Hi,
I'm using our converted attic a lot more now, and have noticed that it's very cold in winter, and heats up in a few seconds when the sun comes out. Clearly it's not very well insulated.
It's a slate roof with a layer of waterproof felt under the tiles (all in good condition) supported on rafters that are 100mm thick and 600mm apart. Ideally, I'd like to fit in as much insulation as possible, and then cover it all with insulation-backed plasterboard.
But the question is, how much insulation is "as much as possible"? As far as I can tell, the two options are:
- 50mm celotex wedged between the rafters, but leaving a 50mm air gap behind for building regs as it won't be air-tight and condensation behind the insulation needs to be ventilated away.
- 100mm sprayed closed-cell polyurethane foam, onto the back of the felt (NOT onto the tiles), thus removing the need for the air gap as condensation cannot penetrate from the inside.
To me, option 2 sounds better as that's twice as much insulation as option 1 (assuming it is not applied by cowboys and the roof is sound). Am I right that it removes the need for the ventilation gap and therefore can be sprayed to this thickness? So far I haven't been able to confirm either way, other than through material provided directly by foam contractors.
Thanks for your assistance,
Miles