Time for another go then.

Pointing his wand up straight in the air, at where he was pretty sure was more or less exactly the same spot as before, Toby swished it into that sideways eight/infinity symbol movement.

"Caelum Saepio!" the Hufflepuff said enthusiastically as he did so.

It was a shame that there was no way of knowing whether their individual spells were working. In fact, it would probably be difficult to instantly work out whether or not the room was insulated yet in general. Only Fuller would know when he checked the room a few hours later.

Still, Toby squinted up at his patch of ceiling nevertheless as if it might give him some kind of clue that the spell was working.