That helps! That is pretty cold. So can you explain what set up you are expecting the dimensions, material and weather protection. I will try and give you the best advice.
The breed type will help also. Silkie or bantams will need some extra support in cold weather.
Generally if your birds are fully feathered they do not require heat even in freezing temperatures. They will fluff up in their house and be just fine. There is also a believe that keeping heat around for the birds during winter discourages them from foraging as they aren't climatised.
If your birds are 5 weeks they may still need a heat source, but you should be very cautious about how you provide the heat. Overheating is a very serious possibility. This is why I feel unable to give general advice without fully understanding your set up idea