Yep, it's a pointless form of insulation, it has to be installed with an air gap above and below and in addition to 90mm of celotex or whatever. There are no advantages to the consumer.
yes, the thermal resistance of a reflective surface facing into a void is not really close to the minimum building regs requrement. Furthermore, it is very easy for air movement / thermal bypass to make that kind of insulation next to useless, and hard to stop the air movement..
In theory it could work well, but in practice it would be like a single glazed window.
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