Insulation for a Vualted Ceiling Roof

[segovia123]

Hi

I am in the process of building a sun room and garage extension which i intend to use as a utility room. Both ceilings are vaulted and will have Velux or similar windows in the roof. My roof trusses are 7*2 @ 400 centers. I have a roofing contractor who will felt batten and fit the tiles. I intend to plasterboard and skim the insides. My questions are

What insulation should I use between the trusses?
Does it need clearance for air flow ?
Does there need to be a membrane on the underside for the insulation to sit on ?
Would you timber panel it out with ply before fitting the plasterboard ?

Thanks

John

 

[noseall]

Warm deck or cold deck, you will still need to fit insulation across the rafters as well as betwixt.

Something like 70mm between with 70mm across would probably do.

Warm deck (insulation above the rafters) requires no additional venting or air gap.

Cold deck (insulation between and below rafters) will require a 25-50mm air gap on the cold side plus a breather membrane to boot.

You ill also need to fit vapour check plasterboards to the ceiling.

 

[segovia123]

Thanks

Forgive my ignorance - whats warm and cold deck ?

I don't fully understand across and between, isn't it just between ?

John

 

[freddiemercurystwin]

You need to stick a slab of insulation between the rafters and then another slab of insulation beneath the rafters and then the plasterboard under that.

Nowadays working out how much heat is lost through a roof takes into account the rafters, they act as cold bridges allowing heat to escape via them. Therefore nowadays insulation is fitted beneath the rafters aswell in a continuous layer to prevent so much heat loss through the rafters. Gettit?

Use a breathable membrane under the tiles so that you don't need any additional roof vents.

http://www.insulation.kingspan.com/uk/pdf/k7.pdf Fig 1C

 

[leew2]

Is this building subject to building regs? I.E. Is it part of an outbuilding less than 30m2 or part of the main house?
If not subject to building regs then you can add as much or as little insulation as you want otherwise you will need to minimum u values.

 

[segovia123]

One is part of the house the other is on the garage. Both are about 24Sq Myt