Looking for more info on the insulation process of a knee wall. I was thinking of insulating the diagonals of the roof line. not what I'm unsure of is, was is the correct type of insulation faced or unfaced for that part of the knee wall?

Thank you for your time

  • This depends on whether or not you already have a moisture barrier. If you do, you should use unfaced insulation so you don't trap moisture between the two barriers.
    – Hari Ganti
    Nov 28 '17 at 19:34

The short answer is that if you're going to randomly add insulation to a wall, adding unfaced insulation is the best answer without more information. The kraft paper facing on insulation works as a vapor barrier and you usually want to avoid trapping moisture in a wall cavity.

If this is in unconditioned space, then definitely use unfaced. If this is in conditioned space, then post a picture of the wall (or describe the wall assembly) and tell us where you live and we can give you more specific recommendations.


If you’re asking about “faced” or “unfaced”, then there is no interior finish on the wall. Otherwise, you couldn’t install batts with the paper facing.

So, if you live in a climate where you are mostly “heating”, I’d use: 1) paper faced batt insulation on the heated side, 2) fasten the paper to the studs, 3) completely fill the stud space with insulation, (i.e.: 4” batts in 4” wall, 6” batts in 6” wall, etc.), and 4) make sure the “attic” is ventilated.

If you live in a climate where you mostly cool, then same thing, except use unfaced.


Do the roof rafters above that slanted ceiling lead to an attic above? This is common in a cape code; I live in one. If so, the answer is NOT to add insulation to those roof rafter bays, though this is common. Many can't pull that insulation out, so they take the fascia off and shove small PVC pipes through those bays to allow air to migrate up. The air from the knee wall attic space needs to escape upwards, allowing fresh air to be pulled in from the soffit vents. Filling those rafter bays with insulation would prevent this from happening.

If those slanted parts are making the room cold, foam insulation board could be installed behind the drywall, or just sandwiched between the existing drywall and another layer.

Attic Venting of Knee Wall in Cape Cod

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.