Insulation for pitched roof WITHOUT loft space

[keyzjh]

I have a pitched roof in a two-up two-down Victorian house which has a cross-section as the attached sketch. A picture in the loft for scale.

In the main loft area (not a conversion), I will be placing 100mm Rockwool (or similar product) between ceiling joists and 170mm laid on top perpendicular.

The roof tiles and membrane are being replaced in the coming days so I can access this area from the exterior side.
Here I plan to use 50mm thick Rockwool between the rafters (roughly 290-300mm spacing between). I considered PIR boards but thought it would be too difficult to get the dimensions spot-on with fitting between rafters, and I only have a small time period between roof being stripped back and laying on insulation, before the roof membrane + battens are installed. The rockwool seems far more forgiving to cut slightly larger and push in to fit between rafters.

My questions are:
1. This is the 50mm thick Rockwool slab I plan to use, it does say it's for sound insulation, but a 100mm thick wool would be too thick to also allow a ventilation gap between the insulation and membrane. Are there any better alternatives I can consider?

Rockwool Sound Insulation Slab - 50 x 600 x 1200mm | Wickes.co.uk

Rockwool Sound Insulation Slab - 50 x 600 x 1200mm

www.wickes.co.uk

2. Regarding complying to Building Regulations, 50mm wool isn't quite 250mm as given in Table C1 in Document L Appendix C. But given the space constraints above, do I have any other reasonable alternative?

3. To sense check, I will also aim to leave a 25mm gap between the insulation and eaves, and 75mm around the boiler flue which vents through the roof in the bathroom. Light and electric cables can go above the insulation.

4. Should I add more insulation where possible in that wedged space in the sketch?

Thanks!

 

[Mr Chibs]

Would think you are better to go with thicker PIR (foam insulation) as it's nearly twice the insulating properties of rockwool.

Get a cheap foam gun from ebay, and fill all the gaps and tape up with aluminium duct tape. (yes, it's a DIY job).

You need to maintain the ventilation gap, whatever you use, so if you can fit 75mm or PIR in between the rafters, I'd do this.

If you are redoing the bathroom?, perhaps you would be able to add some more PIR to the underside of the bathroom before you plasterboard?
You could fill the wedge with rockwool, providing you don't block the ventilation under the roof.

 

[keyzjh]

Would think you are better to go with thicker PIR (foam insulation) as it's nearly twice the insulating properties of rockwool.

Get a cheap foam gun from ebay, and fill all the gaps and tape up with aluminium duct tape. (yes, it's a DIY job).

You need to maintain the ventilation gap, whatever you use, so if you can fit 75mm or PIR in between the rafters, I'd do this.

If you are redoing the bathroom?, perhaps you would be able to add some more PIR to the underside of the bathroom before you plasterboard?
You could fill the wedge with rockwool, providing you don't block the ventilation under the roof.

Thanks! Would 25mm ventilation gap be sufficient?

I understand PIR has better thermal performance than rockwool.
However considering I will need to install PIR externally from above (instead of from below in all the Youtube videos), the installation seems significantly more complicated. First I'd cut the PIR board to size and try to fit between rafters. If the board is too large, it can be trimmed down. But if the board is too small, it will fall through to above the plasterboard, and will require maybe some screws in the rafters to hold it in place, before being able to apply the expanding foam.

I would also have climb above the roof to do this in a short period of time where the roof tiles are removed - don't have the luxury of working indoors!

How does poorly fitted PIR boards compare to a rockwool installation?
Have you seen any videos demonstrating PIR installation from above to between rafters?