The Triumph Dolomite Club - Discussion Forum

For vapour barrier, read semi-permeable membrane; a material which allows vapour to pass through upwards, but is waterproof in the other direction, but really, in a garage this is of little real benefit

A little knowledge is being wielded dangerously in places here, though the science is correct: If the garage is unheated, the incidence of condensation will be infrequent, particularly if there is no moisture generating activity going on in there, and adequate ventilation is provided. Avoid oil/parafin/petrol/meths heaters as each of those fuels creates water when it is burned; in the case of petrol it is 10x its own weight, so don't run the engine with the doors closed, if not for other reasons as well! The biggest generator of moisture in a garage will be you; the average human producing about 3/4 pint of water during an 7 hour sleep cycle, more so when awake and active. If there is a single glazed window in the garage, most of this moisture will condense on that.

Ventilation is the key.

If you aren't scared of using your car on a damp day, you needed worry about keeping it in a well ventilated garage.

Good points there Alun. Following on from your comments, in particular that any water vapour will condense out on the window, is there an argument for having a large up-and-over metal garage door. That way any internal water vapour will condense on the door and run outside through the folded seams ?
My garage has this and it seems to be reasonably dry.
But as has been noted dust and leaves and also insects do intrude, particularly in the Autumn.
Tony.

In building construction a vapour barrier is not usually semi-permeable in one direction, it will normally be a material that prevents the air from passing in either direction. Examples include polythene, foil materials, and bitumen based products.

There are many breathable roofing felts and membranes available now that are water-proof but allow ventilation, and these negate the need for eaves/soffitte ventilation on a roof.

There are a number of insulation products available now that incorporate a vapour barrier within them.

I completely agree that a garage should be well ventilated and, if this is the case, condensation is unlikely to form but the reality is that most of us like a bit of warmth while working in the garage and that, combined with us producing water vapour, could well lead to condensation in the winter months.

I don't wish to appear pedantic but there were a few inaccuracies in the earlier post on the previous page.

If it's prime function, as you describe it, was working, then the moisture from the interior would not be able to reach the outer iron roof to condense. Hence my comment regarding SPMs.

Generally, I specify SPMs on the cold side of insulation, as that is usually where condensation forms, but in a garage we are talking about uninsulated structures. The temperature gradient across a membrane with an iron roof above would be negligible, and any air cavity introduced between would have a relatively low thermal resistance, so if condensation were to form on the underside of the steel, it would likely form on the underside of the membrane also, particularly if it were a true vapour barrier and impermeable.

The only way you could encourage the condensation on the underside of the roof sheets as described and then drip onto the sheet below, would be to use a SPM to allow the moist air to pass through prior to condensing, but for that to occur the air would need to be warmer and able to cool below the dew point once it had passed through and not before. With external temperature being wildly variable, reliance on such a system would be akin to relying on a stopped watch to tell the time; it would be right far less often than it was wrong!

Such a construction would then need to drain the lower sheet at the eaves to purge the water from the building, effectively providing ventilation! Personally, I'd not bother...but then I've only been designing buildings for 25 years

Thanks for your input too Alun. In essence then If i have a waterproof roof ie tiles or felt would you say leave the rest to breath naturally through gaps? Im still not 100% on whether its gong to be wood or brick/block yet, would you treat these types differently regarding ventilation?
Another thing I had planned was a hole in the wall with a pipe to let exhaust gas out if I start a car and fully warm up. I can cap the pipe when not in use or leave open if thats better, is that worth considering?

Cheers

Tony

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Exactly, its a waste of time and achieves nothing.

I live in the north west of Scotland so plenty weather variations to contend with!! Approx 15 yrs ago I built a 20 x 25 ft wooden garage constructed of sections bolted together on a concrete base with a damp-proof membrane under the sections. It had many coats of spirit based wood treatments for about 7 years after which I fixed metal profile sheeting on horizontal battens to the outside walls - this has made a massive difference to the overall feel of the structure, the roof was covered in metal profile sheeting during the original build. A sectional up & over door has replaced the original wooden doors, my only other creature comfort is a domestic humidifier which drains via a pipe outlet into bottom of the "cavity" of the outer walls, the humidifier has been well worth the £100 spent on it. My only regret was not getting profile roof sheeting with an anti-moisture coating to reduce condensation during the early years, once the humidifier was added, you could almost see the interior drying up, the interior is still the internal face of the original wooden sections.