insulation corrrecty installed?

[discoboy]

I've just had a roof re-slated (in N France), and the builders have now finished insulating the underside with knauf paper-backed insulation. This has been stuffed tight inbetween the rafters and is touching the breathable membrane. The brown paper backing is facing into the house and all joints have been taped. They have told me this method is ok, but I'm sure there should be an air gap between the insulation and the membrane? There are also 4 vents on each side, and the roof is just under 200m2
I have the folowing build,
slate
20mm batten
20mm batten along rafter length
breathable membrane
7.5 cm rafter
10 cm knauf insulation
Can anyone confirm who is right?
thank-you,
Chris

 

[freddiemercurystwin]

You do not need a gap with some manufacturers although some manufacturers recommend a 25mm gap, to allow for the membrane to drape. You will need to look up the actual manufacturers recommendations for the actual membrane they used.

The whole point of using a breathable membrane s that you do not need additional ventilation although you can never have too much.

 

[r896neo]

You don't need to leave a gap if you counter batten as they have done. This provides the ventilation gap on the slate side of the roof. It should be complimented with slate edge facia vents though.

 

[mointainwalker]

I'm a bit surprised that professional roofers (?) only used 100 mm of fibre-wool ( U = 0.40), I thought the minimum was 100 mm of PU or equivalent ( U= 0.25 ) or so my roofer told me a couple of years ago.

 

[discoboy]

Many thanks for your replies.
MW, youre right, there should be alot more insulation than 100mm but the rafters are only 75mm deep, and to be fair to the roofer, he did say more was req. He did suggest a french version of kingspan, but added that it was so expensive nobody uses it! You must remember that in France all building materials are very pricey. The price per m2 for the installed insulation was 3.75 euros!!!

 

[mointainwalker]

Well he's right about the cost ( think my 100 mm PU was about E 15/m2 ) but as a professional I believed that he had to stick to the regs , which for insulation is , I believe, RT 2000 although it may have been updated.

Believe me, I know very well the cost of materials and tools here

Edit:

Have just seen your roof size which would have made it expensive, mine is about 105 m2

 

[discoboy]

Yup, the price of materiels in general makes me weep, as do the expats who compete a trades apprenticeship on the boat over! There are so many dodgy builders round here the rogue traders show could have a field day. Wether or not it's the same wherever the British end up in France, or if it's just up north I don't know.
Chris.