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.

How much does it all weigh, can the 100mm roof timbers take it? One for an engineer, as Tony said.

Are you looking to make the rafters deeper to carry the weight, or to make it easier to fit deeper insulation? If the second then you could put horizontal timbers criss cross across the rafters, to reduce thermal bridges, spaced to suit the edges of your boards.

Are you thinking of a different build up for the vertical bits of the room-in-roof walls, and for the ceiling?

Sounds like you are looking to do a thorough job! Have you thought about going for a better U, in return for all your efforts? Is good to have decrement delay so heat comes in later, but also good to have a better U so much less heat gets in (and out). The legal standard for modifying an existing house is between 0.11 and 0.18 depending where you live and what the project involves, though you can aim to be better than the bare legal standard.

How about using PIR instead of woodfibre for the 150mm layer between the rafters, use 40mm of woodfibre below the rafters for thermal mass and acoustics, with plaster(board)? U=0.1ish, decrement delay 5hours ish. Should be ok for condensation as the dewpoint is well outboard of the woodfibre layer.

Perhaps extend your rafters like this? Top chord is your existing rafter. Bottom (new) chord could even be ex 50 x 50.

Posted By: WillInAberdeen
Are you looking to make the rafters deeper to carry the weight, or to make it easier to fit deeper insulation? If the second then you could put horizontal timbers criss cross across the rafters, to reduce thermal bridges, spaced to suit the edges of your boards.

This is to allow for more flexible insulation between the rafters as flexible wood fibre is cheaper than having more rigid. I had not thought about having the extra depth run perpendicular, I assume this is better structurally?

Using 50mm flexible wood fibre between and 100mm wood fibre under would increase the costs but is reasonably.

Posted By: WillInAberdeen
Are you thinking of a different build up for the vertical bits of the room-in-roof walls, and for the ceiling?

The plan is to insulate from eaves to ridge so the internal walls/ceiling would not require anything different.

Posted By: WillInAberdeen
Sounds like you are looking to do a thorough job! Have you thought about going for a better U, in return for all your efforts? Is good to have decrement delay so heat comes in later, but also good to have a better U so much less heat gets in (and out). The legal standard for modifying an existing house is between 0.11 and 0.18 depending where you live and what the project involves, though you can aim to be better than the bare legal standard.

I have but it doesn't look possible within the constraints and having good performance overall rather than great performance in one area and average in the rest.

Posted By: fostertom
Decrement delay - is that in hope of time-shifting the peak of summer overheating?

Yes that is what I am hoping to achieve.

I originally looked at multi-foil but went off the idea as it looked difficult to achieve the gaps between layers and air-tightness and the generally opinion that it's too good to be true.