I live in a house with a finished attic. There's a substantial upper attic (big enough to walk around) and there are also knee walls between the upstairs rooms and the lower attic. There is ventilation at the bottom of the lower attic that connects to the upper attic. I'm concerned that the ventilation is cooling the upper attic, causing a cold knee wall all around the upstairs.
Our attic seems properly insulated for the most part, but one thing is missing: hardboard along the rafters to block off the insulation. This would limit the amount of cold air circulating from the outside to the lower attic and it would also support the insulation.
Bringing any kind of hardboard into the lower attic will be tricky and tedious (the door is 3.5' tall and 2.5' wide). What would be easiest and most effective to use in a space like this?
Also, our floor is tongue-and-groove that also serves as the ceiling for the downstairs. There are holes in the surface (in the grooves or at knots) so warm air can pass from the downstairs directly into the lower attic. Does it make sense to add a barrier to airflow on the floor? What would I use?
Update: the lower attic area has 46 ft of wall and the rafters are 5' long in the lower attic.