How do i overcome this problem

[gasgas249uk]

The chippies who have put on a narrow flat roof above a hallway on a new extention have used 5" ceiling joists. 5x25mm=125mm. Current b regs require 270mm. If i put in that much ill hav to lower the ceiling.

Any ideas please?.

 

[noseall]

What is the problem?

 

[gasgas249uk]

What is the problem?

Thanks for the reply

The measurement from the top of my ceiling joist to the top if my window frame is 210mm , so im thinking that im to light on insulation and the building inspector wont be happy. This is my concern.

Does this make sense to you guys and do you have any suggestions as to

1- Do i need the 270mm everywhere in this flat roof and how do i achieve it .

 

[^woody^]

270mm is the depth if using quilt insualtion.

Use a foil backed board such as Celotex/Kingspan, and you only need 100mm between and 35 over or under the joists

 

[gasgas249uk]

270mm is the depth if using quilt insualtion.

Use a foil backed board such as Celotex/Kingspan, and you only need 100mm between and 35 over or under the joists

Cheers Woody - your a gem

 

[gasgas249uk]

Can the celotex 100mm be inserted between the joists and standards insulation be used for the 35mm?

The standard insulation can be pushed in around the elecs and plumbing?

Sound alright?

 

[mointainwalker]

Woody

i haven't checked the exact u-value of foil-backed PIR, but are you confident of your figures ?

PIR generally has 60% better insulation value than quilt, so I would expect to need 170 mm to be equivalent to 270 fibre=wool.

 

[^woody^]

Yes those depths of insulation are typically accepted/published by LA's as being suitable for flat roofs

 

[^woody^]

Can the celotex 100mm be inserted between the joists and standards insulation be used for the 35mm?

The standard insulation can be pushed in around the elecs and plumbing?

Not really, as the thickness's given were for the same type of insulation. You would have to increase the 35mm if using any other type

In practical terms if you mean quilt as "standard insulation" you wont be able to fix this under the joists and then plasterboard over it

 

[gasgas249uk]

Woody and Mointain walker - thanks

Just to confirm that im reading you correctly

135mm foil backed - celotex min ?

 

[mointainwalker]

I can't answer your question because I don't know if you have to meet a fixed value for the roof or whether it is a package equation taking wall values into account.

I do however disagree with woody, who I think I have read before as saying " PIR is twice as good as fibre-wool " ( apologies if wrong and thinking of someone else).

PIR is not twice as good as wool , it is 60% better and therefore if you require a direct equivalent to the thermal insulation value of 270 mm of f-wool, then you need 170 mm of PIR.

Please note I am only talking about comparative insulation values, not what BCO's will accept or the regs for flat roofs.