Have I got this right...!

[Drew Peebles]

Hi,

If I can have a quick sense-check with the collective knowledge of the forum I'd be really grateful.

I am renovating a old French pile dating from 1100 in the oldest parts to early 1900s in the youngest.

I am working in a newer part of the building and looking to convert a loft. Currently it is oak rafters, battens, and tile - and that's it! It is solid enough but if you stood in the roof on a sunny day you'd be able to see chinks of light around the tiles.

I say this because a lot of the videos I have seen, when looking for advice, have modern roofs with OJB panelling above the rafters.

I have read a lot about venting and different types of barrier and am not sure I fully understand it all.

I can't work from outside - whatever I do will be from the inside.

Anyhow, I am beginning to think that I have a plan. I am thinking...

If I install 100mm celotex between the rafters, use foam and tapes to ensure they are airtight, then install 50mm celotex beneath the rafters (and again use foam and tape), then I can finish with plasterboard and job done. I could even batten between the 100mm and 50mm celotex for an air gap. The space is massive so using thick materials is not an issue.

If I understand correctly... If I do a proper job of sealing the celotex then my roof is airtight and that avoids damp issues inside because there should be no cold surfaces (as I type this I remember there is one velux in the roof already). And the roof 'outside' is ventilated (if I keep the celotex clear of the battens), because ir is an open structure already. Finally, if any rain does get behind the tiles it should run down the foil-faced celotex into a ventilated eave and do no harm. So no need for any high falutin' vapour barriers or the like.

Does this sound sensible? And finally to attach the plasterboard to the rafters presumably entails some pretty long screws (90mm or so) if I don't batten between the celotex layers?

Thank you for any comments (I think!),

Drew

 

[blup]

In principle you could make it a warm roof in the general way you describe. Air borne moisture still passes through warm roofs as I understand it so the design has to accommodate vapour control.
I’d be tempted to leave it as a well ventilated cold roof. Do the French have building regs equivalent?

 

[Drew Peebles]

Thanks blup. Looking at diagrams I think I am proposing a well ventilated cold roof: there is insulation between the rafters, and then beneath them, but no insulation exterior to the rafters. I do want the room to be a bedroom so the room space itself needs to be warm. I would think that the exterior roof will remain well ventilated. As for moisture control, I may have this wrong, but if the room is airtight and well insulated and no warm air is reaching a cold surface then presumably no condensation will form? Hence I mention the velux as if air escapes anywhere I imagine it is there and that is where I might get condensation?

There is French code and I follow it in matters of safety but can be more flexible when it relates to efficiency etc. There will be no inspection; I had to submit an application for any changes that affect the exterior appearance (this is rural France and even then they were 'relaxed', e.g. the mayor swirling a finger at the plan of the garden and asking me to roughly indicate where I would put a rough size of pool) but are not interested about any interior renovation.