Upgrade a wall with a floor overhang (or cantilever) on an existing home by adding rigid insulating sheating and or closed-cell spray foam insulation and air sealing as follows:
- Inspect the existing wall and overhang framing for any deficiencies and make any corrections if necessary. Develop specific detailing for insulating the overhang area.
- Provide four control layers that are continuous over the wall and overhang assemblies: water, air, vapor, and thermal.
- Install exterior wall insulation and interior framing cavity insulation to levels that meet or exceed the current adopted building and energy codes.
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.
When upgrading the wall insulation in an existing home (for example when replacing siding), carefully inspect at any locations in the home with overhanging (or cantilevered) floors. Cantilevered floors in existing homes are frequently found to be uninsulated and not air sealed, causing cold (or hot) areas in the spaces around these floors.
The underside of the overhanging floor is a part of the home’s thermal boundary; it is essentially a continuation of the wall above and below. All of the protective layers found in the wall should also be present on the underside of this overhanging floor. There should be an air barrier, a vapor control layer, a thermal control layer, and a water control layer. The air, vapor, and thermal layers must be continuous with those layers in the exterior wall above the overhang. The water control layer in the wall must terminate at the lower edge of the wall in a way that directs water away from the building. The overhang itself acts like a roof, protecting the vertical wall below it so the water control layer does not necessarily need to be continuous from the exterior wall above the overhang to the underside of the overhang. However, if the overhang is near ground level, the surface of the underside of the overhang should be resistant to rain water that may splash up from the ground in front of the overhang.
Maintaining continuity of the other building enclosure control functions can be challenging at overhanging floors. Below the overhang, the overhanging floor might connect to an exterior frame wall assembly or to a foundation wall. As with roof-wall transitions, effective continuity of the air control layer will rely on careful implementation and an understanding of the many connections that must be made airtight in order to connect the air control layer of the wall above the overhang to that of the overhang itself and to the wall or foundation beneath the overhang. For more information on air sealing and insulating these complex building components, see the guide Cantilevered Floor.
The underside of the overhanging floor might be covered with exterior wall cladding or might be left unfinished. If it is not covered with siding, some other solid material must be provided as means of critter protection.
Figures 1 to 5 show overhang air sealing and insulating details that will be applicable to many but not all overhanging floor conditions. Figure 1 shows a wall-to-overhanging floor transition at the outside corner, that is insulated using rigid insulating sheathing. Figure 2 shows a wall-to-overhanging floor transition at the outside corner, where the overhanging floor is insulated using closed-cell spray foam insulation, which also acts as the air control layer. Figure 3 shows a wall-to-overhanging floor transition with a beam at the outside corner, insulated using rigid insulating sheathing and with the water control layer in front of the rigid insulating sheathing. Figure 4 is a variation of Figure 3 with the water control layer behind the insulating sheathing. Figure 5 shows a wall-to-overhanging floor transition at the inside corner insulated using rigid insulating sheathing. Figure 6 shows a foundation wall-to-overhanging floor transition at the inside corner, where the overhanging floor is insulated using closed-cell spray foam.
The details in Figures 1 to 5 show retrofit situations where exterior rigid insulating sheathing has been added to the exterior vertical walls. Most of the air sealing and insulation details shown would be applicable to overhang retrofits even if rigid foam insulation were not added to the exterior vertical walls.
How to Insulate a Floor Overhang with Rigid Foam Sheathing
- Inspect the structural integrity of the wall and the overhang. Check the framing for any deficiencies, rot, insect damage, etc. Only proceed if any needed repairs are performed. Based on the findings, revise the wall and overhang assembly and review specific detailing as needed. Follow the minimum requirements of the current adopted building and energy codes.
- Perform any needed air sealing of the floor joist cavities. Block and air seal any floor joist bays that are open from the cantilevered floor to the building interior.
- Install rigid foam insulating sheathing at the underside of the overhang (See Figures 1, 3, 4 and 5). Alternately, the floor joist cavities can be filled with closed-cell spray foam.
