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Insulating Existing Floor Overhang

Scope

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.

Description

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.

Retrofit of cantilevered wall showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor
Figure 1. Retrofit of cantilevered wall showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor.

Retrofit of cantilevered wall showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and closed-cell spray foam in the overhanging floor
Figure 2. Retrofit of cantilevered wall showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and closed-cell spray foam in the overhanging floor.

Retrofit of cantilevered wall with beam showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor
Figure 3. Retrofit of cantilevered wall with beam showing details at the outside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor.

Retrofit of cantilevered wall with beam showing details at the outside corner for installing air sealing and rigid foam insulation plus water control membrane in the wall and overhanging floor
Figure 4. Retrofit of cantilevered wall with beam showing details at the outside corner for installing air sealing and rigid foam insulation plus water control membrane in the wall and overhanging floor.

Retrofit of cantilevered wall showing details at the inside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor
Figure 5. Retrofit of cantilevered wall showing details at the inside corner for installing air sealing and rigid foam insulation in the wall and overhanging floor.

Retrofit of cantilevered foundation wall showing details at the inside corner for installing closed-cell spray foam in the wall and overhanging floor
Figure 6. Retrofit of cantilevered foundation wall showing details at the inside corner for installing closed-cell spray foam in the wall and overhanging floor.

How to Insulate a Floor Overhang with Rigid Foam Sheathing

  1. 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.
  2. 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.
  3. 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.
  4. 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).
  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.
  6. 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).
  7. 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.
  8. Install a fully adhered water control membrane at the intersection of the overhang insulating sheathing and wall insulating sheathing (Figure 5).
  9. Attach the furring strips through the insulating sheathing to the structure of the house.
  10. Attach the cladding to the furring strips.

Ensuring Success

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.

Climate

The building assemblies should be designed for a specific hygrothermal region, rain exposure zone, and interior climate. The climate zones are shown on the map below, which is taken from Figure C301.1 of the 2012 IECC.

IECC climate zone map
IECC Climate Zone Map

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.

Framed Wall R-Value Requirements in the 2009 and 2012 IECC
Table 1. Framed Wall R-Value Requirements in the 2009 and 2012 IECC

Framed Floor R-Value Requirements in the 2009 and 2012 IECC
Table 2. Framed Floor R-Value Requirements in the 2009 and 2012 IECC

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.

R-Value Requirements for Vapor Control in the 2009 and 2012 IRC
Table 3. R-Value Requirements for Vapor Control in the 2009 and 2012 IRC

Training

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Presentations

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Videos

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Compliance

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

[Note: Guidance for Version 3.0, Revision 08 is coming soon.]

ENERGY STAR Certified Homes is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy and the U.S. Environmental Protection Agency. Builders and remodelers who are conducting retrofits are welcome to seek certification for existing homes through this voluntary program.

ENERGY STAR Certified Homes (Version 3.0, Rev. 07) requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the 2009 International Energy Conservation Code (IECC).

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. Some states have adopted the 2012 IECC. Visit the U.S. DOE Building Energy Codes Program to see what code has been adopted in each state. For states that have adopted the 2012 IECC or an equivalent code, EPA intends to implement the ENERGY STAR Certified Homes Version 3.1 National Program Requirements for homes permitted starting one year after state-level implementation of the 2012 IECC or an equivalent code. However, EPA will make a final determination of the implementation timeline on a state-by-state basis. Some states and regions of the country have ENERGY STAR requirements that differ from the national requirements. Visit ENERGY STAR’s Regional Specifications page for more information on those region-specific requirements.

The ENERGY STAR Thermal Enclosure System Rater Checklist (Ver 3, Rev 07) specifies

2.1 Ceiling, wall, floor and slab insulation levels shall comply with one of the following options:

2.1.1 Meet or exceed 2009 IECC levels, OR

2.1.2 Achieve <= 133% of the total UA resulting from the U-factors in 2009 IECC Table 402.1.3, excluding fenestration and per guidance in note “d” below, AND home shall achieve <= 50% of the infiltration rate in Exhibit 1 of the National Program Requirements.

2.2 All ceiling, wall, floor, and slab insulation shall achieve RESNET-defined Grade I installation or, alternatively, Grade II for surfaces that contain a layer of continuous, air impermeable insulation ≥ R-3 in Climate Zones 1 to 4, ≥ R-5 in Climate Zones 5 to 8

3 Fully-Aligned Air Barriers. At each insulated location noted below, a complete air barrier shall be provided that is fully aligned with the insulation as follows:

  • At interior or exterior surface of ceilings in Climate Zones 1-3; at interior surface of ceilings in Climate Zones 4-8. Also, include barrier at interior edge of attic eave in all climate zones using a wind baffle that extends to the full height of the insulation. Include a baffle in every bay or a tabbed baffle in each bay with a soffit vent that will also prevent wind washing of insulation in adjacent bays
  • At exterior surface of walls in all climate zones; and also at interior surface of walls for Climate Zones 4-8
  • At interior surface of floors in all climate zones, including supports to ensure permanent contact and blocking at exposed edge

3.2.2 Cantilevered floor

3.2.3 Floor above unconditioned basement or unconditioned crawlspace

DOE Zero Energy Ready Home

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.

The U.S. Department of Energy Zero Energy Ready Home National Program Requirements specify as a mandatory requirement (Exhibit 1, #2.2) that, for all labeled homes, whether prescriptive or performance path, ceiling, wall, floor, and slab insulation shall meet or exceed 2012 IECC levels. See the guide 2012 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

2009 IECC

Section 402 Building Thermal Envelope.

Table R402.1.1 Insulation and Fenestration Requirements by Component.

Section 402.2.6 Floors.

Section 402.4 Air leakage (Mandatory).

Section 402.4.1 Building thermal envelope.

Section 402.4.2 Air sealing and insulation.

2012 IECC

Section R402 Building Thermal Envelope.

Table R402.1.1 Insulation and Fenestration Requirements by Component.

Section R402.2.7 Floors.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

2015 and 2018 IECC

Section R402 Building Thermal Envelope.

Table R402.1.2 Insulation and Fenestration Requirements by Component.

Section R402.2.8 Floors.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

Retrofit: 200920122015, and 2018 IECC

Section R101.4.3 (Section R501.1.1 in 2015 and 2018 IECC). 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.)

200920122015, and 2018 IRC

Section R317 Protection of Wood and Wood Based Products Against Decay.

Section R317.1. Location Required.

Retrofit: 200920122015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 IRC). 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.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

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.

Case Studies

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References and Resources*

  1. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  2. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  3. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  4. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  5. Author(s): Pettit, Neuhauser, Gates
    Organization(s): Building Science Corporation
    Publication Date: July, 2013

    Guidebook providing useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Last Updated: 11/24/2015