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Below Deck Spray Foam Insulation for Existing Roofs

Scope

Insulate an attic in an existing home by installing spray foam (open cell or closed cell) on the underside of the roof deck. This measure will convert a vented attic to a nonvented attic.  The installation steps are as follows:

Inspect the existing roof shingles or roofing membrane for any deficiencies.  If there is any history or evidence of leakage at the roof, this must be corrected since the assembly depends on the existing roof for the water control function.  If the roof is at or near the end of its service life, roof replacement should be considered to ensure acceptable water control.

Install spray foam insulation (and additional loose-fill insulation if desired) in the roof cavity to levels that meet or exceed the current adopted building and energy codes.

Install thermal or ignition barrier (coating, gypsum board, or other material) over spray foam insulation as required by code.

See the following Building America Solution Center guides for more information:

See the U.S. Department of Energy’s Standard Work Specifications (SWS) for more on installing spray polyurethane foam in unvented attics

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

Many older homes have little or no attic insulation. Attic insulation can be installed on the attic floor if the attic will be a vented attic. Or insulation can be installed along the underside of the roof deck, which converts the attic to an unvented, sealed, conditioned attic that can provide a protected environment for HVAC equipment and storage.

An unvented attic assembly should be considered if the attic will be used for habitable space, if the attic contains mechanical equipment or ductwork, if the roof structure is complex (i.e., difficult to vent due to obstructions), and/or if the ceiling plane is difficult to air seal (complex geometry or difficult access).  It is possible create a vented assembly with insulation at the roof deck and spray foam, but this additional step adds cost and is not required for moisture-safe performance.

If the roof has a low slope, there may not be sufficient space for the required insulation and ventilation space needed for a vented attic assembly, especially at the eaves, where insufficient insulation can contribute to ice damming. Therefore, low roofs are good candidates for conversion to an unvented attic with insulation along the roof line.

This unvented attic retrofit assembly applies all of the insulation to the interior side of the roof.  Most roof cladding and water control materials (asphalt shingles, asphalt papers, self-adhered roof membranes, etc.) are vapor impermeable.  In cold and mixed climates, this is effectively a vapor barrier on the “wrong side” of the assembly.  Therefore, to address interstitial (within-the-cavity) condensation risks, special attention should be paid to the type and the levels of insulation in the roof assembly.

The building codes (e.g., §R806.4 in 2009 IRC) specify a minimum R-value requirement for “air impermeable” insulation in a roof assembly, such as rigid insulation or spray foam, to control wintertime condensation.  In colder climate zones, the amount of air- and vapor-impermeable insulation required to control condensation increases.  If using a hybrid approach, it is important to meet or exceed the required ratio of air- and vapor- impermeable insulation to air- and vapor-permeable insulation within the roof assembly.

For this assembly, replacement of the roofing is not required (unless it is at end of service life), but it is vital that the existing roof system provides robust protection from bulk water (precipitation), and that proper flashing is in place.  The low permeance of the roof exterior (cladding and water control layer) combined with the reduced drying due to spray foam means that the sheathing is more vulnerable to damage due to bulk water penetration.

In Figures 1 through 3, the roof cladding is represented as shingles, but other roof claddings would be acceptable provided that the attachment of the roof cladding does not result in horizontal obstructions on the water control layer beneath the cladding.

Sloped roof with cavity spray foam insulation sprayed on underside of roof deck and covered with sprayed-on thermal or ignition barrier coating
Figure 1. Sloped roof with cavity spray foam insulation sprayed on underside of roof deck and covered with sprayed-on thermal or ignition barrier coating. 

Sloped roof with cavity spray foam insulation, strapping, and gypsum board thermal barrier
Figure 2. Sloped roof with cavity spray foam insulation, strapping, and gypsum board thermal barrier. 

lat roof with cavity spray foam plus loose-fill insulation and gypsum board thermal barrier
Figure 3. Flat roof with cavity spray foam plus loose-fill insulation and gypsum board thermal barrier. 

