Can I add radiant barrier on a knee wall?
We frequently hear from people who want to know if they can use radiant barrier on the knee walls in their attic space. If the knee walls are sharing space with an attic that already has radiant barrier on the rafters, then the room on the other side of the knee wall is already benefiting from not gaining heat. However, if you are in a cold climate, then adding radiant barrier to the attic side of the knee wall will help the room retain heat in the winter.
In the photo to the right, the customer was doing an over the insulation (attic floor) installation, so they went ahead and ran the foil up the side of the knee wall so the room would not gain any radiant heat from those rafters across it that were not covered with radiant barrier.
How to Insulate Knee Walls
Knee walls that share space with a conditioned room in your home should have a combination of both traditional insulation and foil radiant barrier insulation.
First you will want to add traditional insulation between the studs (this can be batt insulation, spray foam or even rigid foam insulation). Then, you can add AtticFoil® on top of the traditional insulation by stapling it to the wall studs. You essentially want to wrap the outside of the room with the radiant barrier. Then, you will leave the wall exposed to the attic so the radiant barrier is facing an air space, the attic air space.
Another way to install the radiant barrier on a knee wall is to do so via the Cathedral Ceiling Method. This method has you place the foil closest to the inside of the wall cavity with an air gap. Using foam spacers you can attach the foil behind the drywall and then stuff the remainder of the wall cavity with traditional insulation or foam board. The main objective here is to get the foil closest to the drywall while still maintaining the air gap so the foil can work. This is crucial; once the air gap is compromised, the foil will not work as a radiant barrier. This is why we recommend you use spacers so you can ensure the air gap will not collapse and end up closing off.
Ultimately, no matter which method you choose to go with, you need both traditional insulation and a radiant barrier when you are insulating a living space for maximum benefit and comfort.
Seal & Cover As Much As You Can
The best approach for creating an energy-efficient knee wall is to insulate well and make sure it is sealed air tight from the inside of the room.
Sealing it air tight will prevent warm air from leaking out of the room which helps with energy savings, but it also helps prevent potential moisture problems. Remember, foil does not cause moisture, leaking air does.
Something else to consider is if your knee wall room shares a roof line with the roof of the home, chances are the majority of the heat gain into that room is going to come from the rooms’s ceiling (i.e. the roof of the house). The best way to combat that heat gain is to install radiant barrier foil on the roofline of this room as well. You can do this via the Cathedral Ceiling method which will be fairly simple if you have not already installed the sheet rock.