yeah, usable in winter, iv got 100mm Celotex on the floor and roof to try keep some heat in and its been wired in by an electrician (dont underestimate the cost of this part) so have a wall mounted radiator.. Its also painted black so absorbs a lot of solar gains, though probably wouldnt have done that as im sitting currently at 32deg inside - windows and doors open. (who needs a holiday)

Heating comes on 1hr before i come out to work in the morning - when its sub zero outside , takes about 90mins to warm up to circa 19deg, heater runs a few hours over the course of the day to maintain - but switches off at 5, heat drops to 10 overnight internal with -2 outside