Application of Spray Foam Insulation Under Plywood and OSB Roof Sheathing

Scope

Install spray foam insulation along the underside of the roof deck to create an insulated, unvented attic, which can provide a conditioned space for HVAC equipment that is located in the attic.

DOE Zero Energy Ready Home Notes

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.

The DOE Zero Energy Ready Home National Program Requirements also specify as a mandatory requirement (Exhibit 1, #3) that ducts are located within the home’s thermal and air barrier boundary. See the Compliance tab for exceptions and alternative compliance options.

ENERGY STAR Certified Homes Notes

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. See the guide 2009 IECC Code Level Insulation - ENERGY STAR Requirements for more details.

Description

Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid-1990s. Open-cell spray polyurethane foam or closed-cell spray polyurethane foam (ccSPF) insulation is sprayed along the underside of the roof sheathing to provide a conditioned and insulated attic space that can be durable and efficient in all climate zones.

Moving the air control layer and thermal control layer to the underside of the roof deck has significant advantages in cases where the HVAC system is located in the attic; by locating HVAC equipment and ducts within the thermal envelope of the home, conductive thermal losses are minimized and any losses due to air leaks still contribute to space conditioning (see Figure 1).  Insulating and air sealing at the roof line may also be a more effective means of providing a continuous thermal barrier in house designs that have complex coffered ceilings and numerous holes through the ceiling plane for lights, wiring, etc., which would otherwise make it difficult to achieve the airtightness needed below the insulation layer. In addition it might not be desirable (in hurricane or wildfire areas) or practical (in retrofits) to add roof vents at soffit locations. 

Construction details

Figure 1. Constructing an attic that is unvented and is insulated along the roof line provides a conditioned space for HVAC equipment that is located within the home’s thermal envelope. Reference

When the design choice is made to insulate along the underside of the roof deck, spray foams have advantages over other insulation types because of the ability of spray foams to effectively air seal complex assemblies. The spray foam can also serve as the thermal and vapor control layers in both new and retrofit construction.   

Despite the advantages of using spray foam under the roof deck, there are some potential risks. The primary risks are rainwater leaks, condensation from diffusion and air leakage, and built-in construction moisture. Hygrothermal modeling sponsored by the DOE Building America program and conducted by Building Science Corporation confirmed that even when the roof was modeled with rainfall leaks of up to 1% through the roof sheathing or with initial moisture content of the wood framing and sheathing of up to 18%, roofs insulated with open- or closed-cell spray foam could dry out sufficiently on a seasonal basis. Damage could occur if wood moisture content was above 18%, or if repeated or prolonged leaks above 1% were experienced and if the wood was unable to dry out. However, proper construction techniques, including the following measures, will minimize or eliminate the potential for moisture-related roof damage. (See Grin, Smegal, and Lstiburek 2013 for more details.) 

