Hello Everyone!
I recently had an energy audit done, and the auditor found some big problems with the knee walls upstairs. This is a Cape in Massachusetts.
I’ve been fixing the problem in the north knee wall by putting R-13 fiberglass in the wall, covering with radiant barrier (Reflectix), air sealing the joists under the kneewall, and putting down R-49 fiberglass (R-30 between the joists, and R-19 layed perpendicular) on the floor. It’s already made a huge difference.
This knee wall runs the entire backside (north) of the house.
I’ve run into an issue though, with about 1/3 of it. The master closet and bathroom side of the knee wall are currently insulated with Tuff-R from the inside. In other words, drywall -> Tuff-R -> studs -> knee wall attic space. Clearly that’s not enough.
Can I just add R-13 in the wall here? Do I lose the radiant barrier if it now has an R-13 batt between it and the knee wall attic space?
How should I do this?
Thanks.
Replies
The radiant barrier effect of the Tuff-R is minimal - probably less than R1.
Unless the aluminum face of the Tuff-R is currently toward the attic, you don't actaully have a radiant barrier.
If the alum. face is toward the inside, contactingt the drywall, the radiant barrier is defeated because it needs an airspace on the heated side to act as a radiant barrier. If the alum. face is toward the attic without anything touching it, then the alum. face will act as a radiant barrier in summer to reflect radiant heat from the hot attic. But it does nothing for you in winter when the warm side is in the room rather than in the attic.
So, if you add insulation on the attic side, and the insulation rests against the alum. face, you lose the radiant barrier, but you will gain far more that you lose by the added insulation.
Any batt insulation that you add on the attic side of the knee wall needs to be air-sealed on all 6 sides. This is because batt insulation allows convection air currents to move quite freely thru it, which reduces its insulating value significantly.