- Install a transition membrane from the existing wall sheathing onto the face of the overhang insulating sheathing. Tape the top edge of the air control layer at the inside corner of the overhang (See Figure 5).
- Install the metal wall base flashing (See Figures 1 to 4). The flashing should slope down and away from the wall. Install the transition membrane from the face of the overhang insulating sheathing to the wall air control layer.
- Lap the air control layer at the existing wall sheathing over the transition membrane and over the top edge of the wall base flashing (Figures 1 and 2).
- Install insulating sheathing on the wall (Figures 1 to 5). Wrap insect screen around the bottom of the bottom layer of the insulating sheathing and to the face of the furring strips.
- Install a fully adhered water control membrane at the intersection of the overhang insulating sheathing and wall insulating sheathing (Figure 5).
- Attach the furring strips through the insulating sheathing to the structure of the house.
- Attach the cladding to the furring strips.
Inspect the existing wall and overhang framing for any deficiencies and make any corrections if necessary.
Ensure the thermal, air, vapor, and water control layers of the roof and wall assemblies are continuous.
Ensure proper lapping of building layers to shed water away from the wall and overhang assemblies.
The building assemblies should be designed for a specific hygrothermal region, rain exposure zone, and interior climate.
The map in Figure 1 shows the climate zones for states that have adopted energy codes equivalent to the International Energy Conservation Code (IECC) 2009, 12, 15, and 18. The map in Figure 2 shows the climate zones for states that have adopted energy codes equivalent to the IECC 2021. Climate zone-specific requirements specified in the IECC are shown in the Compliance Tab of this guide.
The insulation levels should be based on the minimum requirements for vapor control in the current adopted building code and the minimum requirements for thermal control in the current energy code. Additional insulation can be added above these minimums to create high R-value wall and floor assemblies. The tables below provide the minimum thermal resistance (R-value) requirements for framed walls and floors specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b), based on climate zone.
Guidance for the ratios of permeable to impermeable insulation for vapor control can be found in Table 3. Table 3 presents information taken from Table R601.3.1 Class III Vapor Retarders of the 2009 IRC (ICC 2009a) and Table R702.7.1 Class III Vapor Retarders of the 2012 IRC (ICC 2009b). It shows minimum thermal resistance values to control condensation using exterior insulating sheathing for Climate Zones 5, 6, 7, 8, and Marine 4. The percentage of the total insulation that is exterior insulation (e.g., insulating sheathing/spray foam insulation) is also shown in the table. These percentages can be used to calculate R-value ratios for ccSPF vs. fibrous fill in “hybrid” double stud walls.
The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
ENERGY STAR Certified Homes requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the 2009 International Energy Conservation Code (IECC) with some alternatives and exceptions, and achieve Grade 1 installation per RESNET Standards (see 2009 and 2012 IECC Code Level Insulation – ENERGY STAR Requirements, Insulation Installation (RESNET Grade 1) and Insulation Installation (RESENT Grade 2). If the state or local residential building energy code requires higher insulation levels than those specified in the 2009 IECC, you must meet or exceed the locally mandated requirements.
