Following the remarkable success of my recent adopting "African " external flying sun-shade sails, bypassing French/Italian external shutters, stopping solar insolation thru the windows, for the recent heatwaves. No fans required so far this summer, and no A/C of course. Keeping windows closed during the day meant the house core temperature only rose about 1 deg C above the early morning minimum temperature. Only energy required was manual hooking outside the "privacy screen" material over downstairs windows and hoisting via a couple of pulleys each window top , the upper storey sail shade.
So for this winter ,my idea is starting with ordinary lined curtains, so only sewn around the edges. Cut 3 slots near the top to introduce some mylar space-blanket/emergency blanket material. Firstly, proof of concept. Kitchen table experiment, a pan of water at 40 deg C on a cardboard box. IR remote thermometer 0.5m away on a similar box, reading 40 degC . Introduce halfway , a single piece of mylar and the reading immediately dropped to the room temp of 24 sometimes 25 degC, remove it and back to
40 degC. No observed difference silver or gold side to the warm pan. So I intend cutting down each 1.5x2m mylar shhet to size, rolling on to some tube , insering in the slots, top end held in place with small bulldog clips. Unroll the mylar and remove the rod. It may even be useful summertime for the reverse situation , if no external shades due to wind say.