Question: Cold Flat Roof Insulation

[Kingo206]

After sorting out our ventilation issue in our flat cold roof, we are now at the point of putting in the insulation.

I am hearing contradicting things at the moment, that we need to use gapotape due to the recent changes in building regs. or can we simply ensure the celotex is cut correctly to size, and any gaps are closed using expanding foam?

Our plan is to the do latter:
We have 7*2 rafter joists, with a 23m^2 area space (25m^2 - 2x1 skylight) see image attached.

We have the following:

- 120mm Celotex
- Expanding Foam
- Aluminium foil tape
- SFTV Reflective vapor membrane layer
- Double sided Butyl Tape

Do you guys think this approach is okay, or do you reccommend any other adhering to the new regs?

 

[Kingo206]

Thanks for moving to the appropriate thread

 

[Mr Chibs]

I filled the gaps with expanding foam, then aluminium duct taped all the joints. This passed the inspection.

 

[Notch7]

Is 120mm Celotex sufficient for 2022 building regs?

you mention you’ve sorted the ventilation issues so I guess you’ve addressed these: an air gap of 50mm and cavities ventilated at each end?

Personally I’m not sure a cold roof is very good with a roof light

 

[Kingo206]

I filled the gaps with expanding foam, then aluminium duct taped all the joints. This passed the inspection.

Thanks your input mate, was this recent, I'm told we need to use a vapour membrane layer.

Is 120mm Celotex sufficient for 2022 building regs?

you mention you’ve sorted the ventilation issues so I guess you’ve addressed these: an air gap of 50mm and cavities ventilated at each end?

Personally I’m not sure a cold roof is very good with a roof light

We have 7*2 joists, so 120mm should be sufficient to ensure minimum 50mm air gap - is that the case?

 

[noseall]

We have 7*2 joists, so 120mm should be sufficient to ensure minimum 50mm air gap - is that the case?

No. The insulation thickness is paramount and should not be sacrificed in order to obtain a certain air gap size. Where a joist or rafter is not deep enough to accommodate both the required insulation thickness and an adequate airgap, one is expected to fix knock-on pieces to the joist or rafter.
We are fitting 150mm currently to our flat roofs. Oddly, I'm sure 130mm achieves the right number, but is impossible to achieve with the thicknesses available. On a warm deck, we just bang down two 75mm (bonded) layers of PIR.

 

[noseall]

After sorting out our ventilation issue in our flat cold roof, we are now at the point of putting in the insulation.

I am hearing contradicting things at the moment, that we need to use gapotape due to the recent changes in building regs. or can we simply ensure the celotex is cut correctly to size, and any gaps are closed using expanding foam?

Our plan is to the do latter:
We have 7*2 rafter joists, with a 23m^2 area space (25m^2 - 2x1 skylight) see image attached.

We have the following:

- 120mm Celotex
- Expanding Foam
- Aluminium foil tape
- SFTV Reflective vapor membrane layer
- Double sided Butyl Tape

Do you guys think this approach is okay, or do you reccommend any other adhering to the new regs?

View attachment 320817

You will likely have to under-draw the ceiling with another 50mm of insulation, prior to plaster-boarding.

 

[noseall]

After sorting out our ventilation issue in our flat cold roof

How did you sort it?

 

[Kingo206]

No. The insulation thickness is paramount and should not be sacrificed in order to obtain a certain air gap size. Where a joist or rafter is not deep enough to accommodate both the required insulation thickness and an adequate airgap, one is expected to fix knock-on pieces to the joist or rafter.
We are fitting 150mm currently to our flat roofs. Oddly, I'm sure 130mm achieves the right number, but is impossible to achieve with the thicknesses available. On a warm deck, we just bang down two 75mm (bonded) layers of PIR.

Hi Noseall, thanks for getting back - I'm abit confused though, so you think 120mm is not sufficient enough?

As stated:
- 7*2 joists
- 120mm insulation
--> Remaining air gap once insulation aligns with joists approx 55mm

Opting to apply:
- Expanding foam to any gaps
- Vapor control layer/barrier (specifically SuperFOIL SFTV Thermal Vapour Barrier)
- Aluminium foil tape
- Double sided Butyl Tape

Your input in regards to the process and what you expect we should do would be greatly appreciated, as I really would like to get this right first time. Thanks again.

How did you sort it?

The ventilation issue we had was that, we initially opted for a parapet roof, but given the height at that point already, increasing to account for the installation of abutments around the perimeter was not viable to our party wall neighbours.

So we took the parapet wall down to eaves level, and simply applied vented fascia around the perimeter to make life easy - We were able to apply an abutment where the extension meets the original house.

 

[Kingo206]

You are right, 120mm is not sufficient for the cold roof system, I have just confirmed this on the phone with my building regs contact.
He confirmed your suggestion to append an additional layer of insulation to achieve minimum thickness of 150mm.

I'm grateful for you guys on here, saved me a lot of time and money!

edit: Is there vapor control layer still necessary after applying the second layer of insulation?!

 

[noseall]

edit: Is there vapor control layer still necessary after applying the second layer of insulation?!

Once you have fitted your first layer of insulation, apply foil tape to the joints and/or squirt foam to the bigger gaps. Fit the second layer across the joists and foil tape these joints too.

Finish it off with duplex plasterboards.

crank3.jpg

• noseall
• 22 Dec 2022

Foil1

• noseall
• 2 Apr 2019

Foil2

• noseall
• 2 Apr 2019

Foil3

• noseall
• 2 Apr 2019

 

[Kingo206]

Thanks @noseall

As I already have the Vapor Control Layer.

So ideally would like to use that, and then I guess plasterboard on top.