How to Install Spray Foam on the Underside of the Roof Decking

  1. Make sure the installation complies with the 2012 International Residential Code (2012 IRC; see the Compliance tab for more details). The requirements of the 2012 IRC Section R806.5 “Unvented attic and unvented enclosed rafter assemblies” are summarized here: Unvented attic assemblies and unvented enclosed rafter assemblies are permitted if all the following conditions are met:
    1. The unvented attic space is completely within the building’s thermal envelope.
    2. No interior Class I vapor retarders (e.g., plastic sheeting) are installed on the ceiling side of the unvented attic assembly (i.e., under the ceiling) or on the ceiling side of the unvented enclosed rafter assembly.
    3. Where wood shingles or shakes are used, a vented air space of at least one-quarter inch separates the shingles or shakes from the roof underlayment above the structural sheathing.
    4. In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation is a Class II vapor retarder or has a Class II vapor retarder coating or covering in direct contact with the underside of the insulation. [Note, the 2012 IRC says “or be coated with a Class III vapor retarder.”  “Class III” is a typo which has been corrected to Class II in the 2015 I codes.]
    5. The attic or rafter assembly meets one of the following conditions regarding the air permeability of the insulation directly under the structural roof sheathing.
      1. Air-impermeable insulation only.  Insulation is applied in direct contact with the underside of the structural roof sheathing.
      2. Air-permeable insulation only.  In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation is installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.
      3. Air-impermeable and air-permeable insulation.  The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control.  The air-permeable insulation shall be installed directly under the air-impermeable insulation.
      4. Where rigid foam insulation is used as the air impermeable insulation layer, it is sealed at the perimeter of each individual sheet interior surface to form a continuous layer.
  2. Install a leak-free roof membrane that is fully adhered to the roof sheathing.
  3. Ensure, when using open-cell spray foam, that a low-perm Class II vapor retarder is installed where required.
  4. Refer to the current state and local building codes for the minimum R-value of air-impermeable insulation required for the roof assemblies in your climate.
  5. Inspect the roof assembly to ensure it has proper drainage protection above the roof deck.
  6. Measure the moisture content of the wood prior to applying spray foam insulation to ensure it has dried to below 18% or to the levels recommended by the spray foam manufacturer.
  7. Ensure the weather conditions and temperatures for installing the insulation are as recommended by the spray foam manufacturer.
  8. Clean the surfaces of the roof sheathing and structural members so they are clear of any debris or dust to ensure proper adhesion of the spray foam.
  9. Cover any mechanical and electrical equipment and wiring prior to applying the insulation.
  10. Provide proper ventilation in the work area during application.
  11. Install the spray foam. It is recommended to hire a licensed professional applicator for the spray foam installation.
  12. Visually inspect the insulation installation to ensure that foam consistently meets the specified depth with no gaps or voids.
  13. Refer to the current state and local building codes for definition and requirements for the ignition and thermal barrier as well as vapor diffusion retarder requirements.
  14. Install additional cavity insulation as needed to meet the desired R-value. See Figure 2.

spray foam under the roof

Figure 2. Open- or closed-cell spray foam is applied to the underside of the roof sheathing and additional fiberglass or cellulose insulation is blown in as a cost-saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck. Reference

How to Install Ducts in an Unvented Attic

When installing HVAC equipment in an insulated conditioned attic, good HVAC design principles still apply:

  • Design a compact, duct layout with short, straight ducts runs. Seal and test ductwork for air leakage.
  • Install a balanced ventilation system such as a heat recovery ventilator or central fan-integrated ventilation with a fresh air intake and timered exhaust. (For more information, see Whole-Building Delivered Ventilation.)
  • Do not install low-efficiency heating systems that draw their combustion air from the attic. Instead install direct-vent sealed-combustion furnaces or heat pumps. (For more information, see Combustion Furnaces, Traditional Split Heat Pumps.)

Ensuring Success

Provide for a very high degree of airtightness, preferably with a continuous membrane adhered to the top surface of the structural roof deck, with a layer of rigid insulation over it to provide condensation control.  The spray foam itself can provide the airtightness when it is installed under the roof decking between framing elements; however, all wood-to-wood joints in the framing must still be sealed.

Keep the roof dry before spray foam is installed. A fully adhered roof membrane will protect the roof before cladding is installed and provide a secondary layer of drainage protection.

Air seal the ceiling plane and seal around penetrations through the ceiling plane for ducts, flues, wiring, soffits, etc., to prevent warm moist air from the house from entering the attic space. See the Guide to Attic Air Sealing for more information on combustion safety, ventilation for indoor air quality, and attic ventilation for durability related to insulating an attic.

Ensure the following conditions are met to prevent risk of moisture damage to roofs when installing spray foam under the roof decking:

  • the installation complies with the 2012 International Residential Code
  • a fully adhered leak-free roof membrane is installed,
  • the roof sheathing and framing are dry below 18% before spray foam is installed, and
  • when using open-cell spray foam, a low-perm Class II vapor retarder is installed where required.