Rater Design Review Checklist
3. High-Performance Insulation.
3.1 Specified ceiling, wall, floor, and slab insulation levels comply with one of the following options:
3.1.1 Meets or exceeds 2009 IECC levels4, 5, 6 OR;
3.1.2 Achieves ≤ 133% of the total UA resulting from the U-factors in 2009 IECC Table 402.1.3, per guidance in Footnote 4d, AND specified home infiltration does not exceed the following:5, 6
- 3 ACH50 in CZs 1, 2
- 2.5 ACH50 in CZs 3, 4
- 2 ACH50 in CZs 5, 6, 7
- 1.5 ACH50 in CZ 8
Footnote 4) Specified levels shall meet or exceed the component insulation levels in 2009 IECC Table 402.1.1. The following exceptions apply:
a. Steel-frame ceilings, walls, and floors shall meet the insulation levels of 2009 IECC Table 402.2.5. In CZ 1 and 2, the continuous insulation requirements in this table shall be permitted to be reduced to R-3 for steel-frame wall assemblies with studs spaced at 24 in. on center. This exception shall not apply if the alternative calculations in d) are used;
b. For ceilings with attic spaces, R-30 shall satisfy the requirement for R-38 and R-38 shall satisfy the requirement for R-49 wherever the full height of uncompressed insulation at the lower R-value extends over the wall top plate at the eaves. This exemption shall not apply if the alternative calculations in d) are used;
c. For ceilings without attic spaces, R-30 shall satisfy the requirement for any required value above R-30 if the design of the roof / ceiling assembly does not provide sufficient space for the required insulation value. This exemption shall be limited to 500 sq. ft. or 20% of the total insulated ceiling area, whichever is less. This exemption shall not apply if the alternative calculations in d) are used;
d. An alternative equivalent U-factor or total UA calculation may also be used to demonstrate compliance, as follows: An assembly with a U-factor equal or less than specified in 2009 IECC Table 402.1.3 complies. A total building thermal envelope UA that is less than or equal to the total UA resulting from the U-factors in Table 402.1.3 also complies. The performance of all components (i.e., ceilings, walls, floors, slabs, and fenestration) can be traded off using the UA approach. Note that Items 3.1 through 3.3 of the National Rater Field Checklist shall be met regardless of the UA tradeoffs calculated. The UA calculation shall be done using a method consistent with the ASHRAE Handbook of Fundamentals and shall include the thermal bridging effects of framing materials. The calculation for a steel-frame envelope assembly shall use the ASHRAE zone method or a method providing equivalent results, and not a series-parallel path calculation method.
Rater Field Checklist
1. High-Performance Fenestration & Insulation
1.3 All insulation achieves Grade I install. per ANSI / RESNET / ICC Std. 301. Alternatives in Footnote 4.4, 5
2. Fully-Aligned Air Barriers 6 At each insulated location below, a complete air barrier is provided that is fully aligned as follows:
Ceilings: At interior or exterior horizontal surface of ceiling insulation in Climate Zones 1-3; at interior horizontal surface of ceiling insulation in Climate Zones 4-8. Also, at exterior vertical surface of ceiling insulation in all climate zones (e.g., using a wind baffle that extends to the full height of the insulation in every bay or a tabbed baffle in each bay with a soffit vent that prevents wind washing in adjacent bays).7
Walls: At exterior vertical surface of wall insulation in all climate zones; also at interior vertical surface of wall insulation in Climate Zones 4-8.8
Floors: At exterior vertical surface of floor insulation in all climate zones and, if over unconditioned space, also at interior horizontal surface including supports to ensure alignment. Alternatives in Footnotes 11 & 12.10, 11, 12
2.6 Floors above garages, floors above unconditioned basements or crawlspaces, and cantilevered floors.
Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.
The DOE Zero Energy Ready Home Program is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy. Builders and remodelers who are performing retrofits on existing homes are welcome to seek certification for those homes through this voluntary program.
Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 2, Item 2) Ceiling, wall, floor, and slab insulation shall meet or exceed 2015 IECC levels and achieve Grade 1 installation, per RESNET standards. See the guide 2015 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.
The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC, can be found in this table.
Section R402 Building Thermal Envelope.
Table R402.1.2 (R402.1.1 in 2012 and 2009 IECC) Insulation and Fenestration Requirements by Component.
Section R402.2.7 (R402.2.8 in 2015 and 2018 IECC and R402.2.6 in 2009 IECC ) Floors.
Section R402.4 Air leakage (Mandatory).
Section R402.4.1 Building thermal envelope.
Section R101.4.3 (in 2009 and 2012). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
Section R317 Protection of Wood and Wood Based Products Against Decay.
Section R317.1. Location Required.
Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)
Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.
Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
There are two levels of floor insulation: high-efficiency insulation, which meets the 2015 International Energy Conservation Code, and ultra-efficient insulation, which is 25% more efficient than this national code. Using high-efficiency and ultra-efficient insulation along with professional installation (e.g., no gaps, voids, compression, or misalignment with air barriers; complete air barriers; and minimal thermal bridging) creates conditioned spaces that require very little heating and cooling, along with even comfort and quiet throughout the house.