Insulating under rafters, service battens & plasterboard

[Eric]

I am at the stage of fitting insulation under the rafters, vcl, 45mm service battens & plasterboard.

 

Is there any particular way the insulation under the rafters should be fitted? Is it better to stagger or is it irrelevant due to service battens?

 

Do the ends of the sheets need to be joined on a rafter or are they ok just on the insulation between the rafters?

 

What is the best spacing for the service battens? 600mm seems quite a bit. The height from the top of the wall to where the ceiling will be is 1280mm.

 

Any photos of previous work or suggestions would be appreciated.

[Redbeard]

Can I ask what the pic above shows? Is it the PIR between the rafters, with tape entirely covering the rafters?

 

I did 20-25 Victorian attic re-fits over the years and they followed a similar pattern:

Fit insulation between rafters. BEIS Best Practice now suggests 50mm vent gap as compared to typical 25mm hitherto.

 

So for typical 75mm rafters, 25mm PIR between rafters. Rafters carefully marked at top and bottom. Offer up 1200 x 2400 x 100mm PIR boards and fix temporarily - 3 or 4 fixings max - with long screws using a piece of slate lath as a 'washer'. Tape all joints and perimeters as VCL or, if desired, add a sheet VCL. Then offer up and fit 25 x 50 slate lath (that's what I used). You could take the view that a hole with a screw in it is not a hole at all, or take the more pessimistic view and pre-drill holes and 'gun' with silicone to form a 'grommet'.

Then I would add 25 PIR between the battens if I did not want a service void. If I did, perhaps 25 PIR and a 25mm void would work. (??)

No time for pics tonight but I will try to find some tomorrow and post them up.

 

I rather more favour wood-fibre now if headroom allows. (It wouldn't have done in most Victorian attics I did). I'll post pics of that too.

 

 

[Eric]

The photo has 170mm PIR in between the rafters, rafters not visible for the tape.

[Iceverge]

Tape the PIR together with foil tape and return it to the walls with tape or airtight sealant to ensure no vapour from the room can migrate into the roof. 

 

Then run 50*50mm battens at 90deg to the rafters for a service void. 400mm CC for 12.5mm plasterboard or 600mm CC for 15mm plasterboard 

 

Run your wires as necessary. (They may need to be uprated, consult your sparky)

 

Infill with 50mm Rockwool batts. They will fit around the wires, provide better sound and fire protection and break the thermal bridge of the rafters. ( I would happily take this to 100mm Rockwool if you wanted to). It is important that you have a very comprehensive airtight layer returned without any holes to the internal walls though. 

 

Then 1 or 2 layers of plasterboard. Seal all penetrations and the perimeter with acoustic mastic for sound protection. 

 

Skim. 

[Eric]

33 minutes ago, Iceverge said:

Tape the PIR together with foil tape and return it to the walls with tape or airtight sealant to ensure no vapour from the room can migrate into the roof. 

 

Then run 50*50mm battens at 90deg to the rafters for a service void. 400mm CC for 12.5mm plasterboard or 600mm CC for 15mm plasterboard 

 

Run your wires as necessary. (They may need to be uprated, consult your sparky)

 

Infill with 50mm Rockwool batts. They will fit around the wires, provide better sound and fire protection and break the thermal bridge of the rafters. ( I would happily take this to 100mm Rockwool if you wanted to). It is important that you have a very comprehensive airtight layer returned without any holes to the internal walls though. 

 

Then 1 or 2 layers of plasterboard. Seal all penetrations and the perimeter with acoustic mastic for sound protection. 

 

Skim. 

Thanks for the info, I am curious as to how I should be fitting the PIR under the rafters though. 
Is there any specific way the sheets should be cut/positioned to line up with rafters etc.

 

It is 15mm plasterboard.

[Iceverge]

36 minutes ago, Eric said:

Thanks for the info, I am curious as to how I should be fitting the PIR under the rafters though. 
Is there any specific way the sheets should be cut/positioned to line up with rafters etc.

 

It is 15mm plasterboard.

 

I think an insulated service cavity with Rockwool comes with lots of advantages over PIR as mentioned. 

 

If you do use PIR Any which way is fine. Less cutting and joints is better. Just use a couple of woodscrews with insulation washers to hold them in place temporarily. Foam and tape the joints. Make sure to mark the rafters lines with a Sharpie on the boards. 

 

Then use the battens screwed through to the rafters to seal the deal. 

 

25*50mm battens will be a pain though. Not very straight and will split easily. 

 

50*50 or 25x75 will be better. 

 

600cc is fine for 15mm. 

 

 

 

[Eric]

I already have the PIR so will use it.

Any advantage fitting rockwool in the service void on top of the PIR & VCL?

[Iceverge]

More insulation is always better. 

 

Better fire and noise protection.