Extreme Kneewall Makeover

Cape Cod and other traditional American construction often have attic areas most commonly referred to as knee walls.  The actual knee wall is the short wall (normally 4 or 5 ft) that is between the living area and the attic.  You basically go outside the home once you enter this space, except for the roof that keeps you dry.  This harsh, outside climate is an unfriendly environment for duct work, water pipes, and stored contents.  It is also the culprit for a large part of the home’s air infiltration and energy loss.

There are two solutions to fixing theses areas- both involve spray foam. First spray the attic exterior walls(Gables) at both ends of the knee wall attic space areas.  Be sure to remove any flooring at the bottom plate so the exterior band can be covered with foam.   Both remedies also involve sealing the eave and spraying 6 to 8 inches of open cell foam on the slopes under the roof deck behind the kneewall.  The foam must cover the top plate and seal to the floor below.

If the slope above the knee wall has fiberglass insulation with a baffle or an air space, baffles should be put in the slopes you plan on spraying prior to foam application.  This will still allow air movement to flow to the fiberglass that is under the roof deck above the drywalled slope.  It will also help to ventilate the flat attic areas above the slopes.

If there is no insulation in the slope or it has been removed, foam can fill the entire slope.  You must be sure to completely fill the slope from behind the knee wall or access the attic to fill the slopes from the top.  Your flat attic areas must have other venting for air flow since you have eliminated the air from knee wall attics.  If that flat attic is accessible, you may consider unventing the attic completely by continuing the foam insulation to the ridge.  You must be able to complete the seal to the other side.

These knee wall attic areas can be transformed into great conditioned storage spaces.  It also extends the thermal boundary so the fight against hot and cold comfort issues is further away from the living area.  Your home’s energy efficiency will also be transformed, resulting in lower utility costs.


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