Most tropical arboreals kept in the hobby do perfectly well between 20 and 26 °C, which is the temperature of an ordinary living room in Denmark, Germany or the UK for most of the year. They do not need a basking spot, and they do not need a heat mat stuck to the side of the enclosure.
Why heat mats on enclosures are a bad idea
A mat against a panel creates a hot spot on a surface the spider can touch. Tarantulas cannot reliably move away from a heat source they are sitting on, and the mat also dries out the substrate directly above it, which drives keepers to add water, which raises humidity in the wrong place. If a room genuinely needs warming, warm the room.
What actually matters
- Stability beats absolute numbers. A steady 21 °C is better than swinging between 18 and 28.
- Avoid cold outside walls and floors. Shelves at chest height are warmer and steadier than the floor.
- Keep enclosures out of direct sun. A clear acrylic or glass enclosure in sunlight becomes an oven very quickly.
- Slow down feeding when it is cold. A spider at 18 °C digests slowly and will refuse prey; that is normal.
If your room really is cold
Below about 18 °C for long stretches, growth stalls and some tropical species become sluggish. A small oil-filled radiator or a heated room, thermostat controlled, solves it for every enclosure at once. If you must use a mat, mount it on the wall behind a shelf, not on an enclosure, and always with a thermostat.
What about summer?
Above roughly 30 °C, ventilation stops being a nice-to-have and becomes the thing keeping the animal alive. A well-ventilated enclosure sheds heat as fast as the room. A sealed one holds it. Move enclosures away from windows, close the curtains and let the air move.