How to Insulate a Roof from the Interior

  1. Inspect the integrity of the roof system (roofing membrane or shingles).  Check for any deficiencies, water damage, active leaks, etc.  Proceed only if needed repairs are performed.
  2. If there is an interior finish at the roofline (cathedral ceiling), remove the interior finish (plaster or gypsum board).  Check the roof framing for any deficiencies, rot, water damage, active leaks, insect damage, etc.  Proceed only if needed repairs are performed.  Based on the findings, revise the roof assembly and review specific detailing as needed.  Follow the minimum requirements of the current adopted building code regarding the wood roof framing construction.
  3. Block the soffit vents; a typical detail is to provide 2x blocking or extended wall sheathing at the exterior wall line, to provide a substrate for the spray foam (see WM354 Roof Eave Upgrade).
  4. Apply spray foam at the underside of the roof sheathing and at the wall perimeter to create an air barrier connecting the wall to the roof, and to provide adequate thermal resistance to prevent condensation.  All attic gable end walls now separate interior from exterior conditions and must be insulated and air sealed.  A typical approach is to insulate the gable end walls with the same spray foam used at the roofline. The area should be free of debris and dust prior to spraying for adequate adhesion.  Install loose-fill insulation with netting over a layer of spray foam in wall cavities if desired.
  5. Install a spray-on ignition barrier or gypsum board thermal barrier over the spray foam as required by code. (See AY-126 Thermal and Ignition Barriers For The SPF Industry).   

Ensuring Success

Inspect the existing roof system, including the roofing membrane or shingles and framing, for any deficiencies and make any corrections if necessary. Consider roof replacement if roof is near end of service life.

Monitor the moisture content of the roof sheathing, when possible, during the construction process. Take measurements before the installation of the air-impermeable insulation to help ensure that the roof deck is dry enough to be covered with spray-foam insulation.

Spray foam is a material that is essentially “manufactured” when applied at the building site. Given the importance of this material’s performance, quality control measures should be set in place. Some key issues include moisture content and temperature of the substrate, applied spray foam layer or “lift” thickness, ratios of the two spray foam components during application, and storage/handling of spray foam components.  Further information is available at the Spray Polyurethane Foam Alliance (http://www.sprayfoam.org/)

Apply insulation to a debris- and dust-free surface to provide adequate thermal resistance to prevent condensation.

Apply insulation to the levels specified in the current adopted building and energy codes.

Given the increased airtightness associated with this retrofit, combustion safety and controlled mechanical ventilation upgrades are required to maintain acceptable indoor air quality.

Climate

The roof assembly should be designed for a specific hygrothermal region, rain exposure zone, and 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 design should be based on the minimum requirements for the currently adopted building code and energy code.  The table below provides the minimum thermal resistance (R-value) requirements specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b) based on climate zone for roof assemblies.

Attic Insulation Requirements per the 2009 and 2012 IECC
Table 1. Attic Insulation Requirements per the 2009 and 2012 IECC

It is important to maintain a sufficient ratio of air- and vapor-impermeable insulation to air- and vapor-permeable insulation within the roof assembly. In colder climate zones, the amount of air- and vapor-impermeable insulation needed to control condensation increases. The table below provides information on minimum levels of air-impermeable insulation for condensation control specified in Table R806.4 Insulation for Condensation Control of the 2009 IRC (ICC 2009a) and Table R806.5 Insulation for Condensation Control of the 2012 IRC (2012a).  For further explanation, see IRC FAQ: Conditioned Attics.

Insulation required for Condensation Control per the 2009 and 2012 IRC.
Table 2. Insulation required for Condensation Control per the 2009 and 2012 IRC.

Where open-cell spray foam is used, the attic space must be “conditioned” by supplying 50 cfm of air from the occupiable space for every 1000 sq.ft. of attic area in climate zones 1, 2, 3, and 4.  This is done to control humidity build-up in the attic spaces (see BSI-077: Cool Hand Luke Meets Attics).

Training

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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 (Ver 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.

