Rafter Level Insulation to latest regs

[FrogParty]

Wondering the best way to insulate a pitched ventilated roof at rafter level to the standard required by document 1Lb (Oct 2010). The recent change to the building regs require U=0.18 on pitched part of roof of attic rooms or loft conversions rather than the previous 0.20.

Noting the varied recommendations made in previous threads (for the original standards), and the variety of configurations accepted by different BCOs (which don't seem to match the older standards either) the whole thing seems rather a black art! I have doubts if the differences between 0.18 and 0.20 are noticeable in terms of fuel bills or comfort, especially given the house walls of solid brick are U = 2, but let's say we really want to achieve 0.18 not just get a tick from the BCO.

I have found some solutions in product literature that are BAA certified and LABC registered (and that should be good, right?) that achieve 0.18 for a non-breathable felted roof (vented from eaves) with nominal 100x50 rafters at 400 spacing.

Kingspan - rigid foam boards -
1) 50mm gap, 125mm K7 (phenolic) between rafters and 38mm K18 overlay. [213mm total thickness]
2) 50mm gap, 140mm TP10 (PIR) between rafters and 38mm K18 overlay. [228mm total thickness]

Web Dynamics - multifoil augmented with other -
3) 50mm gap, 90mm Phenolic (e.g. K7), 20mm gap, TLX Silver across rafters, 38mm baton, plaster board. [225.5mm total thickness]
4) 50mm gap, 100mm PIR, 20mm gap, TLX Silver across rafters, 38mm baton, plaster board. [235.5mm total thickness]
5) 50mm gap, 140mm mineral wool, small (?) gap, TLX Silver across rafters, 65mm baton, plaster board. [302.5mm total thickness]
6) TLX RafterFit between rafters (gaps above and below), 80mm phenolic across rafters, plasterboard directly on top. [192.5mm total thickness]
7) TLX RafterFit, 65mm PIR across rafters, spacer baton, silver backed plasterboard. [202.5mm total thickness]

All solutions, except for those with RafterFit, require extra timber to deepen each rafter by at least 60mm (up to 90mm for wool). This, the minimum thickness (thus loss of room space), and ease of fitting between wibbly/wobbly rafter timbers, makes the RafterFit route look attractive. But not sure about the long nails needed for 6) through both plaster and phenolic boards, could be tricky to handle. Anyone actually done it?

Then, of course, there is Actis Super 10. A couple of loft contractors have quoted to use this, and were certain it alone would pass BC even when I pointed out the changes in building regs. There seems to been a whole load of controversy about using multifoil products (alone), has that been resolved yet? Are Actis going to change their blurb to say "0.18" rather than 0.20? Will BC accept it now there are certified combined solutions, which if they are accurate imply super 10 alone can't be.

Will it take a while before BCOs are insisting on 0.18 compliance, or will they enforce the new value immediately?

As you can see I have done my homework, although I am sure there are other products out there too, but just not sure what path to take. Good practical advice, pros and cons much appreciated.

 

[DevilDamo]

Will it take a while before BCOs are insisting on 0.18 compliance, or will they enforce the new value immediately?

It depends when your BR application was deposited and registered, before or on/after 1st October 2010?

The majority of BCO's will know what products achieve the required u-values and if they don't, it's easy to check with the manufacturer so if you can, provide backup documentation in the form of u-value calculations from the manufacturer. I always use/specify Celotex and I always use their online u-value calculator. I'm sure Kingspan will have something similar.

There's not a lot of difference in achieve 0.18 as opposed to 0.20. Similarly to achieving 0.28 as opposed to 0.30 through a cavity wall. The insulation may be thicker by 5mm, or something. I'd have to run a calculation through the website to check.

What depth and spacings are the rafters?

 

[FrogParty]

BR app not done yet, so 0.18 applies then. But will applications with Super 10 multifoil only be rejected? Seems that they should be even if contractors say they go through OK. Would be nice if multifoil was that effective though, with all the room height saved.

Roof 100x50 rafters, 400 spacing, 1F felt (non-breathable), eave ventilated.

Playing with the Celotex calculator offered solution
8} 50mm gap, 130mm XR4000 between rafters and 38mm PL4000 overlay. [218mm total thickness]

But I also wondered if I could avoid thickening out the rafters by using just 50mm between rafters and a thicker overlay. An 80mm layer would give 0.18, [218mm total inc. plasterboard, same as above] but should be able to have less PIR with something coming from final batons and plasterboard. But then how to fix such a thick layer under the rafters? I guess this is why thicker between and thin across is the norm.

 

[geraint]

i was under the impression that the new regs are for new builds only at this time.. but will like to know either way...

 

[FrogParty]

i was under the impression that the new regs are for new builds only at this time..

The document reads pretty clearly to me:

"New regulations come into force from 1st October 2010 requiring that newly constructed thermal elements (walls, roofs and floors) that are part of an extension to a dwelling should meet the standards set out in the table below. This includes those constructed as replacements of existing elements."
...
"An existing thermal element that is part of a building subject to a material change of use should be upgraded if the U-value is worse than the threshold value in column (a) below to at least the value in column (b) always provided this is technically, functionally and economically feasible."

And in both cases the tables show 0.18 for pitched roof rafters.

When adding a dormer I'm not sure if the unaltered parts of the pitched roof are exempt from the "replacing an element" clause, but they are surely caught by the "material change" of making a room up there. So either way it is 0.18 required in all new BR applications, and any previously granted approval (for 0.20) is only valid until Oct 2011.

What are others going to do to practically satisfy this requirement?

 

[joinerjohn]

The document reads pretty clearly to me:

"New regulations come into force from 1st October 2010 requiring that newly constructed thermal elements (walls, roofs and floors) that are part of an extension to a dwelling should meet the standards set out in the table below. This includes those constructed as replacements of existing elements."
...
"An existing thermal element that is part of a building subject to a material change of use should be upgraded if the U-value is worse than the threshold value in column (a) below to at least the value in column (b) always provided this is technically, functionally and economically feasible."

There is a get out clause there. I've highlighted it for you.