What type of insulation should I use in a knee wall?

Question

We have an unfinished bonus room over the garage on the second floor. A normal door goes to it and same height as rest of the 2nd floors and full sized windows and the house was zoned for the additional room but the original buyers from a few years ago decided to leave it unfinished.

In Southern New Hampshire we are in zone 5 which requires R49 to R60 for uninsulated attic and R13 to R21 for Wall Cavity and R5 to 6 for Wall Insulation Sheeting according to energy code website.

My question is in a knee wall which insulation would be required in the outside wall/roof and above the ceiling?.

Answer 1

As with any answer, you should check your local codes. However, Going with R-30 on top and R-13 on the sides is standard and will give you very good value. If you can, you should plan on putting R-30 into the 'diagonals' too, that is, where the rafters are above the knee-walls but below the ceilings. If you don't have enough room now, you can screw some 2x4s onto the bottom of the rafters to gain more room: it'll be worth it in the end.

Two things to remember: you don't have to or want to insulate away from living space. So, from your diagram, you would insulate vertically up the knee walls, but don't insulate in the area you close off, as you won't gain any value from it. Two, make sure when you insulate the sloped area where the rafters are, make sure you put baffles underneath the plywood before you install the insulation, to leave space for air to flow from the soffit vents up to the ridge vent.