Dormer cold flat roof insulation

[Kojeng]

Hi,

 

(First post, but have been reading many many great posts here!)

 

We are at the design stage of our loft extension (as part of bigger renovation project, more questions to come from me!) and would like to get some thoughts on the proposed build up of the dormer cold roof. The proposal is to have:

 

• 150mm rafter @ 400c/c
• 50mm ventilation gap
• 100mm Kingspan K7 between rafters
• 42.5mm Kingspan K118

 

Giving final u-value of 0.18 for builing regs and reasonable ceilling height of 2.2m.

 

We have K118 acting as our VCL - should we add another VCL layer in between K7 and K118 just in case?

 

Based on posts here this build up has low decrement delay and can get very hot during summer (roof is west facing). We do plan to have air conditioning installed in the loft, but ideally wouldn't want to rely too much on it.

 

As I understand the two insulation products do not provide great sound insulation - will this be a problem during heavy rain?

 

Is there any better configuration to address the decrement delay issue while retaining the same u-value and build up depth?

 

Many thanks!

[Dave Jones]

Make sure there is ventilation at the eaves and the ridge unobstructed by steels etc 

 

you need airflow from bottom to top with nothing in its way.

 

if you have a flat roof can look at mushroom vents.

[JohnMo]

Do you have room at the top of the roofline to install external insulation instead, a single layer of 120 to 150mm, would be easy to install and give better u values.

[Radian]

Have you considered using SIPS? Obviously depends on your dormer dimensions.