HepM

Thanks, I realise that 100mm (or whichever depth) counter batten across existing rafters would be better for allowing for more insulation. Could you advise on which of the ISOVER products to look at? This isn't a reflective type, what has been suggested is PAVATEX ADB - Pavatex

I am looking to insulate a semi-detached chalet bungalow and considering a retro-fit installation of wood fibre insulation between/under rafters. The room-in-roof will be the master bedroom so wood fibre is preferred for it's theoretical performance for heat insulation, decrement delay and sound proofing. The roof consists of concrete roof tiles, bitumen roof felt and 100mm rafters. As this would require a 50mm cavity the options available are to increase the rafter depth for more flexible wood fibre between the rafters with some rigid wood fibre under or 50mm flexible wood fibre between with more rigid insulation under. I am considering the option to increase the rafter depth and I'm under the assumption that this is feasible and straightforward. Is this a common practice or something best avoided? It has been suggested to install a breather membrane between/under the rafters after the 50mm cavity, before any insulation to provide additional protection to the insulation. Logically it makes sense to install this as there are some minor tears to the bitumen felt and a feasible alternative to re-roofing but this isn't something I've seen discussed or suggested elsewhere. Is this common or a waste of effort? I see that manufacturers of wood fibre insulation have their own breather/airtightness membranes, are these special for wood fibre insulation? If not, what should I be looking for as an alternative? The final build-up would be as below which is based on constraints of amount added to roof/space lost (7"), cost and performance with calculated u-value of 0.2 and 10h decrement delay: 1. Concrete roof tiles 2. Tile battens 3. Bitumen roof felt 4. 100mm rafters extended to 200mm 4a. 50mm cavity 4b. Breather membrane 4c. 140mm SteicoFlex 036 5. 60mm SteicoInternal 6. Airtightness membrane 7. 25mm counter battens/service void 8. 12.5mm Plasterboard Any advice and further questions are appreciated.