Although retrofitting a skillion roof is not as common as retrofitting a pitched roof, you can retrofit a Skillion roof under these scenarios: - From the outside, when the roof cladding is going to be changed; or - From the inside if the rafters are exposed.
Recently we got involved on a retrofitting job with exposed rafters.
The customer was after the highest possible R-value that could be installed. They understood that the highest amount of heat loss goes through the roof in an uninsulated house and that it wouldn’t be practical to try and add a top up later on.
There was no need to leave an airgap as the rafters’ depth was 140mm and the insulation was going to be installed in between the GIB plasterboard (the old one and the new one).