Roof build up on top of PIR

[Buzz]

I need some advice on the build up for our roof , we have PIR on top of the rafters with fluffy stuff in-between but what is the order on top of the PIR ?  our local BC has a easy to read guide for cold roof and warm roof build up but nothing for a half and half , the way i understand it would be to go counter batten ,"felt" ,tile batten , tiles ? 

Also does it need to be ventilated ? 

I will talk to the BC when next on site ( they are very approachable ) regarding this but would like to understand a bit more before i have that conversation . 

[Dave Jones]

if you havent got a warm roof then you need to ventilate the space really as it will get condensation inside. Even with a very good vapour barrier like alutrix if you pull the cold inside the building then a dewpoint will form inside and it will sweat.

 

Have you got a restriction on height as making it a warm roof is much much easier and eliminates chances of mould/sweating etc.

 

 

[Buzz]

1 hour ago, Dave Jones said:

Have you got a restriction on height as making it a warm roof is much much easier and eliminates chances of mould/sweating etc.

 

No restriction on height and the pitch is 30 degrees so nothing to steep to work off  , but to go for a full warm roof I would need to add another 330m2 of 100mm PIR and at todays prices is just a step to far . 

So it will need to be ventilated which is not a problem  .

[Iceverge]

Tile 

25mm batten

25mm counterbatten fixed with long screws through to rafters

roofing membrane.

pir tightly fitted and ideally taped at all joints. 

osb

rafters with mineral wool batts between.

plasterboard.

 

You need to avoid allowing interior air coming contacting a cold surface that is also impermeable. in your case you have insulation outboard of your osb keeping it warm so you’re ok provided you don’t put too much insulation inboard of it between the rafters. 

 

what is your target u value and what is the thickness of your rafters?

 

 

[Buzz]

17 hours ago, Iceverge said:

what is your target u value and what is the thickness of your rafters?

I need to hit .14 as that was what has been passed by BC on the SAP  but the original rafters were going to be 225 x 50 but after a re-design from the SE they are down to 50 x 175 , just need to understand what i am going to achieve and how it goes together before i start to build and insulate the roof  

[Temp]

As I recall there is a rule of thumb that to avoid condensation you should have at least 2/3rds of the insulation above the rafters and no more than 1/3rd between. That's 2/3rds of the Thermal resistance not thickness.

 

I believe that can be done easily with something like 150mm PIR above and 100mm Rickwool between but I rushed the sums.

 

Have any thicknesses already been specified ?

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Edited August 21, 2021 by Temp

[Buzz]

7 minutes ago, Temp said:

Have any thicknesses already been specified ?

Yes the original speck was 50mm PIR on top and 225 of multi roll in-between. 

Beginning to think it would be so much simpler to full fill the rafters with wool and put PIR on the inside .

[A_L]

1 hour ago, Buzz said:

Beginning to think it would be so much simpler to full fill the rafters with wool and put PIR on the inside .

 

And much safer in regard of interstitial condensation with the higher vapour resistance layer inside the lower. 175mm of 0.035 mineral wool between timbers and 75mm of PIR below will get 0.13W/m.K

[Buzz]

 

36 minutes ago, A_L said:

And much safer in regard of interstitial condensation with the higher vapour resistance layer inside the lower. 175mm of 0.035 mineral wool between timbers and 75mm of PIR below will get 0.13W/m.K

Thanks for taking the time to reply , as the roof has only been re drawn this week i think its time to put all options back on the table ?

[Iceverge]

Get rid of the Pir. too expensive, messy and wastful.

 

go back to 225mm rafters. Add a proper vapour barrier and a 50mm service cavity.

glasswool is cheapest but i’d prefer blown cellulose.

 

edit: i’ve forgotten the roofing membrane above the osb.

 

Edited August 21, 2021 by Iceverge