Vaulted ceilings - hot/cold and ventilation

17 Jan 2010
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United Kingdom
I am converting a single storey flat roof extension to a pitched (slanting from house) roof.

I plan to use celotex insulation between the joists and then screw plasterboards into the joists.

Is there such thing as hot/cold roofing systems with a pitched roof and with vaulted ceilings?

I have concerned myself because with the plasterboard screwed to the joists and the gap between joists filled with celotex, where will warm air escape to? Surely there needs to be a void?

I will be using the recently favoured membrane under the tiles rather than traditional felt. I understand this helps with ventilation to an extent.
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It sounds like a typical cold (vented) vaulted roof though there are other factors to consider.

A typical detail would show say 100mm between rafters with a 25-50mm air gap above, vented via breathable membrane.
There would be a further 30mm underdraw fitted prior to fixing the plasterboards.

If you are cutting into an existing roof then additional venting is usually required i.e. over-fascia vents etc.

I can't picture this roof. Is it a duo-pitch or a lean-to? If duo, then you need to secure the ridge to prevent spread.
Thanks noseall.

It is a lean to roof - slopes from house. Old flat roof completely removed and started afresh.

The roof is now complete with 2 roof windows. Just need to celotex and then plasterboard.

I have been in contact with Celotex and they recommend the following:

I) Celotex between rafters.
ii) Leave gap of 50mm between celotex and breathable membrane.
iii) Use a vapour barrier between plasterboard and celotex?

I am mostly confused by point iii). I cannot see the purpose of this. Surely this prevents any ventilation? I think their rationale is that ALL hot air should be kept within the room but any that does get through the Celotex should be able to vent out of the breathable membrane.

Would you recommend soffit vents too?

Thanks for your help.
The vapour barrier is indeed to stop moist air within the building migrating through to the cold side of the celotex and causing a condensation problem.
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Is this Vapour Control barrier a Buildings Reg/required requirement?

Also, would you advise just celotexing between rafters or between and below?
Is this Vapour Control barrier a Buildings Reg/required requirement?
Kind of yes, but it is not something they would check or pass off.

Also, would you advise just celotexing between rafters or between and below?
You will need an insulation underdraw yes. The job we are doing at the mo' is asking for 100mm betwixt rafters and 30mm below or across the rafters.
For anyone else undertaking a similar job (lean to tiled, vaulted roof and ceiling).....I have spoken to Celotex and been advised of the following:

1) If you use breathable membrane under your tiles, you do not need to ventilate. By ventilate I mean add soffit vents and ridge vents (if applicable).

2) You only need to leave a 50mm gap between insulation and membrane (under tiles) if you are using a ventilated system - i.e. non breathable membrane under tiles (e.g. felt).

3) If your insulation is foil backed you can achieve a vapour control barrier by just taping all joints with foil tape. You do NOT need to add additional Vapour Control Layers.

4) You require a thermal rating of 0.18. using celotex this is achieved with 100mm between rafters and 35mm under rafters - for non ventilated system.

5) If doing a non ventilated system (MUST HAVE BREATHABLE MEMBRANE) you should leave circa 25mm gap between insulation and membrane.

Hope this helps others.

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