Thread: shed Upgrades wood heater & bitumen black

Condensation is caused because warm air can hold more moisture than cool air. So when warm/moist air is cooled by contact with a cold surface, condensation forms as the air deposits the moisture it is no longer able to hold. How much condensation depends on how much moisture the warm air contains. Unfortunately warm air rises, contacts the cold roof of the shed, deposits it's moistures then the now cold/dry air falls and is replaced by more warm/moist air. Whether the condensation forms drips then becomes a function of the shed:

1/ My workshop is all steel, skillion roof, unlined and uninsulated. The concrete floor has a DPC and so is dry, there is no wet timber, so the air is quite dry. The shed is unheated and fairly drafty, which from another viewpoint means "well ventilated" (and cold). The small amount of condensation that may form (I've never looked!) on the roof never gathers in sufficient quantity to form a drip.

2/ My "hay" shed is all steel, exactly the same materials as the workshop (but far bigger). However it has only three sides, a gable roof and a dirt floor. The air is moist because of the dirt floor and the floor is "warm" because it's under cover, the shed roof traps the rising warm/moist air and the cold steel frame causes condensation (warm air cools, deposits moisture then falls to be replaced by more warm/moist air etc.). The condensation runs down the iron to the next purlin, then runs down the purlin and drips to the floor. There is a dripline below each purlin.

3/ I have a 40' shipping container. This has a suspended wooden floor with wet dirt beneath (it was recently moved so the dirt beneath has not had time to dry out). It is all steel, with the normal square section corrugated walls and roof, no leaks but no ventilation. It RAINS in this shipping container. The problem is that through the day the air warms up, as does the ground below the container. When the roof cools, the warm/moist air cycle happens to a massive degree, producing an almost unbelievable amount of condensation......

To fix the problem;

prevent condensation by stopping the warm moist air from meeting the cold surface i.e. insulate the steel to remove the contact with warm air

or

prevent warm moist air from meeting the cold surface by removing the warm moist air - ventilation and COLD SHED!

or

prevent condensation by keeping the air in the shed dry, so it has no moisture to deposit i.e. seal the floor if it is damp, don't keep items that release moisture (wet timber, washing etc.) in the shed

or

if none of the above is possible (or desirable), then accept that condensation is inevitable but channel it to where it can do no harm i.e all condensation forming steel surfaces needs to slope sufficiently that the drops run along to points of your choosing, rather than dropping off over the machinery!

Which approach will work depends on the situation!

Note that if you have a damp floor, damp timber etc. in the shed, and then you warm the air with a heater it will likely pick up more moisture, so unless you produce enough heat to keep the steel frame warm (very hard to do) your problems may get worse not better!

There is a thin "foil backed foam insulation" product, designed to be stapled to the joists underneath houses (houses on piers!). It's rated R1 from memory. You could probably run such a product across the bottom of the purlins, secured to the purlins with screws or battens, and sealing the joints with duct tape. This would trap a layer of air between the roof iron and the insulation, and prevent the warmer air below from meeting the cold shed frame....... Done right (i.e. trap the air when it's dry!), this may well be your best bet.

Thanks Warb, plenty to think about there!

This is a good thread for me Rattrap, I have just bought shed very close to you so I have some good points to consider, cheers.

I am wondering if the roofing screws would transmit enough of the cold from outside to the inside, as Warb has said hot moist inside meets cold and that's where the first bit of condensing happens and then over the night enough for drips, it might not be the rafters so much as the roofing screws transmitting the cold to the rafters, Are there umm (painted now) Where there localised areas of the white corrosion inline with the screws? If so maybe a bit of expanding foam squirted on each screw (inside) just thinking that u might not have to do the whole rafter if you go that way, What do you think?


Pete