reason they are recommended for unheated spaces.

Where a compressor stops working

The sources disagree on the threshold, and the spread is wide enough to be worth quoting rather than smoothing over. Motoring Research says a compressor unit is ineffective below 15 degrees. Ole Dissing says refrigerant units operate down to 5 degrees and suit northern Europe. Another source says they lose effectiveness below about 18 degrees and cut out around 5 degrees. What all three share is a direction of travel: performance falls away as the garage gets colder, and somewhere between roughly 5 and 18 degrees the machine stops doing much useful work.

What each one costs you in return

Desiccant units use more electricity for the same extraction, and they give out warm dry air as a by-product, which in a cold garage is not entirely a loss. Ole Dissing notes that a refrigerant unit returns roughly two and a half times its energy input as heat. Neither figure is a running cost, and this page does not give running costs, wattages or extraction rates, because they depend on the machine and the building rather than on the car.

The T-Type point

Here is where the general advice and this car part company. A desiccant unit in a small, well-sealed garage is perfectly capable of taking the air well below 40 percent. That is the machine working as designed and it is exactly the outcome an ash-framed car does not want. The humidistat therefore matters more on this car than on a steel one. The setting is not optional, and a unit chosen for its cold-weather capability without a means of telling it when to stop is the wrong purchase for a T-Type.

Heating instead

Warming the space is the intuitive answer, since warm air holds more moisture and the relative humidity reading falls as the temperature rises. It works to a degree, at a price. Ole Dissing states that heating is around two and a half times less efficient than dehumidification for lowering relative humidity.

There is a second and more important limitation. Heating the air does not stop a cold car attracting condensation. Metal has thermal mass and lags behind the air around it, so warmed damp air meeting cold panels, a cold engine and a cold chassis still gives up its moisture onto the coldest surfaces present. Ole Dissing makes exactly that point, and it matches what owners see: a garage that feels comfortable can still leave a car beaded with water. Heating alone is not a substitute for removing the moisture, though gentle warmth alongside controlled dehumidification is a reasonable combination if the building already has it.

The garage, and the ground under it

Brick or stone against a fabric shelter

A brick or stone single garage has thermal mass. It changes temperature slowly, which evens out the daily swings and reduces the number of occasions when a cold car meets suddenly milder air. It can also be sealed reasonably well, which is what makes a humidistat necessary rather than optional once a dehumidifier goes in.

A steel-framed fabric shelter of the Clarke type, of the sort shown in the lay-up guide, behaves quite differently. It has almost no thermal mass, so its interior follows the outside air closely, and its cover is a thin skin that goes cold at night. Condensation forms on the inside of that cover and can drip. It is genuinely useful shelter, keeping rain, frost and sun off a car that would otherwise stand outside, and it is not a sealed enclosure. Trying to dehumidify one is largely futile, because the air you dry is continually replaced. The right approach in a fabric shelter is ventilation and keeping water off the car, not humidity control.

The floor

The floor is the part most owners overlook, and on a wooden-framed car it deserves more attention than the machine does. Moisture rises from bare ground continuously. A plastic sheet laid over grass or hardcore does not stop it; it simply moves the wet to the top surface of the sheet, where it sits under the car and evaporates into the air the car is standing in. A concrete slab is better, though an unsealed slab still passes moisture upward from the ground beneath.

This is where an ash-framed car parts company with a steel one again. A car standing over damp ground is not only collecting moisture onto steel, where it shows as surface rust and can be seen and dealt with. It is taking moisture into timber, where nothing shows for a long time and where the wood holds what it has absorbed for far longer than a panel holds a film of condensation. The ash frame is the part of the car least able to tell you it is getting wet, which is a good reason to treat the ground as seriously as the air.

A properly sealed vapour barrier under the car, laid so that the ground moisture is stopped rather than merely redirected, is the cheapest worthwhile improvement available to most owners. It reduces the load on any dehumidifier, it helps in an unpowered space where there is no dehumidifier at all, and it addresses the source rather than the symptom. The principle is what matters. This archive does not specify materials, thicknesses or installation details, and the choice depends on the building rather than the car.

Running it

A schedule cannot be given from a table, and any page that offers one for your garage is guessing. How much moisture a building takes on depends on its construction, its floor, its ventilation, how much water came in on the car, and the weather outside. Two identical machines in two identical garages will not run for the same number of hours.

Two instruments answer the question properly. A hygrometer tells you what the air is actually doing, and a second one inside the cockpit is worth having, because the cabin of a hooded car is a smaller and stiller space than the garage. A humidistat, whether built into the unit or fitted separately, then holds the air where you have decided it should sit and switches the machine off when it gets there. That arrangement is self-correcting in a way that a timer never is.

Set it, then watch it for a few weeks and read the hygrometers rather than trusting the machine's own display. No run hours or duty cycles are given here because there is no honest basis for inventing them.

What to look for in a unit

Judge a machine by capability rather than by name. The features below are the ones that change the outcome for a stored T-Type. No makes, models, prices or retailers appear on this page, and this archive sells nothing and recommends no product.

• A humidistat. The single most important feature on this car, because it is the thing that stops the air being dried past the point the timber and trim can tolerate. A machine with only a fan speed and an on switch is not suitable for unattended use in a sealed garage.
• Auto restart. Power in a garage is interrupted more often than people expect. A unit that resumes its previous setting after a cut, rather than staying off until somebody visits, is worth having when the car is left for months.
• Continuous drainage. A tank fills and then the machine stops, which is precisely the failure you will not notice. A hose to a drain or a gully removes the need to visit.
• Low temperature operation. Check the stated operating range against what your garage actually reaches in January, rather than against room temperature. This is where the compressor and desiccant question is settled for most unheated British garages.

