re-roofing my 1897 mid-terrace+improve thermal element to 2022 regs

[Neverfinished]

Roof was renewed in the 70's to a cement tile in an all-round poor quality job, now needs doing sympathetically, & back to slate (though possibly with some IN ROOF solar on back elevations).

1. Its a cold roof, see pic 1, showing SE & NE elevations. A main issue is - to up-grade its thermal element extra insulation on interior side of (original) attic room of greater than 40-50mm would be impractical due to already restricted room height & width, see pic 2.

So, I'd 'try' adding insulation above rafters, complemented by the PIR between rafters, (& then makes it a hybrid roof, as Redbeard reminded me on here). So I see raising roof height (by say 50mm) basically being essential, to get to regs, & being 'engineered' by way of a counter-batten etc; but its also necessary my roof 'integrates' with my neighbour to the left side (using a bonding gutter of some comprehensive design I guess, though my B.C. officer says such a bonding gutter, & perhaps additional lead flashing(s), no big deal!)

 

I'm still going to struggle to get to a 0.16 U-value however much insulation I can get in/up there but see little other option.

(Redbeard also mentions a 2-thirds above/ 1-third below-rafter rule of thumb for insulation to avoid the 'interstatial' enemy).

 

Q) Has anyone else any thoughts / suggestions on my meeting my 0.16 U-value for insulation requirements or even approaching - B.C. look sympathetically at draughty old properties . . they say.

 

2. I include below a link showing how TLX (gold, a multifoil) suggest their product be used above rafter in build-up of roof thermal element -including a counter-batten, stated as 38mm (old regs?, presumably now 50mm?), & which also provides ventilation on cold-side of roof. And this would raise my roof +/- 50mm). See its detailing & spec. here:-

 

https://tlxinsulation.co.uk/wp-content/uploads/2021/08/TLX-Gold-Counterbattened-plus-50mm-PIR.pdf

 

3. It says, in this link, that TLX multifoil used to build-up thermal element with this product, including air gap/s, above rafter, followed by 50mm PIR/phenolic within rafter, has a U value of 0.34.

So, if I were to indeed add a further 40-50mm max. PIR (thermal conductivity .022 W/mK) to the inside, under plasterboard . . would this 'build-up' get any where near the 0.16 requirement? (its beyond me to calc. this myself!).

a) Anyone else care to comment further on my thoughs on this insulation detail?

 

b)also what do you/others REALLY THINK about these multifoils - do they actually work (as long as fitting + air gaps adhered to) . . or not, & actually meet U values claimed?

c) Also I'd be interested to hear from anyone about design/detail of a succesful bonding gutter - my roof will be in ~10mm slate but raised say 50mm as explained :  meeting neighbour's cement tile.

                                                                                                      Many thanks, Stephen, Leicester area.

[Peaklander]

I have done a lot of research in preparing for mine. It’s a similar approach and I too will go for 0.16. Are you still looking in here? If so I’ll post-up what we are going to do.

[Mike]

On 14/08/2024 at 15:23, Neverfinished said:

also what do you/others REALLY THINK about these multifoils - do they actually work (as long as fitting + air gaps adhered to) . . or not, & actually meet U values claimed?

I've not kept track of all the arguments, but my impression is that (most of?) those that are still around are likely to be effective if installed properly. Personally I'd only consider one if there are no other viable options, if it had a BBA certificate, and as a top-up, not alone.

 

In France, the recommendation is normally to use the multifoil on the inside, as a top-up below traditional insulation, to reduce the condensation risk. In contrast, the TLX-Gold information that you linked to has it below the tiles, above other insulation, though it does says that there must be a 'well sealed ceiling' to avoid condensation risk. Since making a ceiling vapour tight (and keeping it that way for the next few decades) is not easy, personally I'd use it beneath. Another argument against using it directly under the tiles is that it may get coated with dust, reducing it's effectiveness.

 

On 14/08/2024 at 15:23, Neverfinished said:

I'd be interested to hear from anyone about design/detail of a succesful bonding gutter

Not done it, but rather than a bonding gutter you'd need to create a secret gutter at the lower level, create an abutment with timber with a flashing over the top, and end your tiles with a regular verge.