Baby sleep layers in a cold house - please help me understand!

[NotQuitePeachy]

Due with DC3 in the new year and struggling with what I need to buy for winter sleep layers. We used the heating a lot with previous DCs, but not sure we can afford as much heating this time with how much costs have gone up in the past 5 years. Last winter, our bedroom was regularly around 12C, and I was wearing thermal top and bottoms, fleece PJs, wool socks and had 13.5 tog of duvet. I just can't see how a newborn is going to be warm enough with 3.5 tog, not matter how many layers under it?! Am I mad to consider buying a down pramsuit to use in bed 😅

We have a plug in thermostat controlled oil radiator in my toddler’s room. Not expensive and great at heating his room if it dips below a certain temp. 12 degrees is cold!!

We have a thermostatic heater in my daughters room to keep it at ~20deg over night.
We can't sleep with the heating on as we get too hot and it seems daft to heat the whole house when we only need her room warmer.
She's 4 and sleeps in a poppered vest, PJ's and slipper socks and has a heated blanket. When she's cold she's more likely to throw her covers off.

I'd also look at raising the temp a bit just in your room, whether that's with the TRV on your main radiator or a plug-in one, but they'll be warmer than you think in a sleeping bag – the bag means they have less space to heat, plus it keeps the heat in, just like a 'mummy-shaped' sleeping bag for camping.

That said, babies do vary – DD2 seemed to need one layer more than the grobag chart said as a baby, which sometimes meant a jumper as well as vest and sleepsuit under her sleeping bag. She was slim, so all that fitted comfortably, but they do make cold weather bags with arms, which would have pretty much the same effect.

I think you'll need to make that room warmer somehow, that's just too cold for a newborn.

Ideally you'd want the room to go no lower than around 16 degrees at night, if you are able to use a plug in heater or similar. I would not want to massively layer up a newborn even in a colder room due to the risk of SIDS.
If you can get to the 16 degrees, you can then use a 2.5 tog sleeping bag with sleep suit and vest.
A possible alternative if you are really unable to make the room warmer is the Love to Dream swaddle bags as they do make one intended for rooms under 16 degrees. I believe the extra warmth is from the chest down not the arms bit of the swaddle bag and they have a guide on how to dress a baby under it. Even so, their guide only goes down to 14 degrees so it may still be uncomfortable and difficult to sleep for the baby.

Thank you so much for the helpful replies! I should have said - it's a rented house and I'm pretty sure the roof and/ or insultation is the issue. We do keep it cold because it's annoying to feel like we are throwing money away heating when it doesn't stay in the house, but I'm not sure more heating alone will do the trick. I'll check out the love to dream swaddle bags....

Look at the chart on the gro bags (you can find it by googling) that was the most helpful guide we found. When we had our first, we lived in an old cottage with single glazing and very poor heating (also rented so nothing we could do about it). It was the sort of place where the curtains would blow on a windy day and we could get ice on the inside of windows in the winter. We did use an electric heater in our room at night at least for a few months and then went by the gro bag chart, which worked well.

You can’t put a newborn to sleep in a room that’s 12 degrees. You need to get an oil filled radiator for your room and use it overnight.