The advantage here is that your knee wall is probably already insulated this way so it s just a matter of buying some foam so cheap.
Installing knee wall attic insulation.
Verify that top bottom plates and a fire rated rigid backing have been installed.
Friction fit insulation between and parallel to the framing members in full contact with rigid backing top and bottom plates.
There are as many ways to do it as there are people who do it.
Sheathe the kneewall during framing.
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One of the best methods is rigid foam board with sealed joints over insulation in the knee wall.
Keep the attic outside insulating a kneewall and the attic floor behind the kneewall is possible but tricky.
Seal all the edges penetrations and seams.
Here are the three keys to sheathing an attic kneewall properly.
The disadvantage is that it would be very very hard to reach the desired r value from r 49 to r 60 depending on climate for attic insulation.
Bring the attic inside insulating between the rafters simplifies the air sealing details.
Now that the kneewall is.
This is not very easy in most of the older houses we work on because the hatches and attics are too small to get rigid foam board through.
Cover the whole wall.
Verify insulation material is the correct width and r value.
7 staple the edges of the facing into each of the studs.