Vaulted Ceilings - hybrid or cold pitched roof

[Piers]

We're having a debate with our architect about the merits of installing cold vs hybrid roof and would like some input please.

 

The house will have vaulted ceilings throughout and so we're not too concerned about ceiling heights being impacted by insulation below the rafter.

 

The architect is proposing a cold roof with 150mm Celotex XR4000 between timber rafters, 75mm Celotex GA4000 below, 25mm battens, plasterboard/timber. Steel purlins will be within rafter zone.

 

We're proposing a hybrid with 75mm above rafters, 175mm between (full fill), VCL, battens, plasterboard/timber.

 

Rafters 175mm @400c

 

Architect makes the valid point that the roof will be thicker externally and look bulkier with deeper fascias. Also we'll have to insulate an external balcony to keep a level finish. She also says "The airtightness variation between warm and cold roof is negligible, it mostly comes down to construction standards on site as both can be taped at the joints etc. The detailing of the VCL for a warm roof will be trickier on site for the eaves and balcony overhang vs keeping it internal for the cold roof."

 

I've always thought of cold roofs as "the old way" to build a roof. Warm/hybrid roofs make more sense to me. I don't want to be dogmatic about this - just trying to evaluate if there is an "optimal" way to build it.

 

 

 

[Dave Jones]

cold roof, less to go wrong. hybrid roof is much worse to get right than a normal warm roof.

[nod]

2 hours ago, Dave Jones said:

cold roof, less to go wrong. hybrid roof is much worse to get right than a normal warm roof.

100% agree 

Cold roof is so easy 
 

[JohnMo]

PIR is a pain to install well, very easy to install badly between joists. So not sure I would go there.

 

We added counter battens to increase depth to 350mm then did full fill with spray foam, Then double taped avcl over the whole lot, and glued to the walls. then 50mm battens and plasterboard.

 

If you don't like that idea, go with something like Frametherm 32 between the joists, the underdraw with PIR etc.

 

Whatever you do make sure water vapour cannot makes its way in there.