The UK building regulations require 100mm of loft insulation between joists and 170mm over, ie 270mm of insulation in total.

If it were to be bordered out for use as office, with sistering the joists I would have to use 2"x12"s leaving around 30mm of air gap to stop condensation issues.

This eats a lot into the available headroom, under UK building regulations can I change from 270mm of Rockwool to a thinner amount of Kingspan or alternative?

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You'd need to double-check, but does the 270mm regulation apply to existing houses as well? Otherwise, I've seen plenty of lofts cross battened with 150mm or 175mm deep timbers and boarded on top. Not sure where you want the air gap, condensation shouldn't be a problem here unless you plan on not heating the office... An alternative is to go for a warm roof by putting the insulation in between the rafters instead (where you do need an air gap).

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