(3) Insulation levels in a home shall meet or exceed the component insulation requirements in the 2009 IECC - Table 402.1.1. The following exceptions apply:

      1. Steel-frame ceilings, walls, and floors shall meet the insulation requirements of the 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 inch on center. This exception shall not apply if the alternative calculations in "d" below are used;
      2. 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;
      3. 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 square feet or 20% of the total insulated ceiling area, whichever is less. This exemption shall not apply if the alternative calculations in "d" are used;
      4. 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 insulation levels of all non-fenestration components (i.e., ceilings, walls, floors, and slabs) can be traded off using the UA approach under both the Prescriptive and the Performance Path. Note that fenestration products (i.e., windows, skylights, doors) shall not be included in this calculation. Also, note that while ceiling and slab insulation can be included in trade-off calculations, the R-value must meet or exceed the minimum values listed in Items 4.1 through 4.3 of the ENERGY STAR Checklist to provide an effective thermal break, 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.

 The ENERGY STAR Water Management Checklist specifies:

Water-Managed Roof Assembly

3.1 Step and kick-out flashing at all roof-wall intersections, extending ≥ 4” on wall surface above roof deck and integrated with drainage plane above.

3.2 For homes that don’t have a slab-on-grade foundation and do have expansive or collapsible soils, gutters & downspouts provided that empty to lateral piping that deposits water on sloping final grade ≥ 5 ft. from foundation or to underground catchment system ≥ 10 ft. from foundation.

3.3 Self-sealing bituminous membrane or equivalent at all valleys & roof deck penetrations.

3.4 In 2009 IECC Climate Zones 5 and higher, self-sealing bituminous membrane or equivalent over sheathing at eaves from the edge of the roof line to > 2 ft. up roof deck from the interior plane of the exterior wall.

DOE Zero Energy Ready Home

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 101.4.3 Additions, alterations, renovations or repairs.  Portions of an existing building that are altered in the course of additions, alterations, renovations or repairs must be brought into conformance with the code with the following exceptions applicable to attic/roof retrofit: existing ceiling wall or floor cavities that are exposed provided the cavities exposed are filled with insulation; addition, alteration, renovation or repair projects that do not expose the existing roof, wall or floor cavity; reroofing that does not expose the insulation nor the sheathing.

Section 101.4.5 Change in space conditioning.  This section states that spaces must be brought into full compliance with the new construction requirements if the addition, alteration, renovation or repair changes that space from unconditioned to conditioned space.

Section 402 Building Thermal Envelope. Table 402.1.1 indicates the prescriptive requirements for building enclosure components.

Section 402.4 Air Leakage.  This section indicates that the building thermal envelope (as it is called in the IECC) must be sealed to limit infiltration and that it must be sealed in a manner that is durable allowing for differential expansion and contraction.

2012, 2015, and 2018 IECC

Section R101.4.3 (R501.1.1/R503.1.1 in 2015 and 2018 IECC) Additions, alterations, renovations or repairs.  Portions of an existing building that are altered in the course of additions, alterations, renovations or repairs must be brought into conformance with the code with the following exceptions applicable to attic/roof retrofit: existing ceiling wall or floor cavities that are exposed provided the cavities exposed are filled with insulation; addition, alteration, renovation or repair projects that do not expose the existing roof, wall or floor cavity; reroofing that does not expose the insulation nor the sheathing.

Section R101.4.5 (R503.2 in 2015 and 2018 IECC) Change in space conditioning.  This section states that spaces must be brought into full compliance with the new construction requirements if the addition, alteration, renovation or repair changes that space from unconditioned to conditioned space.

Section R402 (R402.1.2 in 2015 and 2018 IECC) Building Thermal Envelope. Table R402.1.1 indicates the prescriptive requirements for building enclosure components.

Section R402.2.1 Ceilings with attic spaces. This section indicates that the prescriptive requirement for R-38 ceiling insulations is deemed to be met by R-30 insulation when the R-30 insulation extends over the wall top plate at eaves and when the insulation is at full loft and uncompressed over the wall top plate at eaves.  Similarly, R-38 insulation is recognized to satisfy the requirement for R-49 insulation when R-38 insulation extends over the wall top plate at eaves and when the insulation is at full loft and uncompressed over the wall top plate at eaves.

Section R402.4 Air Leakage.  This section indicates that the building thermal envelope (as it is called in the IECC) must be sealed to limit infiltration and that it must be sealed in a manner that is durable allowing for differential expansion and contraction.

