Insulation under roof tiles

[akist]

We are building a small annexe and the roof rafters have gone in. The builder has laid a quite thin "underlay" type material onto the rafters (light cyan/green in colour) and then has installed the battens ready for the roof tiles.

Do we not need something more substantial under the tiles?

 

[Ian H]

Yes, 100mm Celotex between the rafters then 50mm full layer under them.

The green stuff will be a breathable membrane that ‘back int day’ would have been a thick felt.

 

[Leofric]

It depends whether the roof is being insulated at the rafters or at ceiling joist level . Tiles on battens on breathable roofing membrane over the rafters is normal construction as far as that goes.

 

[noseall]

It depends

Correct.
It also depends whether there is ample room for an air gap, (cold side) within the depth of the rafters, to accommodate the thickness of the insulation plus say 25-50mm for said air gap.

Also the thickness between rafters and that which goes across can vary, depending upon the risk of cold bridging through the timber. The thought process is to achieve 150mm thickness of Celotex insulation whist guarding against cold bridging through the rafter. Ians' suggestion is a typical detail (albeit without mentioning the airgap ) but not the only combination.

 

[JohnD]

Do we not need something more substantial under the tiles?

Has this been designed with a habitable room in the roof? Or is it just an unheated loft?

 

[John D v2.0]

(albeit without mentioning the airgap

Just wondering what's the air gap for? Condensation or just to allow the membrane to drape?
I understand a ventilated and possibly drained cavity is needed where there is a risk of condensation/condensation build up, but since there is a breathable membrane above that's unlikely to be an issue.
Although in the latter case If the tile battens are straight on the rafters you would need a small drape in the membrane to allow water to run down the membrane

 

[^woody^]

It depends

Correct.

It depends if the OP is following the plans he has paid for, or whether he's gone for 'Plan B' which is 500 random posts on DIYnot on how to build an extension/out building.

 

[JohnD]

@akist

Has this been designed with a habitable room in the roof? Or is it just an unheated loft?

Without having this answered, all replies are just blind guesses.

 

[Leofric]

I understand a ventilated and possibly drained cavity is needed where there is a risk of condensation/condensation build up, but since there is a breathable membrane above that's unlikely to be an issue.

A 50mm ventilated air space is required above insulation between the rafters to reduce the risk of condensation and to allow the membrane to drape a little below the battens.

Without having this answered, all replies are just blind guesses.

Perhaps akist could tell us all exactly what is being built and then there might be some better answers instead of general principles of 'thermal insulation in roofs/condensation/cold bridging,avoiding the risks '

 

[Notch7]

Without more informstion, Id guess at the annexe having a flat ceiling and therefore insulated at joist level.

Ive found the cinstruction details available on the tyvek site pretty good for insulated pitched roof options, including tiles with a warm roof detail.

 

[akist]

"Without having this answered, all replies are just blind guesses."

No rooms up there, just some space in the centre for storage. Planning to put insulation between the ceiling joists.

You mentioned that there needs to be a way for water to run down over the membrane but the battens will prevent that. Isn't this the case on every roof ? Is there a solution?

 

[^woody^]

Is there a solution?

I'm finding that more clients are requesting domes for this very reason.

 

[stuart45]

Is there a solution?

Don't pull the membrane too tight.

 

[Leofric]

You mentioned that there needs to be a way for water to run down over the membrane but the battens will prevent that. Isn't this the case on every roof ? Is there a solution?

As previously mentioned a slight sag in the membrane allows any water to run down between the rafters. If a low pitched roof is boarded over the rafters ,counter battens under the battens allow any water to run down.