Can Insulation touch breathable felt?

Jim1000

Hello Folks,

I am going to insulate roof. The rafters are 150mm deep. I am looking at the earthwool. There will be plasterboard going on the rafters afterwards.
The roof is new and the felt is very light and green in colour. I have been told this is breathable and I can use 150mm wool no problem.

Is this true?

Many Thanks

Jim

Rabbo

No, it is recommended that you maintain a 50mm air gap between the insulation and breathable roof. This leaves you with only 100mm between rafters so it would be better to use high density PU board insulation (i.e. Kingspan or Xtratherm). You should also use insulated plasterboard to underside of rafters as the 100mm between the rafters wouldn't be considered much. 80mm insulated plasterboard would be typucal in a new house if you can spare the height.

Jim1000

Interesting. I thought as much. I had a look at the felt on line. Here is a quote from fakro
"High water vapour permeability allows the moving of insulation towards the membrane surface without the need for any ventilation gaps in the roof or wall structures. This makes it possible to apply a thicker layer of insulation, which translates into energy savings. This membrane is laid directly onto rafters, insulation or sarking, with the printed side facing outwards"

BryanF

Jim1000 wrote: »

Interesting. I thought as much. I had a look at the felt on line. Here is a quote from fakro
"High water vapour permeability allows the moving of insulation towards the membrane surface without the need for any ventilation gaps in the roof or wall structures. This makes it possible to apply a thicker layer of insulation, which translates into energy savings. This membrane is laid directly onto rafters, insulation or sarking, with the printed side facing outwards"

Does it say something about double cross battening above to allow air-movement?

Calahonda52

BryanF wrote: »

Does it say something about double cross battening above to allow air-movement?

OP, from here:
http://ie.fakro.com/att/foldery/FAKRO_roofing_membranes_2013.pdf

It seems that counter battening is a requirement, from a quick look at the pics in the brochure.

Given that you have a number of holes in the membrane already, as per your other post, ...........:)

Re the insulation, in a roof space rooms such as you describe, I would look at using higher density insulation material such as wood fibre to reduce the problem of solar heat gain in the room, the issue is decrement delay
http://www.designingbuildings.co.uk/wiki/Decrement_delay