Cantilevered floors and floors over unconditioned space in existing homes are frequently found to be uninsulated and not air sealed, causing cold (or hot) areas in the rooms above these floors. Configurations where these overhangs occur include the second-floor overhang of a garrison colonial style house, the underside of a projecting bay, the ceiling of an inset porch, or the ceiling of a vented crawlspace with conditioned space above. There are several methods for air sealing and insulating these types of building configurations. Regardless of which method is used, any seams in the “ceiling” of the cantilever framing cavities should be air sealed before the cavities are insulated and any open floor joist bays that extend from the cantilever into the home must be blocked with some solid blocking material such as rigid foam or plywood and air sealed at the edges. For more information about air sealing cantilevered floors see the guide Cantilevered Floor.

Rigid Insulation below Framing and Cavity Insulation, Finish Material to Underside of Overhang

Figure 1 shows an assembly that is typical for a porch ceiling under a second-story room or the overhang of the second floor of a garrison colonial style home. Any existing finishing material such as plywood is removed from the underside of the overhang framing to minimize the change in elevation of the overhang. The framing cavities are then filled with batt or blown fibrous insulation and rigid foam is attached to the bottoms of the framing joists. Seams in the rigid foam are sealed with tape and a new finish material is installed to cover the rigid foam. This finish material might be fiber cement or engineered wood siding or painted plywood or OSB.

Closed-Cell Spray Foam Insulation in Cavities and Encapsulating Framing

The approach shown in Figure 2 is typical under a first-floor bay or for a vented crawlspace with conditioned space above. The framing is encapsulated with spray foam to decouple the framing from the ground or crawlspace thermal and moisture conditions. The standoffs may be replaced with 2x2 furring if needed for attaching the protection layer (e.g., cement board).

How to Insulate an Overhanging Floor Above Unconditioned Space

1. Inspect the structural integrity of the overhanging floor. Check the framing for any deficiencies, rot, insect damage, etc. Proceed only after needed repairs are performed. Based on the findings, revise the floor assembly and review specific detailing as needed. Follow the minimum requirements of the current adopted building and energy codes.
2. Air seal the joist cavities of the overhanging floor. Block any open bay that extend into the home’s conditioned space, for example that connect to the space between the first and second floors. Use a solid blocking material to block the bays and air seal around edges as described in the guide Cantilevered Floor.
3. Install either loose-fill, batt, or spray foam insulation in the ceiling cavities, then install polyisocyanurate or XPS insulating sheathing to the underside of the framing. Tape the seams of the insulating sheathing to create a robust air control layer, as shown in Figure 1 and Figure 3. ALTERNATELY, install closed-cell spray foam insulation in the cavities and encapsulate the framing, as shown in Figure 2 and Figure 4.
4. Install a protective layer of cement board or plywood over the insulating sheathing. ALTERNATELY, apply intumescent coating or other thermal ignition barrier over spray foam if required by code. To protect the spray foam from rodents and/or birds, attach blocking to the existing framing and attach a protective layer of cement board or plywood as shown in Figure 5.