Loft Roof Insulation - Thermafleece Sheeps Wool Sag Concern

[Rawler]

Hello everyone,

I'm insulating a loft room roof space (ie in the rafters) and going with a breathable system, keeping an airgap below the roof membrane the first layer beneath this is to be Sheepswool (50mm), then likely Woodfibre board. I'm aware of the calculations and appreciate it won't hit building regs U values, but it should be manageable. The main risk is that there is a roof leak and the wool gets wet to a degree where it slumps and blocks the air gap. I appreciate it can absorb and release moisture to a degreee without any effect on shape, but I am working to a tight air gap 25-30mm so not a lot to work with. Has anyone else used this technique? Is it an unnecessary concern? Should I look at cross doweling or netting it from above? I have trialled the latter but it's not taught enough. Any system needs to connect into the side of the existing rafter.

Many thanks

 

[paulc]

Sheep's wool has a thermal conductivity of 0.035 and 0.04 W/mK whilst wood fibre is around 0.038 W/mK. At 50mm thickness, the real world u-values will be so close as to make no difference.
On that basis, why not use wood fibre the full depth and side step any worries about insulation slumping.

 

[Rawler]

Thanks Paulc, it's a good suggestion and I may go down that route. My understanding is the sheep's wool is more breathable and able to regulate moisure better than wood fibre so if there was any slight failure in the roof or extreme temps to cause intersidial condensation it would cope with this better. It has less thermal mass too so cools off quicker in the summer and slows down heat tranfer to the woodfibre which can hold the heat a little longer.