Ventilation space in rafters

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Hi

I am going to use corrigated metal sheeting for a roof of a new house. I was told that i would need to leave as much ventilation space between the insulation and the breathable membrane barrier as possible. The make up of the roof is as follows
  • Plasterboard
  • 50mm insulation fixed under rafters
  • 100mm rafterloc fixed between 225mm rafters.
  • breathable membrane
  • 50mm X 50mm battens
  • Corrigated metal sheeting
This would leave a 125mm ventilation gap. The supplier of the metal sheets has said that the sheets will create a lot of condensation at night and that it would be better to have this ventilation gap in order to preserve the rafters.

Will this gap have a significant impact on the thermal efficency of the roof? any other advice would be welcome.

Thanks
 
Do you mean the old-fashioned wavy corrugated-iron or the new coloured poly-coated metal roofing?

If its the latter, there are ready insulated poly-coated metal "sandwiches" available which is suitable for animal housing, but I don't know if any of these products are approved for use in domestic houses.

What does the planning authority or your permission say?
 
Thanks for the reply

It will be chorus powder coated metal in the traditional wavy style. Tried to get panels but they are not avaliable with that profile.

Planning made no comment on the roof
 

you have very very little sound proofing insulation in that specification.....

the denser the insulant the better thesound insulation. Id serious look at your specification again from the point of view of including OSB board over the rafters, battening over the OSB, and incorporating rockwool between and perhaps under the rafters.
 
You might want to look at the breather membrane / underlay and make sure it is rot proof, if you vent properly at the eaves / ridge and improve your insulation you should be able to fit the sheets with a 50mm vented air space between the membrane and the insulation, the battens should provide enough space between the sheet and the membrane.
 
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