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Condensation under metal roof
I have just had a zinc roof installed on top of 3/4 inch sarking board (3/4" X 8" X 12 foot planks) closely fitted to form an even surface. No underlay was installed. The roof is 60 feet by 20 feet and slopes front to back at 23 degrees (1 in 4).

The sarking sits on 2" X 2" X 12 feet batons and the batons are fitted across 9" X 3" rafters, leaving 2" ventilation across the structure intake at the rear exhaust at the front.

Unsurprisngly there is plenty of condensation on the zinc.

I am currently installing 8 inches of loft insulation between the rafetrs/purlins. Under this will be the plasterboard ceiling.

My question is. Do I need to install a vapour barrier between the ceiling and the insulation/rafters to stop humid air passing through and condensing on the zinc? If so, should it be plastic sheeting or a breathable membrane?

I plan to use heat exchangers to remove humid air from the building

Thanks

Nick
If you have an attic, you roof is not your insulation/vapor/envelope barrier. Your attic floor is.

That is where the sealing and insulating should happen. Prevent the moisture from making it to the roof at all.

Any more information you can provide about the construction of the home would be helpful.
The roof does not have an attic. The attic floor is the roof. It is a sloped roof, in one plane. It has no ridge. The top plate at the front of the house is 5 feet higher that the top plate at the rear. The roof slopes down from the front to the rear, where there is a gutter along the back. It is a very simple construction.

The roof cross section is zinc, then 3/4 inch board (treated timber planks), then 2 inch air gap (intake to rear, exhaust at front),then 9X3 inch rafters with insulation between the rafters.

Below this I think I need to seal with a vapour barrier (plastic?) and finally the finished ceilings in plasterboard.

Do I need the plastic vapour barrier? If so, how well should it be sealed? e.g do I need to seal around the light fittings etc.

Hope you understand this and thanks for your help.

Nick
A plastic vapor barrier is certainly recommended in this application.

Sealing around the light fittings and any other penetrations in the vapor barrier is a must.

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