Insulating using PIR between joists/rafters

[purplemonkey]

hi,

I am trying to rebuild a building and I'm getting to the insulation stage. I notice the kingspan u value calculator offers lots of solutions, but all of them come with insulated plasterboard as a final covering. Is it not possible to simply insulate between rafters/joists and finish with normal 12.5mm plasterboard nowadays?

For my pitched roof, we used 6" rafters with breathable membrane so we can fit quite thick insulation but using the u value calculator it always shows above or below additional insulation.

Same with the walls. We have solid brick walls and the house is small and thin so we really want to maximise internal space. but all solutions shows this additional layer of plasterboard with insulation attached.

I also have a really brain freeze when trying to work out how to work out what u value will be achieved so I'm really at the mercy of this kingspan tool.

thanks in advance.

 

[tony1851]

If you put a decent (min 100) thickness between the rafters, you should just about comply. However,
over time, you may notice the lines of the rafters in the ceiling below, caused by the large difference in insulation value
between the timber rafter and insulation (the timber will be very slightly cooler than the insulation, and so will attract dirt more -
known as 'pattern staining').
A 25mm thickness of Kingspan fixed across the underside of the rafters (and joints taped) finished with 9.5 plasterboard would help to prevent this; and probably cheaper than insulated plasterboard.

 

[purplemonkey]

Thanks for your reply. do you know if there is a calculator I can use to check the Uvalue based on 100 or 120mm PIR in between?

Yes I think you're right about separate kingspan and plasterboard being cheaper, and also a damn sight easier to cut and prepare!
What would be a good method to fix these independently? Just screw one up first then the other?

 

[freddiemercurystwin]

Most people use the Celotex online calculator, it's a bit clunky but you can assess most typical arrangements, for all intents and purposes all PIR insulation types perform pretty similarly and so 100mm of Celotex is near as dammit the same as 100mm of kingspan.

 

[purplemonkey]

Ah yes that calculator is a lot better than kingspan, it allows a lot of freedom of choices. Unfortunately it appears the only way to hit the U value target (0.18) between rafters is with 200mm PIR (gives 0.17).
So looks like I'll have to be doing the plasterboard between and under.

 

[tony1851]

Ah yes that calculator is a lot better than kingspan, it allows a lot of freedom of choices. Unfortunately it appears the only way to hit the U value target (0.18) between rafters is with 200mm PIR (gives 0.17).
So looks like I'll have to be doing the plasterboard between and under.

Surely you don't need 200mm??

 

[phatboy]

Unfortunately it appears the only way to hit the U value target (0.18) between rafters is with 200mm PIR (gives 0.17).

Wait what?! Please share a screenshot of your results, that seems way off!