Loft Conversion Insulation Value

[tleefox]

We are in the process of looking at options for a loft conversion on our house (1930's semi) and I'm struggling to understand in layman's terms what the current Building Reg standards are for this element.

The website below suggest a u-value of 0.18 w/m2k, but I can't get my head round whether this relates to the roof structure as a whole (i.e. combined across the void between the joists and between the sloping trusses) or whether it has to relate to any individual section of the roof (i.e. the insulation between the joists needs to be 0.18, as does the insulation between the trusses).

Hopefully this makes sense! Anybody able to assist?

[worsy]

tleefox said:

We are in the process of looking at options for a loft conversion on our house (1930's semi) and I'm struggling to understand in layman's terms what the current Building Reg standards are for this element.

The website below suggest a u-value of 0.18 w/m2k, but I can't get my head round whether this relates to the roof structure as a whole (i.e. combined across the void between the joists and between the sloping trusses) or whether it has to relate to any individual section of the roof (i.e. the insulation between the joists needs to be 0.18, as does the insulation between the trusses).

Hopefully this makes sense! Anybody able to assist?

http://www.kingspaninsulation.co.uk/Knowledge-Base/Building-Regulations.aspx

Assuming England you will need:

0.18 for the ceiling
0.28 for the walls, note that the dwarf wall on a pitched loft conversion counts as wall.

As you are not converting over an unheated space I don't think you need to worry about the floor but check that.


Edited to say, for your roof, assuming 600mm joists, VCL and Plasterboard, you would need 150mm of Celotex FR5000 or equivalent. Don't forget you should leave a ventilation gap of 50mm.

for the dwarf wall, the joist thickness may prevent you installing anything over 100mm so you might need over joist insulation too. I certainly did for my garage conversion.

i can help if you need me to, just an amateur but did my own calcs for Building Control sign off.

Edited by worsy on Wednesday 9th December 15:54

[8-P]

Out of interest, how does 100mm of Celotex compare to 100mm of standard loft insulation?

Im assuming Celotex or similar provides more insulation because its denser?

[bobtail4x4]

celotex is about twice the insulation value of rockwool.

[worsy]

8-P said:

Out of interest, how does 100mm of Celotex compare to 100mm of standard loft insulation?

Im assuming Celotex or similar provides more insulation because its denser?

My understanding is that it is roughly twice as thermally efficient. I assume the density is the factor also.

[Spudler]

Min 100mm between studs/rafters then 35mm.

[roofer]

Doing this at the moment.

I am using TLX Gold on rafter, with 140 mm Celotex type in rafter, therefore avoiding internal on rafter insulation. Rafters are 197 mm though. Aiming for .16.

[tleefox]

Cheers guys.

[8-P]

Sorry to hijack but can I use celotex under my loft boards, would save raising the floor

[worsy]

8-P said:

Sorry to hijack but can I use celotex under my loft boards, would save raising the floor

Blimey, how shallow are your joists? Normally 200mm of rockwool is adequate, the trouble with Celotex is the foil backing so if you have any moisture you need to ensure ventilation.