Knee Walls
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. When 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. If you want to keep heat in the room (during colder months) though, then adding radiant barrier to the attic side of the knee wall will help the room retain heat in the winter better. If you don’t have foil on the roofline, then you can choose where to install the foil in the crawlspace with the knee wall, depending on what you have in the space.
TIP
This method is the same layers as the Cathedral Ceiling Method - Cold Climates install method, except you don't need to create an air space since the attic is the air space.
How to Install Radiant Barrier on a Knee Wall
Your 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).
Next, 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.
If you don’t have any storage in the crawlspace area outside the knee wall, then you can add AtticFoil™ on the floor of the knee wall area (to prevent heat from the roofline from absorbing into the floor and heating up the living space below that). Otherwise, if you do have storage in the area, you can install AtticFoil™ on the bottom of the roof rafters on the sloping part of the roofline to protect that storage from absorbing heat.
TIP
Adding radiant barrier to your knee walls will help keep the room on the other side cooler in summer and warmer in winter.
For Added Performance of the Wall
For an extra air-tight wall, we recommend two additional materials: adding a layer of foam board and using caulk between layers.
You can use ½” or ¾” foam board BETWEEN the studs and the sheetrock. Roofing nails with a large head to hold the foam board in place before you finish with your sheetrock work well. Basically you will install the foam board like sheetrock and then install the sheetrock using longer drywall screws so you can make it through the foam layer too.
Adding the foam board to the assembly does several things:
- adds R-value
- helps air seal the wall
- reduces thermal bypass
- sound/noise reduction
Finish this optional upgrade by adding a bead of caulk on the face of the studs before the foam layer (or before the sheetrock layer). Be sure to caulk between the bottom plate of the wall to either the concrete slab or the subfloor to reduce air leakage under the wall as well.
You will be amazed at how little heat will get through a wall built using this method.
For similar installation instructions, see Installing Radiant Barrier in an Exterior Wall.