2009 IRC

R316.4 Thermal barrier. This section addresses the thermal barrier requirements when foam plastic is used and its installation.

R316.5.3 Attics. This section lists the exceptions to the use of a thermal barrier, including ignition barrier options for foam plastic insulation.

Section R806.4 Unvented attic assemblies.  This section outlines the conditions for unvented attic/roof assemblies.  Note that table R806.4 indicates the amount of insulation above the roof deck or air impermeable insulation below the roof deck required for condensation control assuming minimum required total insulation as indicated in Section N1102 Building Thermal Envelope.  Higher R-value assemblies will require a proportionally larger amount of air impermeable insulation below the roof deck or insulation above the roof deck for condensation control.

Section R807.1 Attic access.  An attic access is required where the ceiling or roof construction is combustible and where the attic area is more than 30 sf and the height between the ceiling framing and roof framing is more than 30”.  Refer to specific language of this section for required dimensions of the access.

Section R901 Roof Assemblies.  This section outlines the design, materials, construction and quality of roof assemblies.

Section N1102 Building Thermal Envelope. Table N1102.1 indicates the prescriptive requirements for building enclosure components.

Section N1102.4 Air Leakage.  This section indicates that the building thermal envelope (as it is called in the IRC) must be sealed to limit infiltration and that it must be sealed in a manner that is durable allowing for differential expansion and contraction.

2012, 2015, and 2018 IRC

R316.4 Thermal barrier. This section addresses the thermal barrier requirements when foam plastic is used and its installation.

R316.5.3 Attics. This section lists the exceptions to the use of a thermal barrier, including ignition barrier options for foam plastic insulation.

Section R806.5 Unvented attic assemblies.  This section outlines the conditions for unvented attic/roof assemblies.  Note that table R806.5 indicates the amount of insulation above the roof deck or air impermeable insulation below the roof deck required for condensation control assuming minimum required total insulation as indicated in Section N1102 Building Thermal Envelope.  Higher R-value assemblies will require a proportionally larger amount of air impermeable insulation below the roof deck or insulation above the roof deck for condensation control.

Section R807.1 Attic access.  An attic access is required where the ceiling or roof construction is combustible and where the attic area is more than 30 sf and the height between the ceiling framing and roof framing is more than 30”.  Refer to specific language of this section for required dimensions of the access.

Section R901 Roof Assemblies.  This section outlines the design, materials, construction and quality of roof assemblies.

Section N1102 Building Thermal Envelope. Table N1102.1.1 (N1102.1.2 in 2015 and 2018 IRC) indicates the prescriptive requirements for building enclosure components.

Section N1102.4 Air Leakage.  This section indicates that the building thermal envelope (as it is called in the IRC) must be sealed to limit infiltration and that it must be sealed in a manner that is durable allowing for differential expansion and contraction.

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: April, 2014

    Case study describing the first DOE Zero Energy Ready Home in Georgia that achieved a HERS score of 40 without PV and -10 with PV.

  2. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: November, 2012

    Case study describing a deep energy renovation of a home in Florida.

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

    Case study describing a retrofit project in the cold and very-cold climate zones.

  4. Author(s): BSC
    Organization(s): BSC
    Publication Date: April, 2010

    Case study describing a building project in the marine climate.

  5. Author(s): BSC
    Organization(s): BSC
    Publication Date: November, 2013

    Case study describing research about spray foam roof insulation and moisture management.

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): Lstiburek
    Organization(s): Building Science Corporation
    Publication Date: April, 2014

    Article describing correct ways to apply insulation to existing attics.

  6. Author(s): Building Science Corporation
    Organization(s): Building Science Corporation
    Publication Date: May, 2009

    Report discussing how to create livable space in the attic that meets IRC code requirements by either creating a ventilated roof assembly, or and unvented attic assembly.

  7. 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.

  8. Author(s): Loomis, Pettit
    Organization(s): Building Science Corporation
    Publication Date: May, 2015

    This Measure Guideline provides design and construction information for a deep energy enclosure retrofit solution of a flat roof assembly.

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: 11/12/2015