Additional insulation below warm roof

[MartinMountain]

Hi
I'm renovating a property in the French Alps and new roof installation has just been installed above the existing timber structure. The timber joists and planks were existing and have been replaced only where they were damaged by a previous leak.

I am contemplating adding additonal insulation in the void between the roof joists but I don't want to cause any issues by trapping moisture. In some areas there is 150mm existing glass fibre insulation in this void with backing paper which I guess I should remove if I leave the insulation in place.

Does anyone have any advice?

Thanks

Martin

[MartinMountain]

Can I put 150mm glass wool in the void between the rafters above an Intello Proclima membrane and plasterboard?

[Redbeard]

Ideally you want a dynamic condensation risk assessment such as WUFI. This will tell you whether the added insulation will make the plank/membrane 'interface' cold enough for condensation to occur in worst-case circs. A 'rule of thumb' (which seems sensible but I cannot prove or disprove) is that if you do mix insulation between and above the rafters then two-thirds of the R value should be above the rafters and a max of one-third below. If my quick maths when I saw your post last night was correct that's a max of 100 -110mm of, for example, flexi wood-fibre with a lambda value of 0.039W/mK. Your glass wool may have a lambda value of 0.044, so a bit more thickness may be OK. But definitely using WUFI to 'prove' it is the better path.

Edited August 24 by Redbeard
emphasise WUFI advisability

[Mike]

7 hours ago, Redbeard said:

Ideally you want a dynamic condensation risk assessment such as WUFI. A 'rule of thumb' (which seems sensible but I cannot prove or disprove) is that if you do mix insulation between and above the rafters then two-thirds of the R value should be above the rafters and a max of one-third below.

+1 to both of these.

 

And also check that you have at least the legal minimum. In the Alps you need a minimum resistance 4.4 m².K/W (u-Value 0.227) https://rt-re-batiment.developpement-durable.gouv.fr/IMG/pdf/fiche-ravalement-refection-toiture-amenagement-travaux-isolation.pdf