Well, our weather is so mixed up and with this cold snap we are having, I would understand the worry.
At the end of the day, if you want to heat your spiders nothing wrong with it, I would say to heat at night only until we go into full blown winter.
But, I don't think it is cold enough to justify heating for your spider just yet, when the temps at night go below 9-10 degrees, that is when I will start with heating. And then I will only heat the spiders at night and heating off during the day.
Me personally.
I actually start the heating towards the end of May, then I only heat the spider during the night, from 19:00 till 08:30, the heaters get turned off during the day as the day tends to get warmer by itself.
If you have ever sat up for the whole night, after 00:00 the temps drop drastically and keep dropping.
Hence why I say, night time heating, as this is your coldest point of the cycle.
Your spiders will be fine in this weather, at the moment, I have a couple of spiders, inside the house some smaller slings and roaches and 5 larger spiders that is not in the spider room, they out in the "open" and they still doing fine eating and just being spiders.
I have moved the males into the spider room though, as you can see they starting to become a bit sluggish.
It is something you need to learn and get a feeling for.
The first year I did not heat my spiders as they were in the house and the house was fairly warm on it's own(I was still learning), the year after That they were moved outside into the outside room with no heating, I had "frozen" spiders one morning, which I stuck them by the heating source and they were fine, and still kept them in the outside room, just gave them heating pads and they were fine for the rest of the winter.
