Insulating 45 degree internal ceiling returns in loft space

[jc212]

Looking for advice again from you good folks if it's not too much trouble, this time it's in my loft. 

 

For whatever reason the architects thought it would be a good idea to have 45 degree internal plasterboard around the outside walls of the house. See attached photo for what I mean. Only issue with this - there is zero insulation on the back of the plasterboard and these points sit right in front of the soffit ventilation ports. 

 

We put vertical radiators in (not my decision) so literally radiating heat against a surface that's being cooled at high velocity in the loft with fresh air!

 

I am proposing cutting 100mm (or 75mm) PIR and placing this between the rafters in contact with as much of the plasterboard as possible. I would be sure to allow sufficient space 25-40mm in front of the pir so air can circulate freely through the soffit vents.

 

Questions are:

 

What's the best way to detail this

Should I friction fit, or use some kind of adhesive 

Should I foam around the edges in contact with the rafters and then also add foam in the 45 degree gap that will be formed between the PIR and vertical plasterboard

Any other thoughts or suggestions 

[Big Jimbo]

One of the reasons why i hate those slopes when i see them in any property. You are going to have to try and get some insulation in there, while maintaining airflow under the tiles if you have a cold roof.

[Mattg4321]

gapotape looks great. expensive though. Needs foaming in at very least imo. Airtight.

[jc212]

Not seen that, looks absolutely perfect. Id be struggling to reach down to foam in the gap between plasterboard coming up from the room, sounds as though applying this it would expand and fill it. But in principle this would make sense putting PIR here? 

[Iceverge]

72.5mm insulated plasterboard chamfered top and bottom and screwed through the existing plasterboard into the rafters. 

 

Foam either a good low expansion foam, tape and skim the joints to wall and ceiling. 

 

Don't bother taking down the existing plasterboard. No space gain in the room and will create an enormous mess.

Edited 18 hours ago by Iceverge

[jc212]

I was planning on leaving the inside finish as is (would prefer not to reskim etc). Preference would be to insulate in the loft space 

[Redbeard]

12 hours ago, jc212 said:

I was planning on leaving the inside finish as is (would prefer not to reskim etc). Preference would be to insulate in the loft space 

 

All my thoughts as I read the posts so far pre-supposed that you would be dropping the plasterboard and working from below.

 

On the basis that you can't see what you don't see I would not like to guarantee how well you can seal the joints. Variables include rafter pairs which are not a completely consistent distance apart, rafters 'on the twist' and so on. I have never seen a sloping soffit insulation job done 'from the apex void' where it has been completely tight. Even when access is available it's not necessarily wonderful. Many rooms-in-the-roof insulated from above when re-roofing are testament to this. 

 

For an excellent job I would kit up with PIR,(if that is your material of choice - others are available - air-tightness tape and a/t primer, bite the bullet, drop the ceilings (sorry!), clear up the mess and insulate the slope leaving a minimum 50mm ventilation gap below the felt. 0versail the sloping insulation past the flat 'loft floor' and carefully detail the junction between PIR and 'fluff'. Are those 150 rafters?

[ProDave]

You are never going to do a perfect job sliding it down from above, but if you don't want to strip the plasterboard off it is a case of doing the best you can, accepting it won't be perfect.

 

I would try solid batts of rockwool type material.  they should be stiff enough to push down the gap but with a bit of flex to hopefully take up the irregularities.  Buy a small quantity and try it, if they work buy the rest.

 

PIR is so solid with no give, you will end up with gaps around all the edges.  If you use that, the most important thing is get the face of the PIR down flat on the plasterboard,  i,e, no gap between the plasterboard and the PIR,  then foam in as much of the edges as you can reach from above.  Buy some thin plastic tube and experiment using that to extend the reach of your average foam gun.

[saveasteading]

4 minutes ago, ProDave said:

solid batts of rockwool type

Agreed.

It's better to have a good job with medium insulation properties than gaps in a better insulator.

You could perhaps maintain the 25mm air gap by first inserting a board to the roof slope, then removing it.

Rockwool type material should be the waterproof variety in case there is ever a leak, as soggy wool would be a bit problem.

 

 

[FuerteStu]

1 hour ago, ProDave said:

 

PIR is so solid with no give, you will end up with gaps around all the edges.  If you use that, the most important thing is get the face of the PIR down flat on the plasterboard,  i,e, no gap between the plasterboard and the PIR,  then foam in as much of the edges as you can reach from above.  Buy some thin plastic tube and experiment using that to extend the reach of your average foam gun.

 

I had an awkward spot to do this and used earth sleeving (16mm cable size) over the nozzle. Worked surprisingly well. Releasing the foam slowly and steadily as i pulled it out. 

 

 

[Mr Punter]

To do this properly, strip the plasterboard, insulation between the rafters to leave a 50mm gap beneath the felt, 75mm insulation under the rafters, plasterboard.  A bit of a pain but it means you maintain good loft ventilation and prevent associated mould / damp risk.