Utility Room ceiling insulation

[danbert]

Hi

I'm after a cheap and cheerful way of adding a few degrees to the utility room in the winter. It's basically an extension of the garage (single skin wall, flat cold roof and attached to the house.)

I want to run an oil radiator or similar, and install reflective insulation on the inside of the roof, but due to the way the rafters run it will be almost impossible to provide a ventilation gap above the insulation (no soffits).

I wondered if attaching foil insulation or even kingspan/celotex across/between the rafters but leaving an open gap (a couple of of inches) at opposite ends would provide any heat retention at all, or if all of the heat would just escape through the gaps into the void above? It doesn't need to be toasty, just take the edge off.

Any opinions or other suggestions would be most welcome.

Many thanks