Kingspan insulation thinckness on extension

[chez4836]

Hi, I am building an extension with a part flat roof and also pitched roof construction. The joists are 150mm x 50mm - 400mm centres on pitched roof and 200mm x 50mm on flat roof. Want to have insulation between rafters. Due to the fact that I need an air gap between insulation and breather membrane [cold roof], can I calculate what thickness Kingspan/Celotex I need. I was hoping for 120mm, then have an air gap of 30mm. Not too sure if this reaches the 0.2 U value though. Flat roof is 2Mx4M, pitched roof is 1M x 4M. Flat roof covered with 18mm ply, normal concreete tiles on pitched roof. Happy to put layer of insulation quilt under rafters before I put up 12.5mm plasterboard. Thanks in advance for any help - new to this site.. Chez4836

 

[GaryinAshby]

My BCO recommended 165mm between the rafters (90mm and 75mm sheets) or 100mm between and 35mm underneath for my cold (flat) roof. Whatever you go with, the minimum space for through ventilation above the insulation is 50mm.

 

[chez4836]

Cheers, I am thinking now to go with 100mm between rafters and the thermal quilt underneath [thermal resistence of 6.1]. That way, I can still keep the recommended 50mm air gap.

 

[freddiemercurystwin]

For the exact actual thickness required contact Kingspan Technical http://www.insulation.kingspan.com/uk/contact.htm , have to hand rafter sizes and centres and they'll tell you exactly. Ask them to email you a confirmation for you to give to Building Control. They'll do it whilst you're on the phone take about 3 minutes.

 

[Chukka63]

same as gary.just done a loft conversion and used 120mm between and 40mm under the rafters.

 

[DevilDamo]

You could use Celotex's u-value calculator, which is VERY good...

http://www.celotex.co.uk/Technical-Services/Further-Information/U-value-Calculator