Improving loft room insulation?

I have a largish loft room (approx 8m * 5m * 3m at roof apex) as part of the house extension, which is partially above the garage. The previous owners have installed white polystyrene insulation of unknown thickness between the rafters and then plasterboarded and skimmed over this.
In the summer the room gets very hot, and in winter very cold. I've been told I have two options:
1. Tear down existing boards and polystyrene and install Celotex/Kingspan type insulation, then re-dryline and skim.
2. Install insulated plasterboard on top of what's already there, and skim.
I would rather go for option 2 as it would be quicker and cheaper, but is it going to be sufficient to insulate the room properly, and what thickness of insulated plasterboard would be best?
TIA
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Nobby wrote:

Nobby: Just a thought. White 'bead' polystyrene can absorb water. If there is not a proper vapour barrier on the warm side of the wall there is a possibility that moist/warm house air can seep into the wall and then the water condense out there, within the insulation. Wet insulation isn't much good! Also trapped moisture can cause rot/mould. Might be best to check it out thoroughly? The rule here in a colder but similar climate to the UK, is that the vapour barrier be not more than one third through the wall/ceiling/roof etc. from the warm side. Thus when we added one inch of styrofoam to our ceiling, when refinishing them, we had the original insulation comprising six inches of f/glass batts placed within the ceiling rafters, plus another six inches of blown in loose fill, all above the vapour barrier and were well within the 30% guideline. Our 'attic' is only about 36 inches high btw. so little storage up there. But the low roof slopes makes for a safe easy roof to work on. Also make sure the cold side of insulation is sufficiently ventilated, to ensure that miscellaneous moisture can be dissipated. But not familiar with UK roof construction that may be a superfluous comment. Insulated plasterboard may well incorporate a vapour barrier; in form of plastic or foil, probably just inside the plaster layer? I have heard of people when installing a new vapour barrier etc. 'slashing' the old one which would now be deeper into the wall, to avoid potential condensation within the insulating layers. We did considerable reading on the subject of vapour barriers when building our first house in 1960 and this one in 1971. Neither have suffered moisture problems due to it; however the third owners of house number one applied vinyl siding over my original cedar clapboard! :-( and also blocked up some eave and other crawl space ventilation; and then did have some trapped moisture problems! Few comments anyway. Cheers. Terry.
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