Insulated garden room/office. How cold does it get in winter?

[Koulibiak]

I want to overwinter some plants that are not frost hardy in a garden room/office at the end of my garden. It has bifold doors and is fully insulated, but generally not heated (there is reverse a/c but it is rarely in use).

I can start the a/c on heat if needed from time to time, but I’m wondering whether that’s going to be necessary. How cold is it likely to get? We rarely go below -5C here. I’m guessing the insulation means the room will stay at least a few degrees, maybe even 10C above outside temperature?

Google and AI searches haven’t really shed any light on this.

So, if you have an insulated, recently built garden room, how cold does it get in the depths of winter? Does it reliably stay above freezing point? Or, do you know how many degrees above outside temperature the room stays at? TIA 🙏

I have a fully insulated garden room, finished last year. It hasn’t dropped below 12 degrees or so, no matter how cold it gets outside.

I have a little oil filled radiator in there which I switch on for 30 mins or so in winter if I want to sit up there.

Nobody can give you an accurate answer without knowing the specifics of your garden room / what insulation used etc. However yours sounds very similar to mine and the answer is it will get cold inside on sub zero days but I’d guess not below 10 degrees. Kind of depends what I’m doing in there to generate warmth. I put my banana plants and a few other less hardy things in there each year and they’re happy as can be.

@Walkacrossthesand @NonmagicMike thank you both! That’s really helpful.

Can't help with garden room, but we have a leaky conservatory with badly fitted bifolds that's attached to our (thankfully, rental) house and only a curtain between it and the lounge - 3 sides of wall to ceiling glass. Lowest it got throughout last winter was 5 degrees above outdoor temps. Which, at its coldest, was -2.

It is South facing though, which makes a massive difference.

In my unheated garage, I have a small electric heater, with a thermostat set to 5C. It rarely comes on, but is positioned near the pipes. Insulation slows heat loss, but does not prevent it, so an unheated building will eventually get cold. If you have windows there will be solar gain on sunny days. If the heater only comes on when the room approaches freezing, then, in most parts of the UK, it will not use much electricity. It will also help reduce damp.

An oil-filled radistor is particularly safe, as it never gets hot enough to start a fire, even if, by mischance, a piece of paper or cloth falls on it.

The integral thermostats on heaters are not very accurate, so I use a separate one. You can get some that fit into the socket.