Re-roof as part of loft conversion

[tomcat_]

Hi all,

I'm starting a loft conversion in a victorian terrace house.

as part of the conversion i will be reroofing the front pitched roof that will stay as is.

I will be replacing the old felt with a tyvek and new artificial slates. However i'm getting a bit confused with the different views including that of my building inspector. Rafters are about 84mm depth.

- My contractor wants to have 50mm gap underneath the tyvek then 50mm PIR and then 30mm foil. (with 25mm gap either way)

- My building inspector is happy with only 25mm gap underneath the tyvek if i counterbatten from the outside. (parapet walls in both sites so the slightly raised front it should be hardly noticed)

- What i would like to do and based on the tech specs that i have read on the tyvek (or other breathable membranes) is to counterbatten on the outside and install the tyvek taut then i should be able to push insulation all the way up to the membrane between the rafters and finish under the rafters with foil backed insulated plasterboard. This way i have an unventilated pitched roof and about 110-120mm PIR insulation which i believe is going to offer better u value than ventilated PIR and foil combination.

Is there a common practice on the reroofing, and is unventilated insulated pitched roof going to work in older properties?

Thanks for your help

 

[^woody^]

If you design so as to prevent air getting into the roof structure and also remove any voids, thus removing the possiblity of air being able to condense on a cold surface within the roof structure, then you have removed the condensation risk and so don't need to ventilate. The problem is, many/some BCO's dont understand this.

 

[tomcat_]

Thanks for your reply.