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Cold roof: Insulation, ventilation and BR u-values


oranjeboom
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Just about to order some more PIR having sorted out my plan of attack.

 

Half of this pitched roof is going to be 'vaulted' (i.e. Just one pitch of the roof with internal wall coming up right up to the ridge). So planning to fill between the 75mm (yes 75mm!) rafters with 25mm PIR and then a further 200mm PIR under the rafters. That leaves me with a 50mm ventilation gap.

 

It's a slated roof with battens (not countered or any sarking) but have Tyvek breather membrane which I think means I could even reduce ventilation gap to 25mm?

 

Build Reg Table 2 seems to say that roof u-value  should be 0.18:  https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/540327/BR__PDF__AD_L1B_2013_with_2016_amendments.pdf

 

This is a retrofit rather than new build but I want to get as good as I can get u-values and not simply satisfy BR! I've read so many conflicting roof values ranging from 0.11 to 0.2! Just need a sanity check that I'm on the right track here.

 

Celotex calculator tells me I will end up with around 0.1 so with my EWI and 3G windows I am hoping i am on the right path. Everyone's specs are different but with BR values being pretty dire it's difficult to guage what a decent u-value would be! Should I try and improve on 0.1? I may be able to add a further 50mm but would mean fitting the plasterboard becomes a bit more cumbersome (Would probably have to fit battens underneath PIR and then fix PB to that).

 

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4 minutes ago, oranjeboom said:

Just about to order some more PIR having sorted out my plan of attack.

 

Half of this pitched roof is going to be 'vaulted' (i.e. Just one pitch of the roof with internal wall coming up right up to the ridge). So planning to fill between the 75mm (yes 75mm!) rafters with 25mm PIR and then a further 200mm PIR under the rafters. That leaves me with a 50mm ventilation gap.

 

It's a slated roof with battens (not countered or any sarking) but have Tyvek breather membrane which I think means I could even reduce ventilation gap to 25mm?

 

Build Reg Table 2 seems to say that roof u-value  should be 0.18:  https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/540327/BR__PDF__AD_L1B_2013_with_2016_amendments.pdf

 

This is a retrofit rather than new build but I want to get as good as I can get u-values and not simply satisfy BR! I've read so many conflicting roof values ranging from 0.11 to 0.2! Just need a sanity check that I'm on the right track here.

 

Celotex calculator tells me I will end up with around 0.1 so with my EWI and 3G windows I am hoping i am on the right path. Everyone's specs are different but with BR values being pretty dire it's difficult to guage what a decent u-value would be! Should I try and improve on 0.1? I may be able to add a further 50mm but would mean fitting the plasterboard becomes a bit more cumbersome (Would probably have to fit battens underneath PIR and then fix PB to that).

 

 

Just a note that Seconds & Co seem to be well-stocked with PIR at present.

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When you get down to U=0.1 I think it's much, much more important you work out the detailing right. I'm thinking particularly how the continuation of insulation and airtightness works at eaves level, but also how well the insulation is installed.

 

It's easy to come up with an authoritative U - work out the R for the rafter section, allowing for a %age reduction for the timber, then add that to the underside insulation, then get the U.

 

Goes without saying and I guess you can't do it, but "proper" warm roof over the top would be better. Would 75mm rafters be beefy enough?

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75mm rafters have been beefy enough for the past 70yrs and BCO did not mention them. They supported heavy concrete tiles before but have replaced them with fibre cement slates, so quite a bit lighter.

 

I'll be doing the fitting myself as most of the contractors I've had in have simply been clueless/"well this is how i have always dunnit.." mentality. So I'll be making sure to get the detailing and fitting correct.

 

As for 2nds & Co, I sometimes find that some of their pricing is still a bit on the high side especially with this so-called PIR shortage. But I'll take another look, or maybe delay until tomorrow for all those wonderful Black Friday sales.¬¬

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2 hours ago, oranjeboom said:

 

Celotex calculator tells me I will end up with around 0.1 so with my EWI and 3G windows I am hoping i am on the right path. Everyone's specs are different but with BR values being pretty dire it's difficult to guage what a decent u-value would be! Should I try and improve on 0.1? I may be able to add a further 50mm but would mean fitting the plasterboard becomes a bit more cumbersome (Would probably have to fit battens underneath PIR and then fix PB to that).

 

 

- Improving a U = 0.1 construction with 50mm of PIR will save between 1 and 2 kWh per annum per m2  in a U.K. climate

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Just had a quote back from 2nds and co. They're actually £200 more than Insulation Giant. I know even the new boards are usually a bit warped etc, but curious how they price their used/2nds stuff. Probably pricing in the 'PIR shortage' thing....not sure what the actual lead time is with new stuff but hopefully sooner than the 5weeks that most suppliers are advertising still.

 

BTW, anyone seen any spectacular Black Friday prices on any PIR?? Rockwool seems to be 'reduced' on some sites.

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