Insulation and heat are very personal choices. You will get a variety of answers on this post, and all or none of them will be perfect for you. One thing to consider will be the breed of chickens that you have, and the type of coop you are putting them into. A larger coop (which will decrease pecking issues if they have to stay inside for a couple days because of weather) will be harder to keep warm with only 6 chickens. A smaller coop will be easier to keep warm, but will be harder to ventilate properly and you could end up with some frostbite or pecking issues. It's a trade off, I guess.
For me, I have 4 BOs in a 6x8 coop and I insulated AND used a ceramic heat emitter. Even still, my coop got down to -5F inside. When it got down below -25F outside, which was about 2 weeks in January, I supplemented with another red light on an extension cord and could keep the coop around 11-13F inside. I had no problems with frostbite. More important than insulation is making sure that the coop is free of draft. Seal up seams, wrap the framing with paper if you can. Even when the coop was at 0F, it didn't feel so bad inside as there was no draft. Ventilation should be high up and close-able in bad weather. My chickens refused to go outside into their run if there was snow on the ground. Luckily, the run is covered, so the snow there was minimal. Still, they spent a lot of time inside the coop even tho their pop door was open. One thing that I really like is that I hardwired my heat lamp fixture into the wall and can operate it with a switch. Now I wish I had wired it for TWO lights. Having the fixture installed in the wall helps me sleep at night without fear of fire.