PIR board in rafters

Moggaman

Hi.

I have 7x2 rafters in my new build. Room in roof scenario.

Ahead of insulating these rafters I want to analyse which is the best method or end result…1. A flexible insulation which fully fills the rafter or 2. A 125mm PIR board with 50mm ventilation gap.

i have insulated plasterboard going on the rafters also.

The benefits of the flexible stuff is that it is quite easy to friction fit. But my worry is that it’s conductivity is higher.

I don’t know how to accurately fit PIR board.. do u cut it 5mm less wide and foam the gap or cut it accurately and hope it fits with no room for foam

My DEAP does show 180mm full fill rafters. Is there any issue with full fill in terms of ventilation?

thanks for reading this and I appreciate your responses.

Biker1

https://www.boards.ie/discussion/2058208767/pir-board-in-rafters

Regardless whether it's PIR or a mineral wool insulation you will require the ventilation space above it unless the outer roof was counter battened and over fascia ventilator used, only then can you full fill the space. What did your BER assessor specify?

Moggaman

Yes in fairness the detail has double battening from the BER assessor.

Dudda

If you've counter battens then you don't need the ventilation and can full fill the rafters. It is slow and time consuming to get the insulation to fit tight between the rafters. Is this something you intend to do or the builder? You'd probably do a bit of a better job yourself as you'd take more time and be more particular. A builder would be more inclined to just throw it up. If it's the builder I'd probably use the flexible stuff and try and have thicker insulation to offset the lower performance of the flexible insulation. By the sounds of it you're not planning a service void for cables or do you intend to have any Mechanical Ventilation Heat Recovery (MVHR).

Moggaman

https://www.boards.ie/discussion/comment/117893798#Comment_117893798

I am planning to do it myself. Sspoke to roofer this morning. I reckon the additional counter battening could cost me 1k so I will go 125mm PIR with 50mm air gap. No service void but inten on bringing any electrics down from the flat ceiling or up from the floor.

he also mentioned PIR board with cuts in it (accordion like).. u cut the board 5mm strong and squeeze it in. Sounds ok to me.

Dudda

The the accordion insulation is what I'd have got the builder to fit. That's the flexible stuff I was on about not fibreglass. The accordion insulation is not as good as PIR. If you are doing it yourself I'd take my time and fit the PIR. You'll get good at it after a few joists. You could purchase a few rolls of foil tape and tape over the joists basically joining both PIR insulation boards. That would be a good finished job then.

Best of luck with it whatever you end up doing.

Moggaman

https://www.boards.ie/discussion/comment/117897163#Comment_117897163

Thanks . Is there a name for the foil tape?

Biker1

https://www.boards.ie/discussion/comment/117901074#Comment_117901074

As you should be installing an airtight membrane there is no advantage to taping this way.

Moggaman

https://www.boards.ie/discussion/comment/117901220#Comment_117901220

Yes Airtightness membrane going in

SC024

https://www.boards.ie/discussion/comment/117895712#Comment_117895712

cut it tight & then get a lump of timber & hammer & tap it in.

Moggaman

I’m about half way through. It’s a dog of a job. Bought myself a good mask.. lots of particles floating about…

it now looks like I need to put in a sun tunnel to bring a bit more light to the upstairs hall. I will be insulating the tube up to the slates but how are these sun tunnels dealt with In terms of Airtightness… thanks

GreeBo

https://www.boards.ie/discussion/comment/118082481#Comment_118082481

You'll need to tape the tube to the PIR insulation but I'm not sure how you are going to stop the cold bridging, it will all depend on the construction of the dome part of the sun tunnel. The velux ones for example are behind a glazed window so will be better than some crappy 2mm plastic dome.