I put a spare storage heater in mine. It’s on a timer so it only comes on for a couple of hours a night, keeps the workshop warm an dry.
Mark P.

Hi Chris

It does depend on your workshop construction. My draughty wooden shed is always a lot drier in winter than my brick detached garage! If the workshop is fairly airtight and well insulated, it may be better to use a dehumidifier.

My workshop was constructed by lining and insulating one half of the double garage, using studding walls, insulated wall/floor/ceiling with Celotex. I knocked the plywood and glass panels out of the wooden back door, filled the openings with Celotex and covered the door with 6mm waterproof plywood skins.

With this insulation, the diurnal temperature variation is usually only 1 degree C or so inside the workshop, whatever the outside weather, until I go in there.

I run a dehumidifier 24/7/365 in the workshop. This is controlled by a humidity controller module from fleabay (around £15 from a supplier in China), built into a diecast box with mains socket on top. It is set to turn on when the humidity rises a little above 50%. So electricity consumption depends how high the external humidity is, at times it might be largely off for weeks; with damp weather it is on more often, but I think certainly less than 50% duty cycle even then. I think the overall electricity cost is minimal, especially considering the degree of protection to my machines and other tools. As a result of this, I now simply don't get rust in my workshop! The dehumidifier also gives a small degree of background heat, probably only 150W maximum at 50% duty cycle, but this can be enough to raise the workshop temperature by perhaps 2-3 degrees in winter.

The use of a dehumidifier does rely on having a fairly airtight building. If the building is very leaky (my shed), you will be trying to dry out the world! I would like to reduce run a dehumidifier in my (woodworking) shed, but I think it would require substantially better draught proofing. Note also that dehumidifiers do not work well at low temperatures – they need to be above a few degrees C to work, depending on type.

Martin

Back in the 60's we put up a 'polythene tent' in an old very drafty 20ft x 12ft wooden shed to varnish a boat in a dust free space. I was very surprised that the inside was so warm with minimal heat simply by excluding the drafts.

My workshop, over 30 years old now, is a wooden shed which I insulated with 50mm rockwool cavity insulation batts between the framing- walls, ceiling and floor. I also used a large sheet of polythene – well it came on a roll – which I stapled up on the roof and ran down the walls to the floor to make an airtight tent and vapour barrier before covering roof and walls with plywood. There are cutouts for windows and door. It has a raised wood floor which has a polythene damp proof membrane under the insulation. It requires very little heat – oil filled radiator -to keep it dry in winter and none in the summer. The heater is on the off peak, with a thermostat set at 7.5C plus an overide switch for me in the day. Yes, I occasionally forget to turn it off!

I am tempted to add a dehumidifyer. I have one for our campervan which has a drainage pipe that we run outside so never need to empty the water resevoir. But I see no need as there would be very little saving on electricity consumption

I do cheat a bit as the shed is split into 2 sections so the outside door opens into the first section which acts as a lobby with an inside door, which is more or less airtight, to my 10ft square workshop section. I am at present helping a friend to duplicate my setup but he has no lobby space so we are fitting a door fame inside the outer door just deep enough to fit a cheap, sadly inward opening, interior door to obtain the air tight fit

My shop is windowless and fairly small, 10'9" x 6' 9": external 19mm cladding on 50mm frame, with 12mm internal cladding. Roof is rubber on 12mm ply, on 50mm frame with 12mm ply internal ceiling. The only bit that is not insulated wirh glass fibre is the 18mm ply floor (It stands on 8" x 2" bearers, these are protected from the weather by a fence at the back, and a low wall on two other sides. When the ambient temperature approaches freezing, I switch on a 60 watt tubular heater under the steel bench. After a couple of days, the bench is just warm to the touch. There is a rarely used 6" fan with a rainproof external cover , mounted high up, with a couple of small fixed vents low down in the wall. Mostly, unless raining, or cold, I work with the door open!

(To indicate ambient temperatures/humidity, this is in East Anglia, U K)

Over the twelve years of its life, rust is VERY VERY rarely seen. (I got more rust by exhibiting out of doors, on a rainless day, at a village fete!).

As KWIL says, keep the temperature above the dew point, so condensation does not occur. And remember that damp air is heavier than dry, so floor level vents will help it leave the premises, with , hopefully, drier air entering via a high level weatherproof vent.

Obviously, no form of heating that involve combustion within the shop should be used.

A woodburner, MIGHT be OK since the products of combustion go up the chimney, and it will aid ventilation, but beware of any leaks, YOU DO NOT WANT TO SUFFER CARBON MONOXIDE POISONING!

Having said all this, if you are in an area of the world with constant high humidity, a dehumidifier may be needed.

Hope that this is of some help

Howard

Chris Evans

Several issues here

How airtight is your workshop?

If your workshop is not moderately airtight, you would be trying to dry out the world, as noted above. For example, my workshop is approximately 4.7 x 2.3 x 2.2m in size, or 23.8 cu. m. The saturated content of air at say 20C is 17.3g per cu m (see **LINK** ), so total 411g, or 411ml, for my workshop. Of course I am only trying to reduce the humidity down to 50%, so I need to extract about 200g per air change. So, if my dehumidifier tank has a capacity of perhaps 3 litres (not sure exactly what it is), it will cope with 15 changes of saturated air. If you are emptying your dehumidifier several times a day, you either have a very large workshop, or it is not at all airtight, or you have a lot of damp material (wood, paper?) stored there (which will dry out before long).

In practice, it is dependant on the weather: in the most damp conditions, I might empty the dehumidifier once every 5 to 7 days. In drier conditions it might be once every few weeks. Dehumidifiers do not generally work efficiently below a few degrees C, but the coldest conditions tend to be the driest.

For reference, my workshop has 2 standard wooden doors with no special attempt at sealing, it is otherwise well sealed, and located in central southern England.

Note the dehumidifier will give a double benefit of both drying the workshop, and giving a small amount of background heat. The heat is both from direct power consumption, and additionally the recovery of the latent heat of evaporation of the recovered water. Assuming my dehumidifier reservoir capacity is 3 litres, then taking the latent heat of evaporation at 2265kJ/kg, I will have recovered almost 6.8MJ for each tankful, which is equivalent in total to the use of a 1kW heater for 1 hour 53 minutes.

As you note, keeping the air moving can help – this is where my draughty old wooden shed wins out over my brick garage. The real issue here is with changing weather conditions – after a period of cold weather, when a damp warm front comes over, the warmer moist air meets cold metal and causes condensation and rust. So the fan helps, mainly in that it assists the workshop contents to keep up with changing external conditions.

Another method used is local heating, perhaps attaching a powered resistor to large lumps of machinery to keep it slightly warm. For example, a rise of 10 degrees (perhaps more than necessary) would effectively halve the relative humidity at the surface of a machine. Like many physical and chemical processes, the water content of saturated air roughly doubles for each 10 degree C temperature rise.

Martin

Edited By Martin Whittle on 08/12/2015 09:50:28

I used to have a problem with "sweating" machines, usually after a cold night followed by a warm spring morning. When I needed to enlarge my workshop I demolished the old one and built all new. The concrete base was insulated with 3mm foam (for wooden floors) over a polythene membrane then flooring grade chipboard. I lined the inside with plywood and insulated the whole thing with 75mm rigid foam and have not had a problem since. No background heat, just a slow rate of change of the interior temperature when not occupied and a small amount of ventilation from the small gaps around the door.

Martin