Are there Thermal Curtains that aren't blackout curtains?

I’m looking for curtains to keep my small exposed office warmer. A quick search on thermal seems to only return ones that are also blackout curtains. Not a feature I want or need. But I understand the two probably go together.

Office is 12’x7’ with 3 sets of windows 36" wide. 2 walls exterior and a third is the garage. It is a cold room with limited ducting for heating. I am using a duct booster fan I’ve had for years.

You could buy a sheet of bubble-wrap and tape it over the window.

Interesting and I already have a roll downstairs left over from the move.
Bubble wrap is pretty close to the solar covers made for pools.

Only draw back is with a curtain, it is easy to open when I’m in the room. Bubble wrap taped over, not so much.

The usual point need to be made.
You are trying to top heat escaping though the window. It escapes in two ways.

  • Radiant heat from the room just vanishes through the glass. This is not quite a simple as one might think. Ordinary glass adsorbs infra red radiation aka heat, and this is mostly how the radiant energy gets lost.

  • Air heats the glass and the energy conducts through the glass and vanishes outside. This is a huge energy loss. Inside the room a convection cycle will set itself up circulating the air, constantly drawing warm air over the window and cooling it. Any sort of fan or forced air circulation in the room will just make matters worse.

You can reduce the radiant heat transfer with a low emissivity coating on the glass. May be hard to retrofit however. An opaque curtain intercepts the radiant heat, and re-radiates it. So at least half comes back into the room. Multiple layers improves things. A thermal curtain might include a silvered lining. That is also trying to reduce the emissivity of the curtain, so that it doesn’t radiate so much from that surface. That does need to be opaque. You don’t necessarily need it, but it does help.

Mostly you are going to use curtains to stop air movement near the window. To stop air movement you need to constrain the size of the air gaps to stop convective cells forming. That means really quite small gaps. Well under an inch. This is why lined curtains work well, they add another layer of trapped air in a small gap. Also why curtain are pleated. It isn’t just to look nice.

Curtains with gaps above and below can set up convection that just channels air past the window. This why you have pelmets and/or curtains that touch the floor.

Can you sew? I bought this fabric to back some existing curtains recently, though I haven’t gotten a chance to break out my sewing machine yet. The biggest complaint from other buyers is that it’s not room-darkening.