Insulating Floor Below Kneewall

Any penetration between the conditioned interior air and the knee wall space needs to be sealed tight.

Insulating floor below kneewall.

When insulating a bonus room floor you ll usually be working from below. We prefer cellulose but we use fiberglass sometimes too. Most of the air leakage we tackle is below the sole plate above the floor and in the floor joists directly below the knee wall. In most cases a room with a kneewall has a sloped ceiling that extends from the top of the kneewall to normal ceiling height about 8 feet off the floor.

We are very thorough air sealing these areas with excellent results. Remember to seal air leaks at any electrical boxes in the kneewall and ceiling below the insulated floor. A kneewall is sometimes defined as a short wall under a sloped ceiling a typical kneewall is between 3 and 5 feet high. Below my kneewall floor are can lights and no blocking in the joists so it would be extremely difficult to air seal and insulate below the deck.

Locate the blocking under the kneewalls. A job we finished last week dropped from 4060 to 2150 cfm50 just with knee wall air sealing we were able to get 8 of one wall that had no top plate. I don t believe it would be practical to move the insulation to the roofline. Added bonus the floor of this knee wall leaked too so there is a line of foam just above the floor joist.

Further a perpendicular roofline intersects the kneewall. If the area of the heated bonus room above the garage is smaller than the area of the garage ceiling you ll need to install blocking between the joists to provide an air barrier where the floor insulation stops. An insulated kneewall is effectively an exterior wall so you ll need a layer of rigid foam drywall osb or thermoply on the back side of the kneewall to prevent cold air from degrading the performance of the insulation. My kneewall has gable vents and the roof has no soffit vents.

Remember to seal air leaks at any electrical boxes in the kneewall and ceiling below the insulated floor. An insulated kneewall is effectively an exterior wall so you ll need a layer of rigid foam drywall osb or thermoply on the back side of the kneewall to prevent cold air from degrading the performance of the insulation. Underfloor insulation if you have you an access hatch that allows you to get into the crawlspace void below the floorboards this is a relatively easy process if not you will have to take up. Just like above rough cut a foam block to fit and seal with foam sealant.

The third point is the knee wall and crawlspace floor drywall. Blown insulation in the knee wall and top attics.