U value calculation, help!!!!

[davidward89]

Who can make head or tale of this then?! I'm sure it's just me being thick but I'm not understanding the "timber rafters with spacing" sections and how this applies practically.

Its a calculation for insulating the pitched roof space in my house, ready to make them in to usable rooms. Roof construction is tiled, breather membrane. I spent ages considering sheepswool and hemp batts etx but unfortunately it purely comes down to price and head space!!

My rafters are 75mm deep.

Any help would be great, especially drawings for a simpleton like myself!!

 

[davidward89]

My understanding is that it's just stating that my rafters add to the thermal value and it's split the 75mm up in to 25mm and 50mm, as opposed to this meaning I need to do some sort of extra battening?!

 

[Cherryburn]

You can't fill all of the space between the rafters with insulation, you need an air gap behind the insulation to allow any water vapour that gets through to escape, and to make sure that your roofing membrane can continue to "droop" slightly between the rafters so that any water on the outside doesn't get stuck around your roofing battens and start to rot them away. So the 75mm isn't "split up" so much as you have a 25mm air gap, 50mm of insulation between the joists and then another 80mm below the joists.

Practically it can be a good idea to cut some spacers to make sure you maintain the air gap, but as long as the boards are flush with the bottom of the rafters it should be fine. The tricky part is cutting them so that they're a tight fit, and then filling any gaps where you can't. Then your 80mm layer goes below, and you tape the joints of those boards to make a continuous vapour control layer. The main principle of insulation systems like this is that you want to make it hard for water vapour to enter the system, and easy for it to escape. The better you stick to that principle, the less likely you are to have any issues.

 

[a twig]

I would agree with the above - pointing out that generally to achieve modern building regs requires 150mm of insulation, typically assuming 125 mm rafters with 100mm of Celotex between the rafters and 50mm of Celotex below.

So if you only have 75mm rafters then you’d be looking at 50mm of Celotex between the rafters and then 80-100mm (product dependent) below to meet the requisite u values

You can play with the numbers here

https://u-valuecalculator.com/gb/calculation/PR3_1

 

[davidward89]

You can't fill all of the space between the rafters with insulation, you need an air gap behind the insulation to allow any water vapour that gets through to escape, and to make sure that your roofing membrane can continue to "droop" slightly between the rafters so that any water on the outside doesn't get stuck around your roofing battens and start to rot them away. So the 75mm isn't "split up" so much as you have a 25mm air gap, 50mm of insulation between the joists and then another 80mm below the joists.

Practically it can be a good idea to cut some spacers to make sure you maintain the air gap, but as long as the boards are flush with the bottom of the rafters it should be fine. The tricky part is cutting them so that they're a tight fit, and then filling any gaps where you can't. Then your 80mm layer goes below, and you tape the joints of those boards to make a continuous vapour control layer. The main principle of insulation systems like this is that you want to make it hard for water vapour to enter the system, and easy for it to escape. The better you stick to that principle, the less likely you are to have any issues.

Thank you, I fully understand all this, it was literally the two boxes that I've now realised are referring to the fact that the timber rafters themselves are adding to the thermal value, these being split up in to 25mm and 50mm on the table above - the 25mm giving 0.1923 and the 50mm giving 0.3846 thermal resistance.
It's all good

 

[davidward89]

I would agree with the above - pointing out that generally to achieve modern building regs requires 150mm of insulation, typically assuming 125 mm rafters with 100mm of Celotex between the rafters and 50mm of Celotex below.

So if you only have 75mm rafters then you’d be looking at 50mm of Celotex between the rafters and then 80-100mm (product dependent) below to meet the requisite u values

You can play with the numbers here

https://u-valuecalculator.com/gb/calculation/PR3_1

And a twig thanks. The calculation was worked out with me inputting how much I could put between the rafters (50mm) and then inputting my required u value and it calculated the 80mm underneath. And building control have approved this so good to go .
Thanks for your reply

 

[paulc]

Here is an interesting, although somewhat expensive product to put along the edges of Celotex type boards - http://www.gapogroup.com/
Doesn't appear to be available in 50mm widths, but there are alternative products available. Personally, when I've needed thick layers of Celotex, I've built them up from 25mm sheets, foaming & taping the joints and staggering them.

 

[mh12]

https://historicengland.org.uk/images-books/publications/eehb-insulating-pitched-roofs-ceiling-level-cold-roofs/heag077-cold-roofs/

The above is a very useful insight to insulating historic building at roof/ceiling level.
David, you've mentioned about having looked at natural insulation methods at and below rafter level but the cost differential was not favourable hence you've decided to go down the PIR route.
I don't know whether the risk of interstitial condensation is less of an issue with non breathable insulation materials at ceiling/sloped roof level than it is with solid wall insulation.
None the less I suppose it is an importance consideration.
From my limited understanding any insulating fitted between the rafters ideally should be flex type in order to achieve snug fit and avoid thermal bridging/cold spot unless the spec between the rafters is symmetrical horizontally and vertically in which case you can fit the cellotex block PIR board.
Are you thinking of having some sort of VCL between the gyproc boards and the cellotex to limit water vapour travelling through?
Insulating older buildings-- so many considerations, makes the headspin. Historic England take on the subject is breathability of insulation is very importance consideration.