Combining Rigid Phenolic Insulation & Fibreglass in attic - Virtues of same?

boots1

Folks,

Began insulating my attic space yesterday & while the rafters are deep enough to accomodate 125mm kingspan & still retain a 50mm ventilation path, the truss tye (horizontal section of the truss) will only accomodate 80mm kingspan (phenolic boards). I cannot squeeze 125mm into it as there is 4 x 1 bracing running across all the tye sections. I was therefore going to fit the 80mm between the tyes & run 200mm fibreglass over the tyes in the void above, & composite plasterboard on the underside of the truss. Is there any value to be gained by using a combination of the two (rigid phenolic & fibreglass) to increase the amount of insulation fitted? I know one option may be to run the rigid boards all the way up the rafters to the to the ridge, but with the positioning of the bracing & tyes, it would be a right pain & wouldn't necessarily lend itself to a good snug fit in any event as it gets very awkward in the confined space. As I type this I'm wondering also, is there any value in fitting the 80mm tight between the tyes & laying loose rigid boards directly on top of these in the void, thus increasing the thichness??? Thanks in advance, boots

ardara1

you're not restricted to height above the ceiling so just fill it full of fibre and layer over the joist with fibe again (100 + 200 over to achieve 0.0.15 U-value) - it's a hell of a bit cheaper than using rigid boards.
If you want to go better and improve detailing - line the underside with a layer of rigid insulation - PIR or Phenolic.