Why do I get condenstation

[jack55]

I have a shed which I just use to store wood and bits and pieces in. It is not heated. This morning after a few days of hard frost I noticed the inside of the roof was covered in condesation.

The roof has 18mm tongue grove chip board cover in two layers of roofing felt. The eaves are open so theres plenty of ventilation.

http://jackclark55.webs.com/shed No2-5.JPG

http://jackclark55.webs.com/shed No2-11a.jpg

http://jackclark55.webs.com/wood shed.jpg

I don't work in this shed so theres no real heat source. Any one got any ideas on what i can do to stop the condesation?

 

[motownmartin]

I had this problem until I insulated with foil covered insulation boards.

I put it down to the sun warming up the shed walls which brings out the damp into the air and forming condensation on anything that was cold, I don't this for sure but the problem has now gone.

 

[RogerS]

Search the forum for "dew point" and you'll find a load of similar threads.

 

[wizer]

Is condensation harmful to health? All our windows suffer from it. There's no mould just every so often beads of condensation.

 

[jack55]

condesation on windows I can live with, dripping all over the wood I'm storing in a shed to keep dry is another matter. I suppose it could be the sun warming up the roof, hasn't been above freezing point all weekend so the rest of the air in there will be cold.

 

[pren]

I've got the same problem :evil:

My crappy plastic 5x3m shed is like a sodding rain forest at the mo! Why I didn;t get round to re-building it with timber and insulation during the summer I have know idea.

I'm finding myself using machines as quick as possible so I can get them back under covers to avoid water gettting on them. My bandsaw table looks like the bleedin moon at the moment from all the rust!

 

[grainoftruth]

Sorry, bit of a long answer but it is saving me from X Factor so the pleasure is all mine. It's all to do with humidity and temperature. Basically air contains water in the form of vapour, the warmer the air the greater capacity it has to hold water. Unfortunately the same is true in reverse; as the temperature of air drops it's capacity to hold water reduces.

So, what is happening in your shed is as follows. The air in the shed is ambient, ie the same as whatever the outside conditions are on any particular day. By day the temparature tends to be a little higher and, as it's autumn and raining quite a lot, the relative humidity is close to 100% (ie it is completely laden with water). When the temprature drops, either night time or just a cold snap, the air in your shed cools down and cannot hold the volume water it has in it any longer - this is known as reaching dew point. So the water condenses out of the air and usually onto the coldest surfaces - your roof would quite probably be the coldest surface.

Unless you put heating in your shed there is only marginal gain from insulation or foil backed boarding or anything else. The air will still cool down when it gets cold outside and the water will still condense out. If you have any metal in your shed, like tools, the water will condense on to these first.

By far the simplest solution to your problem is a dehumidifier. I would recommend the desiccant type as these are the only ones that work effectively at really low temperatures. There are two leading brands of desiccant dehudifier, I have one of each in my garage and shed and they work fine.

One last point, your shed will need to be relatively well enclosed for a dehumidifier to work.

Steve

 

[MikeG.]

Oh No!!.........not again! A dehumidifier simply masks the problem. Ventilation, insulation and trace heating eliminate the problem, for a fraction of the price.

Moving air dries stuff. We are still drying clothes on the washing line, so long as there is a breeze. Pull an occasional, gentle breeze through your insulated workshop and your condensation problems will disappear. The point of the insulation in an unheated workshop is that the very cold surfaces, which would otherwise attract condensation, are eliminated. The amount of heat you need to put in to maintain a reasonable temperature when you are working also reduces.

When I rule the world, I will ban dehumdifiers (except for flood-damage), uPVC-anything on houses, and patio heaters. On day two I'll start on limiting plane numbers..........

Mike

 

[Digit]

Wot abaht lathes then Mike? :lol:

Roy.

 

[yo_chuci]

gets my vote... roll on april

 

[grainoftruth]

Mike,

Your right if you put some heating in the workshop, as you suggest, but without that the insulation is a waste of time.

As to ventilation, I agree with ventilation generally but it won't stop condensation, it will onlt speed up how quickly the condensation evaporates as soon as the air warms. Just look at a car outside a house, it will often be soaking wet in the morning during winter, regardless of whether there has been rain. This is because the dew point has been passed as the temperature drops - plenty of ventilation but still you get condensation. Later in the morning the wetness disappears, partly due to the ventilation but mostly because the air temperature has risen and so it can reabsorb the moisture.

The cause of the condensation is unquestionable. I suggested one way to deal with it and there are other ways. If you don't like dehumidifiers then fine. They work well for me; I have permanent drains fitted to them that discharge outside, they add a little heat to my workshop too and I don't get any condensation or rusty tools. I have them on auto so they come on automatically if the relative humidity gets too high.

