Loft insulation and boarding

[padstar]

I am lofting floorboards downstairs for a new floor structure so plan to reuse the original boards in the loft to give useable storage areas.

The loft is not yet insulated and I had toyed with insulating between the rafters but am now sticking to traditional quilt between the joists.

What is deamed sufficient. I wanted to keep it simple and go for 100mm between the existing joists then cross batten with 4x2's to allow a 2nd 100mm layer of quilt before laying the floor boards. Is that good enough?

Also what centers can I get away with on the cross timbers - I don't want to go through the floor boards and through the ceiling. Is 600mm pushing it?

 

[Dewalt2003]

you are going to be putting more of a load onto the ceiling joists which are only designed to hold your ceiling up.
in reality the more you store in aloft and the more you walk about especially in the centre of ceilings, you are risking
cracking, plasterboard nails popping, and general damage. the best way is to use 5 inch timbers and bear onto outside wall plates and cross on load bearing wall. then board out making original joists redundant from load

 

[padstar]

In reality i will be up there 1-2 times a year with very little stored up there. I plan to span from LB walls where i can but in instances where i cant is this really a problem.

On the insulation is 200mm a decent level

 

[JohnD]

Yes

Insulation brings diminishing returns, so each additional layer brings less benefit than the one before

 

[jeds]

In reality i will be up there 1-2 times a year with very little stored up there. I plan to span from LB walls where i can but in instances where i cant is this really a problem.

On the insulation is 200mm a decent level

It depends on the situation and how overall heat loss is calculated? In the old days it was about minimums, but these days everything is calculated as a whole.
That said, for an isolated situation, 300mm is the average. If you have space for 300mm that's what you should go for. If you are upgrading an existing situation then 200mm might be acceptable.

On the loading; lofts are normally rated for a 0.25kN/m² live load. That's the load over and above the load of the materials - rafters, boarding etc. It's normally enough for general storage but be careful of large quantities of paper - paper is heavy stuff.