14 would not concern me.
Adding insulation around the hutch given they are in an heated shed probably isn’t going to change the temperature in the hutch by much given it’s your whole shed that you are heating. If the temp in the shed drops then surely given the whole of the front of the hutch is open then any warmth in the hutch is just coming out and colder air is coming in anyway.
Insulating the hutch is only going to make a real difference if there is a heat source actually inside the hutch itself (and not in the shed, such as in my situation) and you’re trying to trap that heat in and/or if you’re trying to keep out draughts (which you aren’t).
Therefore it goes back to what I have said before, keep the shed ticking over and then add extra hay in the hutch if you feel they need more to snuggle into on a colder night.
You may decide to throw a blanket over the hutch or even over half the front if it is dropping too low in the shed as it may help reduce transfer but I’m not sure even that is necessary at all with the shed being heated (and your shed is not getting too cold) and definitely not at this time of year/when it is starting to warm up anyway