How to Insulate Overhangs and Cantilevered floors

Key considerations:

Overhangs have more surface area exposed to the outdoors relative to other parts of the house and the more surface you have exposed, the faster you loose heat.  Floors exposed directly to the outdoors are prone to air leakage because they are typically covered with just one layer of sheathing.  See also band joists, rim joists, and between floors. 

Possible downsides:

• Pipe freeze ups (see recommendations below)
• Cold and drafty living space 
• Poor energy efficiency
• Cold floors
• Cold interior walls
• Excessive heat loss in ductwork

Recommendations:

Insulate the overhanging portion of the floor to the same level that you would insulate a roof or attic in order to compensate for the additional surface area losses.   Avoid installing plumbing in a cantilevered floor.   If you have to run plumbing out into a cantilevered floor for a bathroom or radiant heating, make sure to build airtight and insulated “boxes” around the plumbing to reduce heat loss and the chance for pipe freeze ups.  Leave the box “open” to the interior side so the plumbing gets the benefit of the warmth of the house.  You want the plumbing to be trapped in the warm side of the cantilevered floor with plenty of R-value below. Where the floor joists pass from outside to inside make sure there is a thermal and air “block” in the plane of the exterior wall at the end of each floor bay.   

1. Overhangs and cantilevered floors are “peninsulas” that jut out from the main house and have higher surface area exposure than other parts of the house.  More surface area to radiate and conduct heat to the outdoors. 
2. The floor bay goes from inside to outside and needs an airtight stop in the plane of the exterior wall to prevent cold air from entering.  If this stop is not complete then...
3. Cold air has easy access to the rest of your house.  Traveling between floors until it finds plumbing, ductwork, interior walls, penetrations, etc.
4. Any plumbing in an overhang should be inside an insulated box.  Insulation-Guide has seen many pipe freezes in this location.

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