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Insulating and Air Sealing Existing Floors above Unconditioned Spaces

Scope

Install rigid insulation and finish material below the framing and cavity insulation of a building overhang
Install rigid insulation and finish material below the framing and cavity insulation of a building overhang

Insulate a floor over unconditioned space with conditioned space above (such as a cantilevered floor, the second floor overhang of a garrison colonial house, the underside of a projecting bay, the ceiling of an inset porch, or the ceiling of a vented crawlspace) by adding insulation and air sealing as follows:

  • Inspect the overhanging floor framing to verify existing conditions and develop specific detailing for insulating the overhanging area.
  • Provide four control layers that are continuous between the overhanging floor and wall assemblies: water, air, vapor, and thermal.
  • Install insulation to levels that meet or exceed the thermal requirements of 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

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.

Rigid Insulation is installed below the framing and cavity insulation of a building overhang or cantilever then covered with finish material
Figure 1. Rigid Insulation is installed below the framing and cavity insulation of a building overhang or cantilever then covered with finish material.

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).

The ceiling over an unconditioned space is insulated with closed-cell spray foam that fills the ceiling cavities and encapsulates the framing
Figure 2. The ceiling over an unconditioned space is insulated with closed-cell spray foam that fills the ceiling cavities and encapsulates the framing.

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.
Foil-faced polyisocyanurate insulating rigid foam sheathing is installed below the floor framing of this house built on piers
Figure 3. Foil-faced polyisocyanurate insulating rigid foam sheathing is installed below the floor framing of this house built on piers. (Image courtesy of Building Science Corporation)

Spray foam insulation air seals the underside of the flooring of a home built on piers
Figure 4. Spray foam insulation air seals the underside of the flooring of a home built on piers. (Image courtesy of Building Science Corporation)

Application of intumescent coating/protection layer
Figure 5. Application of intumescent coating/ protection layer. (Image courtesy of Building Science Corporation)

Ensuring Success

No information at this time.

Climate

The floor assembly 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 zones
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. (See 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). Additional insulation can be added above these minimums to create high R-Value floor assemblies. The table below provides the minimum thermal resistance (R-value) requirements for framed floors specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b), based on climate zone.

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

Training

Right and Wrong Images

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Presentations

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Videos

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CAD Images

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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

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. 08) 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 08) 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 7
  • 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 - 2018 IECC and IRC Minimum Insulation Requirements: The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, and 2018 IECC and IRC, can be found in this table.  

2009 IECC

Section R402 Building Thermal Envelope.

Table R402.1.1 Insulation and Fenestration Requirements by Component.

Section R402.2.6 Floors.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

2012, 2015, and 2018 IECC

Section 402 Building Thermal Envelope.

Table R402.1.1 (R402.1.2 in 2015 and 2018 IECC) Insulation and Fenestration Requirements by Component. 

Section R402.2.7 (R402.2.8 in 2015 and 2018 IECC) Floors.

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building thermal envelope.

Retrofit: 2009, 2012, 2015, 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.)

2009, 2012, 2015, and 2018 IRC

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

Section R317.1. Location Required.

Retrofit: 2009, 2012, 2015, 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

  1. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: December, 2011

    Case study about a deep energy renovation of a home in the marine climate.

  2. Author(s): BSC
    Organization(s): BSC
    Publication Date: February, 2009

    Case study describing a building project in the hot-humid climate zone.

  3. Author(s): BSC
    Organization(s): BSC
    Publication Date: January, 2010

    Case study describing a building project in the hot-humid climate zone.

  4. Author(s): BSC
    Organization(s): BSC
    Publication Date: April, 2009

    Case study describing a building project in the hot-humid climate zone.

  5. Author(s): BSC
    Organization(s): BSC
    Publication Date: July, 2010

    Case study describing a building project in the hot-humid climate zone.

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: 12/15/2015