Pouches and crystals

Moisture-absorbing pouches, tubs of crystals and silica gel bags have a real use and a hard limit. In an enclosed cabin, in a boot, or tucked under a hood, they take the edge off the damp in a small volume of still air, and they need no power. They are incapable of drying a garage. The volume of air is far too large, the building keeps replacing it, and a handful of pouches on the floor achieves nothing measurable.

CarCapsule suggests swapping them roughly every sixty days from October to April in a humid climate, and notes that silica gel can be dried in a low oven and reused. Treat them as cabin housekeeping alongside proper humidity control, not as a replacement for it, and keep an eye on any that saturate and start to leak.

The battery

Two things make the battery worth its own section on a T-Type. The first is that these cars are positive earth, which matters the moment a modern charger is connected. Get the polarity wrong and the damage is not confined to the battery. Check what you are doing before you clip anything on, and confirm that the charger is suitable for a positive earth installation and for the type of battery fitted.

The second is the difference between the two approaches. Disconnecting the battery and leaving it, ideally somewhere cool and dry rather than on a cold concrete floor, is simple, and it stops the small parasitic drains that flatten a battery over a winter. A modern maintenance charger left connected is usually better, because it monitors the battery and delivers current only when the battery needs it. An old trickle charger does something different and less kind. It pushes a steady current regardless of state of charge, which over months can cook a battery dry rather than preserve it.

Charging inside a fabric shelter deserves a word of caution. It means mains electricity, a charger and its heat, and lead-acid gassing inside a small enclosure made of fabric, standing on ground that may be wet. If that is your only option, use a properly protected outdoor-rated supply, keep the charger off the ground and clear of the cover, and prefer removing the battery to a garage or outbuilding for charging over leaving a charger running unattended in the shelter.

The hood and sidescreens in storage

What can be said with confidence is limited, and it is better to say so than to invent a rule. Hood material and its stitching take a set where they are folded, and folds that stay closed for months trap moisture against the fabric and against the frame. A hood left up is under gentle tension and unfolded, which suits the material and lets the cockpit breathe less freely; a hood left down is folded and stacked, which keeps the cockpit open to the air of the garage and puts creases into the same places for a long period.

Owners take both views sincerely and this archive has no test evidence to settle it. If the hood goes up, it should go up dry and go up properly, not damp and half-tensioned. If the hood comes down, it should be dry, folded as the maker intended rather than crushed, and the cockpit given some means of drying, which is where cabin pouches earn their place.

Sidescreens are more straightforward. They are best stored flat, dry, and out of the car rather than stacked into the cockpit where the panels can press against each other and the glazing can mark. Somewhere indoors, at a stable temperature, away from direct heat and away from anything that will bear on them, is the sensible answer.

Questions owners ask

What humidity should an MG T-Type be stored at?

No single authority gives a figure for an ash-framed car, and the published bands disagree. Auto Classica Storage and Ole Dissing give 40 to 60 percent, CarCapsule gives 40 to 50 percent citing corrosion research from the US National Institute of Standards and Technology, Cotes gives 35 to 55 percent, Munters gives 45 to 50 percent, and Dantherm gives 55 to 60 percent at 18 to 20 degrees for a professional facility. The band where nearly all of them overlap is 45 to 55 percent. This archive reasons from the timber rather than asserting a figure of its own: a T-Type has a wooden body frame as well as steel, leather and rubber, and 35 percent would be too dry for it.

Can a dehumidifier be too dry for a wooden framed car?

Yes, and that is the point most storage advice misses because it is written for steel monocoques. Published guidance warns that below about 40 percent relative humidity, wood, leather and rubber dry out and crack. Timber shrinks as it dries, and an ash frame that loses moisture can loosen at its joints while the trim and the rubber age faster. A desiccant unit in a small, well-sealed garage is entirely capable of taking the air below 40 percent if it is left to run unchecked, which is why a humidistat is not an optional extra on this car.

Desiccant or compressor for a cold garage?

Desiccant units are agreed to work down to near freezing, which is what an unheated British garage in January actually is. Sources disagree on where a compressor, or refrigerant, unit gives up: Motoring Research says ineffective below 15 degrees, Ole Dissing says refrigerant units operate down to 5 degrees and suit northern Europe, and another source says they lose effectiveness below about 18 degrees and cut out around 5. For a cold garage a desiccant unit is the safer bet on capability, at the cost of more electricity for the same extraction. Either way the unit must be on a humidistat so it stops before the air gets too dry for the timber.

Should I disconnect the battery when storing a T-Type?

If the car will simply be left, disconnect it, and remember these cars are positive earth, which matters when a modern charger is connected. A better arrangement is a modern maintenance charger left connected, which monitors the battery and stops rather than pushing current continuously the way an old trickle charger does. Check that the charger you use is suitable for the battery type and for a positive earth installation, and think carefully before leaving any charger running unattended inside a fabric shelter.

Read next

• Servicing

  Winter storage: the lay-up checklist

• Structure

  The ash frame

• Winter use

  Driving a T-Type through the winter

• Servicing

  Chrome & brightwork