I basically want to stop something getting too cold
You can if you like add to the requirements that the space I wish to
maintain at the specified temperature is around 6 x 8 x 12 feet, and is
enclosed by brick walls, wooden roof and small single-glazed windows.
ah. The Garden Shed, or thermal equivalent thereof.

On a similar note, there was a drugs raid in Liverpool where an address
came to the attention of the police via their helicopter - it had been
snowing, and everywhere was white except for this one roof, which was
completely clear (and probably steaming).

As you can guess, a visit a short while later revealed a cannabis farm in
the attic with heat lamps ablaze etc.

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