Choice of insulation for under rafters on attic trusses.

[Chanmenie]

I have almost finished the insulation between the rafters of my chalet bungalow, which is made up of 2 layers of PIR 100mm and 75mm the rafters are 225mm so I have the 50mm air gap over the top.

I am undecided on what is best to use under the rafters, where the sloping ceilings are I will brace the trusses with 12mm ply then fit 30mm PIR across the rafters, my quandary is what to do in the loft voids due to the 4x1 bracing, should I continue with the 30mm PIR and cut and tape it around the bracing or add 1” battens to the underside of the rafters and use superfoil, if the latter I’m concerned about joining and taping the superfoil to the PIR where they meet between the uprights and over the truss cord, to achieve airtightness.

The truss designer has confirmed its ok to remove the diagonal 4x1 bracing where the sloping ceiling are and replace with 12mm structural ply, but that still leaves lots of bracing in the voids to work around.
 

[Thorfun]

30 minutes ago, Chanmenie said:

I have almost finished the insulation between the rafters of my chalet bungalow, which is made up of 2 layers of PIR 100mm and 75mm the rafters are 225mm so I have the 50mm air gap over the top.

I am undecided on what is best to use under the rafters, where the sloping ceilings are I will brace the trusses with 12mm ply then fit 30mm PIR across the rafters, my quandary is what to do in the loft voids due to the 4x1 bracing, should I continue with the 30mm PIR and cut and tape it around the bracing or add 1” battens to the underside of the rafters and use superfoil, if the latter I’m concerned about joining and taping the superfoil to the PIR where they meet between the uprights and over the truss cord, to achieve airtightness.

The truss designer has confirmed its ok to remove the diagonal 4x1 bracing where the sloping ceiling are and replace with 12mm structural ply, but that still leaves lots of bracing in the voids to work around.
 

what do you mean by 'loft voids'? if you're insulating between and under the trusses why do you need to insulate anywhere else? here's what our loft looked like after insulating.

 

 

we then fitted Intello Plus and service cavity battens in to the trusses. we don't plan to plasterboard the loft but the battens help hold the membrane in place.

[Chanmenie]

I am insulating from the ridge to the eves, so the voids either side and above the living space are insulated, 

The voids / space either side of the bedrooms can then be used for storage and means I won’t have to insulate the MVHR pipes 

[Thorfun]

But surely the walls are insulated so the insulation to the eaves just carries on down to the wall? 
I’m still confused as to why you need to do it. Surely insulating the trusses finishes the wrapping of the whole building in insulation?

[Chanmenie]

10 hours ago, Thorfun said:

But surely the walls are insulated so the insulation to the eaves just carries on down to the wall? 
I’m still confused as to why you need to do it. Surely insulating the trusses finishes the wrapping of the whole building in insulation?

Yeah as in my picture the insulation goes from ridge to eves, I don’t understand what’s confusing about it ? 
You have obviously use PIR under the rafters, and you have the loft voids either side. 

[Eric]

What does it say/show on your building warrant drawings?

I think I understand what you are saying but it would be better fully insulated with pir as Thorfuns photo above.

[Chanmenie]

9 minutes ago, Eric said:

What does it say/show on your building warrant drawings?

I think I understand what you are saying but it would be better fully insulated with pir as Thorfuns photo above.

What are building warrant drawings ? 
I did the building reg drawings, but I’d meet the regs without the additional layer underneath.

someone suggested it would be easier to use the superfoil than the PIR under the rafters but then he is an insulation salesman.

I think as you said, I’ll stick with PIR and cut and tape it around the bracing

[Eric]

I’m not sure if this attached photo will help show the levels of insulation (internet photo)

 

[Thorfun]

1 hour ago, Chanmenie said:

You have obviously use PIR under the rafters, and you have the loft voids either side. 

yeah. I used glass wool between the rafters and then put PIR below. I thought you said you were putting 30mm PIR underneath the rafters which is probably where my confusion is coming from!

 

On 02/12/2023 at 09:43, Chanmenie said:

where the sloping ceilings are I will brace the trusses with 12mm ply then fit 30mm PIR across the rafters

so I just presumed you were doing the same as we did which is why I questioned about filling the loft void!

 

[Roger440]

Well im glad im not the only one that completely confused as to what the OP is asking!

[Chanmenie]

Thanks for the responses guys, I think I caused some confusion calling the empty space either side of the uprights “loft voids” 

so unlike Eric’s diagram I am putting PIR (175mm) between rafter from the ridge to the eves so the entire roof is insulated.

Ive decided to use PIR under all rafters from ridge to eves instead of the suggested superfoil.