Dear reader,

I am working on a garage which is rendered unusable for several months of the year due to condensation forming on the metal roof, and then falling onto the garage contents.

I am about to carry out the following work. Could contributors add their comments if they have carried out similar work themselves.
On metal corrugated roofed building with inclined roof supported on timber roof trusses.
Attach 1000gauge DPC blue polythene sheeting on the inside of the building. Attachment points to be insulated and water sealed.
Sheeting to be continuous where possible, and passed under roof eves to allow water to be discharged over and clear of external walls.
Any joints to be overlapped in favour of gravity 'waterfall'
Polythene to be supported to avoid sagging and thus collection of 'pools' of condensate on the polythene inside face.

I would be extremely grateful to hear from anyone who has carried out this remedial work.

Thanks
Paul.

it will just condense on the plastic or next coldest surface
the moisture is in the air you need insulation to stop the moist air hitting the cold surface
what sort off ventilation do you have ??

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we are all ------------------still learning

I did something similar for a confined storage area which covers about half the roof area, I fitted polystyrene up to the roof and then covered with DPM, no evidence of condensation at all in that half.

But, BA is right, if you've still got the same amount of damp air it'll need to settle somewhere.

Your solution will be heating to above dew point and/or maintaining adequate ventilation....I'm currently working on the latter in the hope of having my first condensation free winter!

wrinx

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www.ilmostro.co.uk

Hi Paul,

Basically, you're doing a tanking exercise? Google tanking!

SSM

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Measure twice, cut once!

Dear Wrinx,

Thanks for your contribution. I am gradually building up a practical solution to the problem of condensation forming on cold metal roofing sheets. Your comments are a valuable step forward.

Could you expand on your remedial work to cure condensation as follows;
Thickness of polystyrene sheet.
Method of attaching polystyrene sheet to roofing/rafters....i,e, adhesive, screw bolt etc
Gap between poly' sheet and corrugated roof.
Gap between the poly sheeting and polythene DPM.
Method of attaching poly sheeting to polystyrene sheets
Was the storage area heated.
Was the area condensation free when the external air temp was below freezing.
Was the storage area ventilated.
Assume flat poly sheet used against corrugated steel roof and not a spray polyurethane.
any other important features.

I look forward to your reply.

Thanks
Paul

I jammed 50mm polystyrene in between the wooden roof supports (think they're about a metre apart), no gap to the roof. This was covered with polythene DPM stapled to the roof supports. Gap to the polystyrene is zero to no more than about 10-15mm where it sags a bit .

Then the whole lot is secured in place with some 30mm batons screwed at 90 degrees to the roof supports, every 500-600mm or so.

Storage area is in a large and unheated garage, although I did look at some air-source heating, might still do this.

The garage wasn't very well ventilated thanks to excessive use of expanding foam by the previous owner. I've removed some of this and will get rid of all of it to promote airflow.

The roof is an alternating mix of steel and poly sheets, both suffer(ed) as badly as each other with condensation...the idea (as I'm sure you know) is that the polystyrene creates a warmer surface which moisture will not condense on...but as mentioned earlier, it is still in the air so just settles on the next coldest thing. In my case the rest of the ceiling is still bare but I intend to cover it all once the ventilation issue is solved.

For the size of my garage (8mx4mx2.5m) I think a 200mm extractor fan should create enough circulating air with some vents in the doors, failing that I'll fit another fan!

I just hope it works...my garage is VERY damp in poor weather conditions.

wrinx

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www.ilmostro.co.uk

A dehumidifier would help just as much as heating and use a fraction of the electricity.
There can be too much ventilation. If you are sucking air out with a fan that air has to be replaced by new air coming in. If damp is rising through a concrete floor the air going out will be more humid than the air coming in but if not and there are no leaks then you could just be sucking out warm, dehumidified air and replacing it with cold, damp air.

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Mike

I found dehumidifiers to be expensive...certainly in the space I've got.

wrinx

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www.ilmostro.co.uk