Insulating Existing Floors over Garage

Scope Images
Rigid foam insulating sheathing installed over an existing garage ceiling with a new gypsum board fire protection layer installed over the foam
Rigid foam insulating sheathing installed over an existing garage ceiling with a new gypsum board fire protection layer installed over the foam
Scope

Upgrade a garage ceiling when a portion of the interior of the house is located directly above the attached garage by adding insulation and air sealing as follows:

  • Install insulating rigid foam sheathing to the underside of the existing garage ceiling, which typically has a finished surface of gypsum board or lath and plaster, or directly to the underside of the ceiling joists if the garage has an unfinished ceiling and the joist cavities are exposed. If the joist cavities are exposed, the layer of insulating sheathing will need to be foil-faced polyisocyanurate or XPS and the sheathing should be detailed to serve as the air control layer with seams taped and edges caulked or taped.
  • Install blocking for reinstallation of existing services and attachments on the ceiling.

The U.S. Department of Energy’s Standard Work Specifications has additional information on insulating floors over garages.

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

Adding insulating rigid foam sheathing to a garage ceiling can improve the comfort of floors above an attached garage and increase air sealing between the garage and the house. The insulating sheathing can be installed over an existing ceiling (Figures 1 to 3) or if there is no ceiling the foam can be attached directly to the ceiling joists.

Either way, a layer of gypsum board must be installed to cover the insulating sheathing for fire protection. If the ceiling cavity lacks insulation, additional blown or batt insulation can be installed prior to installing the insulating sheathing and gypsum board.

Other recommendations for insulating and air sealing a garage ceiling when there is living space above the garage can be found in the guides Floor Above Garage and Air Sealing Attached Garage.

Rigid foam insulating sheathing installed over an existing garage ceiling with a new gypsum board fire protection layer installed over the foam
Figure 1. Rigid foam insulating sheathing installed over an existing garage ceiling with a new gypsum board fire protection layer installed over the foam.
Rigid foam insulating sheathing installed over an existing garage ceiling that is first air sealed with caulk
Figure 2. Rigid foam insulating sheathing installed over an existing garage ceiling that is first air sealed with caulk.
Rigid foam insulating sheathing installed over an existing garage ceiling with retrofits to air seal exterior wall before adding exterior wall insulating sheathing
Figure 3. Rigid foam insulating sheathing installed over an existing garage ceiling with retrofits to air seal exterior wall before adding exterior wall insulating sheathing.

How to Insulate a Garage Ceiling

  1. Inspect the existing garage ceiling.  If ceiling drywall is in place, patch and air seal at seams and around existing penetrations as needed to establish a robust air control layer, as shown in Figures 1 to 3. Blow in loose-fill cavity insulation if desired.
  2. ALTERNATELY, if the ceiling joists are exposed, install either loose-fill with netting, batt, or spray foam cavity insulation.
  3. Install blocking for re-installation of existing services and attachments on the ceiling.
  4. Install insulating sheathing (foil-faced polyisocyanurate, XPS or EPS Type II) as shown in Figures 1 to 3.  Install over existing ceiling gypsum board or if there is not existing gypsum board, then attach directly to ceiling joists. If the ceiling joists are exposed or the existing garage ceiling is removed, the layer of insulating sheathing must be foil-faced polyisocyanurate or XPS and should be installed as the air control layer, so all seams should be taped and sealant should be applied around all wiring and other penetrations
  5. Install 5/8 inch Type X gypsum board or equivalent as the ceiling covering. Mud and tape seams.
Ensuring Success

For a house with an attached garage, air leakage from the garage to the conditioned space of the home through the garage walls or ceiling can draw unwanted pollutants into the home.  Steps should be taken to ensure that the health of the occupants will not be compromised by the retrofit. Airtightness of all walls, ceilings, and doors that are common between the attached garage and the interior of the house should be verified and improved, if needed. Fire protection must be provided as required by the current adopted building code.

Ensure the air control layer of the garage ceiling is continuous.

Maintain a proper ratio of vapor and air impermeable to vapor and air permeable insulation. 

For information on interface of attached garages with a common wall, see Rigid Foam Insulation between Existing House and Garage Walls.

Climate

In house designs where there is living space above the garage, the garage ceiling should be treated as the floor assembly of the rooms above and 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. 

Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18.
Figure 1. Climate Zone Map from IECC 2009, 12, 15, and 18. (Source: 2012 IECC)
 
 
Climate Zone Map from IECC 2021.
Figure 2. Climate Zone Map from IECC 2021. (Source: 2021 IECC)

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
CAD

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, Version 3/3.1 (Rev. 09)

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 and Insulation Installation (RESNET Grade 1). 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.

DOE Zero Energy Ready Home (Revision 07)

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.

2009 International Energy Conservation Code (IECC)

Section 402.4 Air leakage (Mandatory).

Section 402.4.1 Building Thermal Envelope.

Section 402.4.2 Air sealing and insulation.

Section 402.4.5 Recessed lighting.

Table 402.4.2 Air Barrier and Insulation Inspection Component Criteria

20122015, and 2018 IECC

Section R402.4 Air leakage (Mandatory).

Section R402.4.1 Building Thermal Envelope.

Section R402.4.5 Recessed lighting.

Table R402.4.1.1 Air Barrier and Insulation Installation

Retrofit: 

2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 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 International Residential Code (IRC)

Table R302.6 Dwelling/Garage Separation.

Section R302.5 Dwelling/garage opening/penetration protection.

Section R302.6 Dwelling/garage fire separation.

Section R315.2 Where required in existing dwellings.

Table 702.3.5 Minimum thickness and application of gypsum board.

Section N1102.4 Air leakage (Mandatory).

Section N1102.4.1 Building Thermal Envelope.

Section N1102.4.2 Air sealing and insulation.

Table N1102.4.2 Air Barrier and Insulation Inspection Component Criteria

 20122015, and 2018 IRC

Section R302.5 Dwelling/garage opening/penetration protection.

Section R302.6 Dwelling/garage fire separation.

Table R302.6 Dwelling/Garage Separation.

Section R315.3 Where required in existing dwellings. (Location in 2015 and 2018 IRC)

Table 702.3.5 Minimum thickness and application of gypsum board.

Section N1102.4 Air leakage (Mandatory).

Section N1102.4.1 Building Thermal Envelope.

Table N1102.4.1.1 Air Barrier and Insulation Installation

Retrofit:

2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 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.

References and Resources*
Author(s)
Pettit,
Neuhauser,
Gates
Organization(s)
Building Science Corporation
Publication Date
Description
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.
*For non-dated media, such as websites, the date listed is the date accessed.
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

Building Science Measures
Building Science-to-Sales Translator

High-R Floor Insulation = High-Efficiency or Ultra-Efficient Floor Insulation

Image(s)
Technical Description

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.

High-Efficiency or Ultra-Efficient Floor Insulation
Sales Message

High-efficiency floor insulation helps provide added thermal protection. What this means to you is less wasted energy along with enhanced comfort and quiet. Knowing there is one opportunity to optimize performance during construction, wouldn’t you agree it’s a great opportunity to meet or exceed future codes?

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