The Swiss solved ye olde sash window headache.
With nature's helping hand.
This thermal insulator is packed with hundreds of nature's amazing little heaters; the cherry stone.
Basically a little chunk of hardwood with a bubble of air trapped, the cherry stone has the unique ability to store, and gently release heat.
On top, it neatly fits into any surrounding what is perfect for the traditional sash window and the modern, flush bi-fold door.
A nifty, very efficient, thermal barrier against unwanted draft that saves energy, money and the chippie...