Garage ceiling

Yes, in theory you could fix Kingspan or another PIR insulation board over the existing plasterboard on the garage ceiling. It won’t add much weight, and assuming the ceiling is fixed to standard 8 × 2-inch floor joists, they should comfortably support it. Alternatively, you could use SuperFOIL insulation, which may be easier to install than PIR. If fitted and taped correctly, it can also provide a continuous airtight vapour-control layer. I’d recommend a minimum of 30mm PIR. A 40mm SuperFOIL system can provide strong thermal performance, but the exact equivalent depends on the specific product and installation method. The SuperFOIL can be stapled into place, then counter-battened, plasterboarded and skimmed in the future. Whichever system you choose, check the manufacturer’s installation and fire-performance requirements, as it’s a garage ceiling it will be subject to fire regulations.