Warm Roof insulation options

I was hoping i could get some advice from some of you with experience as i'm getting too much contradictory advice.

So we are building a dormer house, with half of the house been a full two story. I had agreed with the engineer previously to go for a full warm roof. It was going to be
Slates, counter battened, pro clima breathable membrane, 24mm steico woodfibre, 175mm rafters, rafters filled with Metac 180mm, air tightness membrane, 50mm counter batten filled with metac, plasterboard, plaster.

The plan was to have the airtightness membrane brought right up to the apex. I had the roofing materials bought early due to planned price increases.

We now have the woodfibre on. We have had input from an air tightness installer who has suggested that it would be a complete waste to bring the metac right up to the Apex. He has suggested that we only bring the metac up the ceiling slopes of the living spaces and then using ordinary rockwool over the tops of these ceilings and the air tightness membrane should only be brought over the living spaces.

If we follow his advice the attic space above the living space would not be airtight and would not be insulated except for the woodfibre. It would probably mean that i have wasted my hard earned money on the woodfibre and the metac.

What do you suggest i do ???

Poor Uncle Tom

That's a modified cold roof build up, not a warm roof build up.

Poor Uncle Tom

Poor Uncle Tom wrote: »

That's a modified cold roof build up, not a warm roof build up.

Would you go for the full warm roof or the modified cold roof all things considered ?.

Poor Uncle Tom

Deleted User wrote: »

Would you go for the full warm roof or the modified cold roof all things considered ?.

It really depends on:

Site conditions,
Height restriction for planning permission?
Use of the roof space,
Budget,

I don't know any of the above in relation to your situation so I can't answer.

For me it was the right call, but I was making full use of the entire roof space in the job I'm referring to.

Poor Uncle Tom

Poor Uncle Tom wrote: »

It really depends on:

Site conditions, Windy
Height restriction for planning permission? Already built to planning permission
Use of the roof space, Probably a metre at most of the slope is used in living spaces
Budget, The hardware is already bought

I don't know any of the above in relation to your situation so I can't answer.

For me it was the right call, but I was making full use of the entire roof space in the job I'm referring to.

See above.

My worries would be if i go full warm roof, i would be heating an unused attic. If i go full warm roof and then also try and insulate above the living spaces, this would be additional costs.

If i go cold roof, the space above the slopes would have to be ventilated. It would also mean the fibreboard was pointless.

Poor Uncle Tom

In your 'warm roof' scenario mentioned above, how are the structural roof timbers ventilated?

Poor Uncle Tom

Poor Uncle Tom wrote: »

In your 'warm roof' scenario mentioned above, how are the structural roof timbers ventilated?

There not, there is 25mm of woodfibre on top of the rafters. There is MHRV in the rest of the house. Those rafters are full filled with metac.

Calahonda52

OP
re
pro clima breathable membrane
which one exactly pls?
have u slating lathes and c/b's?
air tightness membrane
which one exactly?

Calahonda52

Calahonda52 wrote: »

OP
re
pro clima breathable membrane
which one exactly pls? Solitex Plus
have u slating lathes and c/b's? Yes 2" conter battens and slating lathes
air tightness membrane
which one exactly? Pro Clima Intello Plus, not installed yet

See above