Steve

 

[kasandrich]

With an insulated workshop, how much background heat would be needed to prevent condensation? would a 1 foot 45watt tube heater on basically during hours of darkness during these winter months?

 

[MikeG.]

if you put some heating in the workshop......... without that the insulation is a waste of time.

As to ventilation, I agree with ventilation generally but it won't stop condensation, it will onlt speed up how quickly the condensation evaporates as soon as the air warms.
Steve

Not so Steve.

As I explained, the role of insulation on an unheated building is to prevent the cold-surface problem.

Ventilation works. Go back to your car analagy.......5 celsius and no breeze, your car will be covered in condensation in the morning. 5 celcius with a wind, no condensation. That hasn't worked by blowing the moisture away, either, or by "speeding up evaporation as the air warms".

Look, it works. I've been designing and building outbuildings for 30 years, and have never had any condensation problems. I have never had to resort to a de-humidifier. Get the building right and condensation just isn't an issue. A friend of mine uses a couple of light-bulbs in his engineering workshop as background heat, switched on by a thermostat. I don't ever put any heat into my shed unless I'm in it. I leave my cast-iron bandsaw bed uncovered year round, and none of my tools has any oil or wax on them. I have never ever had a spot of rust........and my workshop isn't some magic super-insulated hi-tech super-shed.

Mike

 

[grainoftruth]

Mike,

I'm impressed. I used to live in a victorian house with sliding sash windows and never had any condensation problems at at all - that was down to natural ventilation and the background heat in the house etc. Reinforces your point.

I now live in the Chilterns and my house, garage and shed is under a huge canopy of oak and beech trees. This causes natural damp conditions and greatly reduces natural ventilation. I have tried ventilating my garage and shed and trace heating but neither worked for me. I was dogged with condensation problems until I resorted to a dehumidifier, that worked for me. I did try traditional design solutions and I do have some experience of building designs, I've been a surveyor for the last 35 years.

The only thing I disagree with that you say is that insulation avoids cold surfaces. This is only true if you have some heat source to keep the temperature in the shed or outbuilding above ambient.

Steve

 

[Steve Maskery]

So Mike, how do you maintain moving air, without losing all your heated air?

I'm interested in this as, despite being about to lose my present workshop, I am mentally planning my next, even if it won't be for some time. I want it to be right. My present one is really part of the house, insulated like the house, room above, rooms on most of both sides, double glazed and radiator on the house heating, so comfortable (apart from the concrete floor - wooden next time).

S

 

[MikeG.]

Steve M,

The heating is intermittent (just when someone is working in the shed), so ventilation can be shut off at that point. When the shed is empty, the ventilation does it's stuff (ever so gently........we aren't talking gales here). As an aside, every house has the "heating V's ventilation" issue.......typically with trickle vents over windows being open at the same time as the heating is on. There are extract fans available with heat-exhangers built in, designed to move the air but not take the heat out of the building, but that would be over-kill in my view.

Steve (Bucks),

if you don't believe me regarding surface temperatures, go and put one hand on the cast-iron bed of your PT or tablesaw, and the other on the surface of your bench. Or, one hand on the roof of your car, and another on the upholstery.

Mike

 

[Steve Maskery]

Thank you.
S

 

[JohnBrown]

Mike wrote:

f you don't believe me regarding surface temperatures, go and put one hand on the cast-iron bed of your PT or tablesaw, and the other on the surface of your bench.

I was always taught that the reason your cast-iron bed "feels" colder than the surface of your bench was because cast iron is a way better conductor than wood, and therefore capable of conducting the body heat out of your hand much more quickly. The same reason why carpet feels warmer under(bare)foot than ceramic tiles or lino.

However, I'm following this thread with interest, because I want to get round to insulating my miniscule wooden shed sometime soon...

 

[MikeG.]

John,

I am not sure of the physics of this, but there is more to it than you suggest. If you hang a towel next to a planer thicknesser, and then allow the conditions to deteriorate such that condensation occurs........it will occur on the bed of the PT rather than on the towel. Condensation in your bathroom will be on the mirror and tiles, and not on the door or wooden toilet seat.

Mike

 

[JohnBrown]

I think it's the same principle at work, the mirror is able to conduct the heat away from the moisture laden air more efficiently than the towel.

I don't know all the physics of it either, but I do know that if cast-iron was intrinsically colder than other materials in the same ambient temperature, or if towels were intrinsically warmer, we could all save a lot of money on heating and refrigeration. Just make a big cast-iron box to store the butter and milk in.
Hang some towels over the radiators and turn off the central heating.

Obviously I am ignoring thermal intertia in this.