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It might help if we clarify a few points here for Tony and any of the others considering building a garage.
All the air around us has moisture in it and as the temperature drops towards freezing the amount of moisture the air can hold drops. You will often hear the term “Relative Humidity” which refers to the amount of moisture in the air compared to what the air can hold at that specific temperature. When the air cannot hold all that moisture, it condenses as dew.
If you take the situation of an unheated garage then as the air is gradually cooled while maintaining the moisture constant content, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will be saturated, is called the dew point. If the air is cooled further, some of the moisture will condense.
Now if you understand all that then let me make some comments about the types of construction proposed here.
Timber walls. Fine particularly if they are not continuous and allow ventilation to the outside for as the air cools inside the garage that warmer, near saturated; air will probably pass through the walls to the outside and be replaced by cooler less saturated air from the outside. If the timber walls are continuous then the dew point, the point at which the condensation forms is likely to remain within the wall itself and that moisture will be absorbed by the timber. Unless the timber is treated or is of a type that can cope with moisture then it will eventually rot. (Probably not within the lifetime of the structure proposed here). However condensation can, and will, still form on the underside of the roof.
Double brick or block walls. The dew point will always be towards the outside of the wall and in the case of the double brick the condensation will run down the inside of the outer brick wall.
If you look very carefully at the photograph of Geoff’s “reasonable sized shed” you will see the flexible lining under the iron roof. The prime function of that lining is to form a vapour barrier. Condensation will be forming under the iron roof and will be dripping down onto that vapour barrier which will then carry it to the outside edge of the roof where it will be discharged into the open.
Here is a photo taken in my garage/shed cum workshop.
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If you look up at the underside of the roof you can see the vapour barrier which shows lines which mimics the shape of the corrugated iron roof above it. All the condensation forms above the vapour barrier and below the iron roof and is carried to the outside as it is in Geoff’s shed.
There are various types of vapour barriers available, from, as Geoff describes - foil backed continuous plastic sheeting, through to bituminised paper based products (which I have used).
I would always use some such product under any roof (even my wood shed where I store my wood for the winter fires which is open on all sides has it under its roof) and I would always put something under the floor slab (in this country it is usually plastic sheeting) as well, to stop the damp rising up from below.
Tony says he is erring towards a cavity wall of some sort which should work. I would recommend that no matter what sort of roof he eventually selects that he places a vapour barrier under the external roof skin and if it is a concrete floor, a vapour barrier under that.
He will then have a nice dry space in which to store and work on his cars.
Robert
Sorry, but I have to disagree with most of this.
The vapour barrier will actually stop water vapour in the air, in the garage/shed, from meeting on the cold surface of the underside of the corrugated sheets. So condensation will not form above the vapour barrier and below the corrugated iron sheets, as stated. Unfortunately, as the vapour barrier is mostly touching the corrugated sheets, the surface of the barrier will be almost as cold as the sheets and the condensation will just form on the underside of the barrier. In this case the vapour barrier will have little, or no effect. It would have been better, although still not ideal, to have fixed the barrier to the underside of the purlins and taped the joints, then at least there would be an insulating air gap in between the sheets and the barrier, and the surface of the barrier would have been relatively warmer and less likely to form condensation on its surface.
Vapour barriers in building construction are mostly used to stop interstitial condensation (condensation that forms within the thickness of a wall wall/roof structure) and should be placed towards the inside of the building. For example, if you have a roof consisting of corrugated sheets, with Rockwool insulation between the joists/rafters, and a boarded ceiling, then a layer of polythene above the ceiling boards will prevent condensation forming where the dew point is reached within the structure. This might be within the Rockwool insulation, or on the lower surface of the corrugated sheets, depending on the inside humidity levels and temperature, and the external air temperature.
Vapour barriers, or more accurately Damp Proof Membranes, are always a good idea under a concrete floor slab, and, if possible, turn them up the walls and link them into the Damp Proof Course. Heavy gauge polythene is best for this.