Got me thinking so I looked at my roof the other morning. It had frosted during the night and my roof was covered in "ice" basically. When I got to the front of the house, there was one section (between rafters) that was melted. In my upstairs, I have knee-walls/low ceilings that are covered in t&g oak planks. No insulation between that and the roof. However, the one section that was melted was where I've removed some planks for some remodeling which was allowing the heat into the attic. So apprarently just a layer of 3/4" t&g wood is a pretty good insulator.