Loft conversion

[catlad]

Can anyone advise on current U.values for vaulted ceilings\roof for a loft conversion?
I believe they have gone up recently and I'm not up to date!
Thanks.

 

[George_engineer]

England, Wales or Scotland?

Assuming England a 'new fabric elements in existing dwellings' for a roof is 0.15 W/m^2 K

Page 25 of Part L (https://assets.publishing.service.g...uploads/attachment_data/file/1099626/ADL1.pdf)

However - even now building regulations represent the bare legal minimum and you can save lots of money over the years by putting in as much as is feasible.

 

[jeds]

Agreed, in simple terms; the old basic standard was 100mm PIR between joists and 40mm continuous PIR below joists. That gave you a u-value of 0.19. The current minimum u-value is 0.15 which can be achieved by increasing the 100mm between to 150mm between. I would, at least, make the continuous layer 50 or 60mm, which will give you 0.14-0.13 respectively.

 

[^woody^]

The issue however, is fitting all the new thickness of insulation in.

Previously, lofts typically needed rafters upgraded to 150mm deep, so a 100mm sheet of insulation would fit with a 50mm air gap.

Now you will need to either convince the BCO to accept no air gap, or deepen the roof section.

 

[George_engineer]

There are some get outs

- if 5% of internal space is taken up you can reduce insulation (although not sure how that translates to a roof. Suppose a deeper section removes the useable floor space)
- as it's a weighted average you can increase insulation in the wall section of a dormer (u-value 0.18) or in the dormer roof to allow a reduction in the pitched roof insulation, which is likely to be more problematic.

 

[jeds]

Indeed, there are lots of ways to achieve a u-value but the structure plays a large part in what can or can't be done. Alternatively, add the continuous layer on the external face and make the roof a hybrid/warm roof, eliminating the ventilation gap. This is all subject to detail design.

 

[tony1851]

The issue however, is fitting all the new thickness of insulation in.

Previously, lofts typically needed rafters upgraded to 150mm deep, so a 100mm sheet of insulation would fit with a 50mm air gap.

Now you will need to either convince the BCO to accept no air gap, or deepen the roof section.

Or convince the BCO that, under the sloping section, headroom would be reduced to an 'unreasonable' degree, and therefore the thickness of insulation should be modified accordingly.
Some inspectors are more reasonable than others.

 

[catlad]

Thanks guy's
The reason I ask is I'm fitting 3 velux skylights for a customer, and he is thinking of doing the insulation himself. He has an old slate roof with 3x2" rafters which will take a bit of modifying to accommodate all that insulation, He has a single skin party wall with his neighbour ( terrace house ) would 50mm insulated plasterboard be enough of an upgrade or would he need to build a stud wall?

 

[catlad]

England, Wales or Scotland?

Assuming England a 'new fabric elements in existing dwellings' for a roof is 0.15 W/m^2 K

Page 25 of Part L (https://assets.publishing.service.g...uploads/attachment_data/file/1099626/ADL1.pdf)

However - even now building regulations represent the bare legal minimum and you can save lots of money over the years by putting in as much as is feasible.

Thanks George
I have been telling people for years to insulate their roofs. Some do some don't but they all pay in the end one way or another!

 

[catlad]

Or convince the BCO that, under the sloping section, headroom would be reduced to an 'unreasonable' degree, and therefore the thickness of insulation should be modified accordingly.
Some inspectors are more reasonable than others.

Yes Ive told him to employ a private bco. and come up with a plan.

 

[^woody^]

Or convince the BCO that, under the sloping section, headroom would be reduced to an 'unreasonable' degree, and therefore the thickness of insulation should be modified accordingly.
Some inspectors are more reasonable than others.

This is one instance where trying to get around the requirements is not as beneficial as meeting them.

 

[catlad]

Yes I agree. He has inherited this project from the last owner with steels and a staircase.

 

[tony1851]

This is one instance where trying to get around the requirements is not as beneficial as meeting them.

Yes, bang one's head on the roof more often but be warmer.