roof space insulation

[Carpenterjohn]

I want to insulate my roof void. Details are: 1895 terraced house, recently reroofed, continuous eaves ventilation plus some ridge ventilation, 100mm ceiling joists, approx area 6x4 metres; current insulation 100mm fibre glass.
To use more fibreglass or other conventional insulation would need a piggyback build up of joists to 200mm or more. Could I use the \\\'superquilt\\\' such as Actis tri-iso-super 9, or Screwfix\\\'s quilt? These are supposed to equal 200mm fibreglass, with a U-value better than 0.2.
Can I lay boards directly over the quilt for storage and access? There are no power cables in the attic other than the lighting circuit (all pendant luminaries).
Anyone able to help here?

 

[JohnD]

you can cross-batten the joists with additional timbers at 90 degrees to the existing ones. that way, you can put another layer of insulation between them without losing the timbers, and can (if you want) put boards on top of the extra timbers to walk about on. i have done that and am quite pleased, it spreads the load and makes it more rigid. but it does not make the flooring any stronger as a whole, so you can't use it as a habitable room or a loft conversion. It is Ok for storing the Christmas tree and your collection of Judy Garland albums or world cup videos

I believe it is also possible to put e.g. 100mm rigid insulation boards on top of the joists and lay flooring boards on top of them. If you step on insulation boards themselves I expect your foot would go through it. I have never done it myself and don't know if it is a good way.

 

[Carpenterjohn]

Thanks, JohnD. Laying additional joists at right angles to the existing ones would be neat, and the spacing could match standard Cellotex cavity batts. I'm concerned that the ceiling is already sagging so if I went for doubling up joists they'd best be piggyback full length for extra strength. I've just checked the old joists, they're closer to 3" than 4". Cheapskate Victorians. Don't want to take the ceiling down as I'd lose the nice cornice and don't want the mess.