Not sure if this is exactly what you're looking for, but Reflectix is pretty close. I've used it to keep my pot hot and make it easier to hold after cooking, it's decent, but not thermos level insulation.

https://www.amazon.com/Reflectix-BP48010-48-Inch-10-Feet-Insulation/dp/B000BPAULS

If you ever order groceries online (like from Amazon Fresh or Prime Now), you can get this stuff for free from the bags they put your groceries in

Use reflectix bubble wrap. Go across the front of the studs. Then nail your paneling over the wrap. Tape all the seems with metal tape. You want a tight moisture and insulating shield. I recommend you google “sauna times”. They have a ton of info. I’ve only used harvia heaters.

Reflectix BP48010 Double Pack Insulation, 48 in. x 10 ft https://www.amazon.com/dp/B000BPAULS/ref=cm_sw_r_cp_api_i_7Ha1Eb7PC03BK