Cold winter rest indoors

How to give your cacti and succulents a rest indoors during winter without catching a cold ...

I have a greenhouse were I keep most of my plants during the winter rest from October to April. The temperature vary from +15 to +1 C during this time, and an average of ca 7 C. For most cacti this is a perfect place, but for some this could be a dangerous place. Therefore I need to keep a dozen plants indoors where it is warmer. For example Pilosocereus, Buiningia, Discocactus, Arrojadoa, Floribunda and other comming from warmer areas.

But the problem for me had been that it was too warm indoors. The radiator under the windows are very hot in December - February and close to the plants it can be 22-24 C.

So I built a windowsill with ca 20 cm high wood/plastic frames around, except towards the window. The aim was to catch the cold comming down from the window, and to prevent the heat comming up from the radiator. In this way I reduced the temperature 3 degrees to 17 C. And by opening the window a millimeter (not more!) the temperature went down to 12 C! The cost for the small windowsill is very low and easy to make. It could be possible to grow and flower not only Brasilian species like the ones mentioned above, but also Mammillaria, Notocactus, Gymnocalycium and some more.
The new windowsill and my small "Brasilian collection". The temp is ca 15 C in the "box" but in the room it is 22-25 C.

Pilosocereus and Arthrocereus in winter sun!