insulating a tin roof

[David P]

Ive noticed that the insulation posts get mixed responses so I thought I'd throw this one in...
I am building a large shed/workshop with a pitched roof that i want to be as dry as possible inside.
I have also inherited a load of building materials that I want to use, particularly, many sheets of flat galvanised steel 2x2 and stirling board. Ive also got loads of tubes of silicon.

I want some advice on the best way to use the galvanised sheet as a roof, and what order to add insulation, vapour barier, breather, interior surface (plasterboard or stirlingboard) so that condensation isn't a problem.

what I have in mind is this (from outside inwards)
galvanised sheet seperated 2" from rafters by 2x2 to allow air circulation.
breather paper over rafters.
insulation between rafters
vapour barier under rafters
interior surface screwed to underside of rafters over vapour barrier.

this seems a bit weird to me - is there any point in the vapour barrier?

as the tin roof will be really noisy in rain is there any way that the tin could be fixed directly to an insulating board (using the silicon i've got), without trapping moisture that will rot the rafters.


would it be better just to board the rafters, and slap the tin on top, then seal beneath the rafters
what would you do?

 

[masona]

The way I did my summerhouse roofing was using 4"x2" rafters, P5 Grade T&G chipboards (moisture resistant) with roofing felts on top then metal roof sheetings, underneath I've used 2" insulation slabs with the 2" air flow at the back for air circulation to soffit vents, any condensation under the metal roof sheetings will be collected onto the roofing felt & out to the gutter. Under the metal sheetings will also be vented as well.

Metal sheeting > roofing felts > Grade 5 chipboards > 4"x2" rafters > 2" insulation > plasterboards

 

[David P]

any condensation under the metal roof sheetings will be collected onto the roofing felt & out to the gutter. Under the metal sheetings will also be vented as well.

Metal sheeting > roofing felts > Grade 5 chipboards > 4"x2" rafters > 2" insulation > plasterboards

masona. seems like an ok plan - only thing is, from what ive been reading on the subject I get the impression that because the felt would be on the cold side of the roof you will be just as likely to get condensation on the underside of that, which will be trapped in the wood and cause rot.

seems you should never have an inpermeable membrane on the cold side - which tin is. or is roofing felt a minor insulator in itself like wood is?

perhaps a good solution would be to use some sort of rigid insulating board that isn't absorbent and doesnt rot, like expanded polyurethane, directly under the tin in place of the t&g board.

 

[masona]

only thing is, from what ive been reading on the subject I get the impression that because the felt would be on the cold side of the roof you will be just as likely to get condensation on the underside of that, which will be trapped in the wood and cause rot.

Somehow I don't think so because the felt is flat on the boards and it's not really any difference on a close boards roofing rafters of a house. Sorry I forgot to mention my roofing felt is stuck down with bitumen
I have repairs a few roofing felt on sheds and so far I haven't seen any condensation under the felt yet. Maybe the warmth of the timber is enough to keep it off(?)

perhaps a good solution would be to use some sort of rigid insulating board that isn't absorbent and doesnt rot, like expanded polyurethane, directly under the tin in place of the t&g board.

Nothing wrong with that at all

 

[Nige F]

Also think of the consequenses for a Cat on a hot tin roof ..........RSPCA will be after you