Roof insulation / air gap design

[Pappa]

Hi Guys

 

 

My Structural Engineer has suggested this for me for my 31 degree pitched roof, attic conversion

Let me know your thoughts please, in particular about the air gap - He said as long as the roofing membrane is not touching the PIR, its fine.

Rafter dimensions is 74 x 46 mm @ 430 centers.

 

Thanks in advance

 

P

[ETC]

Which way is this drawn?

If the rafters are shown on a slope you will need continuous ventilation from the low to high end.

Plus 50mm gap is normal.

[Pappa]

1 hour ago, ETC said:

Which way is this drawn?

If the rafters are shown on a slope you will need continuous ventilation from the low to high end.

Plus 50mm gap is normal.

Apologies. I got carried away with the drawing.
This is the corrected diagram - side view, as though looking down from the ridge beam down to the eaves:

 

 

 

Are there minimums for the air gap?

Head height is at a premium in the attic so Structural engineer is trying to save space where he can.

 

Thanks

 

P

[Redbeard]

15 minutes ago, Pappa said:

Are there minimums for the air gap?

 

Yes:

 

https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf

 

Proctors used to do (may still do?) a membrane which claimed to function with no air gap. I have sadly seen numerous examples where serious condensation and mould have occurred with a nominal 25mm air gap (OK, so poor installation and possible insufficient size of eaves vents may also have been contributing factors. For 25mm extra loss of headroom I honestly no longer think it is worth the risk. If headroom is a particular issue on stairs (re BR req'ts) consider sacrificing 25mm of insulation in that area only.

[ETC]

4 hours ago, Pappa said:

Apologies. I got carried away with the drawing.
This is the corrected diagram - side view, as though looking down from the ridge beam down to the eaves:

 

 

 

Are there minimums for the air gap?

Head height is at a premium in the attic so Structural engineer is trying to save space where he can.

 

Thanks

 

P

Grand. 50mm air gap (or as near as possible) needed between the insulation and underlay. There are some underlays which say you don’t need ventilation but they do have some caveats.

[Iceverge]

PIR between timbers. I'm not a fan. 

 

I'd you really must go for the thinnest option nothing comes close to insulated metal panels. You can get them in tile effect.

 

https://www.insulatedpanelstore.co.uk/rt45mm

 

80mm and a 50mm insulated plasterboard and you'd have your U value of 0.16w/m2K done and dusted in less than 145mm all in.

 

 

 

 

[Iceverge]

 

You could sub in k118 kooltherm plasterboard and probably shave another few mm if you were really in the mood for setting fire to money. 

[Pappa]

1 hour ago, Iceverge said:

I'd you really must go for the thinnest option nothing comes close to insulated metal panels. You can get them in tile effect.

 

Good shout, will definitely keep that in mind for future projects.

For now, I've already got the roof structure in place and its a conservation area so I have to use rickety old roof slates.

 

I'll take your advice @ETC and @Redbeard and keep the 50mm (49mm) gap.

Please can you sense check this:

 

 

This gives U Value of 0.176

Using 0.022W/mK PIR.
 

Thanks in advance

 

P

[Thorfun]

You can put the air gap above the rafters using certain membranes and fully fill the rafters. 

[ADLIan]

If you fully fill the rafter with insulation you then need to counter-batten and batten for the tiles/slates.

[Pappa]

2 hours ago, Thorfun said:

You can put the air gap above the rafters using certain membranes and fully fill the rafters. 

 

15 minutes ago, ADLIan said:

If you fully fill the rafter with insulation you then need to counter-batten and batten for the tiles/slates.

 

Thanks guys

Any work I'm going to do will be from inside only and I don't fancy disturbing the existing membrane/rafters/slates as the roof has only been renovated in the last few years by previous owner.

 

I'll stick with the air gap and the above design.

 

Thanks again

 

P

 

[Iceverge]

On 30/04/2025 at 15:38, Pappa said:

 

Good shout, will definitely keep that in mind for future projects.

For now, I've already got the roof structure in place and its a conservation area so I have to use rickety old roof slates.

 

I'll take your advice @ETC and @Redbeard and keep the 50mm (49mm) gap.

Please can you sense check this:

 

 

This gives U Value of 0.176

Using 0.022W/mK PIR.
 

Thanks in advance

 

P

 

How're you planning on fixing the plasterboard? 

[Pappa]

9 hours ago, Iceverge said:

 

How're you planning on fixing the plasterboard? 

Screw through the insulation in to the rafters using plastic insulation fixings to minimise thermal bridging.