Boarding loft

[Andydow]

Hi I’m currently adding an extra 200mm of loft insulation to the 100mm already down to give me 300mm advised.

I want to have a section in the middle boarded out, I’m reusing boards already down. My existing joists are only 85mm deep so I need to raise c215mm to clear the new insulation. Loft legs aren’t long enough, so timber is preference.
I see 2 options
1. 220mmx38mm x2.4m , I’ll need 10 of them to do the area, laid perpendicular and loft roll between 600mm centres, but I’m concerned about additional weight
2. 89mmx38mm x2.4m, with leg supports cut from 69mm x 38mm x 130mm long at 1.2m resting on original joists, using plates and brackets to secure to original joists and new raised joists. More work but should be lighter
Thoughts on options or any other suggestions?

 

[PolarNorth78]

Loft leg XLs are 300mm tall so why are those a no go?

 

[Andydow]

Didn’t want to have such a large gap between the insulation and boards to max height plus mixed reviews on them warping out of shape

 

[cdbe]

Put celotex on top of joists and loose lay your boards on top.

 

[Andydow]

If I have 100mm fiberglass which will be compressed a bit as the joist are only 85mm what thickness of celotex would you recommend

 

[cdbe]

I believe the performance of celotex is roughly twice that of mineral wool for the same thickness plus you eliminate all the (relatively poor performing) support timber, gaps and issues of mineral wool settling.

Mines an upside down system - I have 200mm joists filled with mineral wool and boarded on top and my ceilings are underboarded with 100mm PIR. With the heat rising up from the ground floor, CWI and decent windows we rarely need heating on upstairs.