Loft conversion insulation

[beeches1]

I currently have a 4x4 timber joist across a 3.2 span supporting the 4x2 ceiling joists below. I am going to strip this all out and replace with 7x4 main joist and 7x2 to replace the old ceiling ones. No problems with this but above that the existing rafters are 4x4 resting on 8x8 purlins.

So main question is for insulation I am proposing 75mm cellotex between the rafters, then screw and glue additional 3x2 timbers on to the existing rafters with 2nd layer of cellotex on that. Top off with vapour barrier then cross battens before plasterboard.

Has anybody got any doubts about the idea of screwing the additional rafters on? Will that get approval? It's a bit like a diy glue-lam.

 

[cdbe]

You would normally put as much as you can between the rafters (while maintaining the necessary air gap of around 50mm between insulation and felt), then the rest under as a continuous layer to meet the required u value and prevent cold bridging. The celotex website will help you work it out. Celotex is better than celotex plus rafter so you gain nothing from deepening rafters, although it is a bit fiddly fixing through plasterboard and 100mm celotex into the rafters.

If you tape the celotex joints with foil tape you don't need a vapour barrier.

Don't know much about 4x4, 8x8 and 7x4 timbers (an older property, or a ship?) but it's easy enough to size new ones with the tables you can find online.