Attic Knee Wall Ventilation

The peak attic is insulated above the bedroom ceiling and has gable end vents at each end.

Attic knee wall ventilation.

Most builders install fiberglass batts between the studs and some type of blown insulation between the floor joists. Filling the space behind the knee walls with insulation helps to lock in your home s thermal energy. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. Install a top and bottom plate or blocking at the top and bottom of all knee wall cavities.

Intake vents located at the lowest part of the roof under the eaves allow cool. Kneewalls short walls under sloped ceilings are common in story and a half homes and in bonus rooms above garages. Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls. Ventilating knee wall attics is challenging.

The problem with many kneewalls is that they have fiberglass batt insulation with nothing covering them on the attic side as shown in the photo below. With that said it s important to allow outdoor air to enter the attic. Install insulation without misalignments compressions gaps or voids in all knee wall cavities. If you can gain access to the triangular attic behind the kneewall it s often possible to inspect the rafter bays to see if the builder included ventilation baffles.

If these are absent they can sometimes be added by sliding them in place above the batts especially if the sloped section of roof is relatively short. I read lots of guides and blogs understanding that roof deck sheathing needs to be ventilated from soffits at bottom edge of roof preferable to ridge or gable end vents. What happens then is that the batts don t make contact with the air barrier the drywall air moves through and around them and they fall out of the attic kneewall. In an ideal world we install soffit vents as intakes make sure the slopes can breathe with ventilation baffles and exhaust through a ridge vent.

Rather than leaving this space empty homeowners can fill it with insulation for additional thermal protection. Old school roof vents which look like rectangles on the roof work pretty well too.