These recommendations were based on hygrothermal modeling conducted by Building Science Corporation and sponsored by the DOE Building America program, which modeled the impacts of rainfall leaks or initial wood moisture content on open- or closed-cell spray foam-insulated roofs in several climate zones. Modeling showed that the roofs were able to dry out sufficiently to avoid durability issues if the initial moisture content was under 18% or where rainfall leaks of up to 1% were modeled. Damage could occur if wood moisture content was above 18% or if prolonged leaks above 1% were experienced and if the wood was unable to dry out. However, making sure that wood is dry before initial installation, proper construction techniques including installation of a water-proof roofing membrane, and ensuring that the roof can dry to the inside will minimize or eliminate the potential for moisture-related roof damage. (See Application of Spray Foam Insulation under Plywood and Oriented Strand Board Roof Sheathing for more details.) 

When installing HVAC equipment in an insulated, conditioned attic, design a compact duct layout with short, straight ducts runs. Seal and test ductwork for air leakage. Install a balanced ventilation system. Install high-efficiency direct-vent, sealed-combustion furnaces or heat pumps.

Climate

In cold climates, interior relative humidity can directly affect the sheathing moisture content with open-cell or closed-cell spray-foamed roofs and Building Science Corporation recommends that winter-time relative humidity in homes located in Climate Zones 6, 7, or 8 should be maintained below 35% (Grin, Smegal, and Lstiburek 2013).

Open-cell spray foam should be a Class II vapor retarder or be coated with a Class II vapor retarder in cold climates 5, 6, 7, and 8 (2012 IRC R806.5. Note, the 2012 IRC says “or be coated with a Class III vapor retarder.”  “Class III” is a typo which has been corrected to Class II in the 2015 I codes.) Class I vapor retarders should not be installed on the ceiling side of any spray foam installed on the underside of the roof decking in any climate zone as the vapor retarder will prevent drying to the inside in case of a roof leak.

See the Compliance tab for more information on the IRC’s climate-specific requirements for insulation values and what amount of that insulation should be air impermeable in each climate zone.

IECC Climate Zone Map

Training

Right and Wrong Images

Presentations

None Available

Videos

  1. Application of Spray Foam Insulation Under Plywood and OSB Roof Sheathing
    Publication Date: July, 2015

    Video describing how to air seal attic rafters with spray foam.

CAD Images

None Available

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.

2009 and 2012 IRC

The 2009 and 2012 IRC Section R202 defines vapor retarders class information.  A vapor retarder is defined as “a measure of the ability of a material or assembly to limit the amount of moisture that passes through that material or assembly.”  Vapor retarder classes are defined by the IRC using the desiccant method with Procedure A of ASTM E96. These classes are:

Class I: 0.1 perm or less
Class II: 0.1 perm to 1.0 perm
Class III: 1 perm to 10 perms

The IRC has had information on unvented attics for several editions.  The 2012 IRC Section R806 contains the following requirements, with slight modifications from the 2009 edition, with the most notable addition being identification of vapor retarders by class in R806.5 items 2 and 4. 

R806.5 Unvented attic and unvented enclosed rafter assemblies.  Unvented attic assemblies and unvented enclosed rafter assemblies are permitted if all the following conditions are met:

  1. The unvented attic space is completely contained within the building thermal envelope.
  2. No interior Class I vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.
  3. Where wood shingles or shakes are used, a minimum ¼” (6mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.
  4. In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation shall be a Class II vapor retarder or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation. [Note, the 2012 IRC says “or be coated with a Class III vapor retarder.” “Class III” is a typo that has been corrected to Class II in the 2015 I codes (BSC personal communication 6-27-14).]
  5. Meet one of the following conditions, depending on the air permeability of the insulation directly under the structural roof sheathing.
    1. Air-impermeable insulation only.  Insulation shall be applied in direct contact with the underside of the structural roof sheathing.
    2. Air-permeable insulation only.  In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control.
    3. Air-impermeable and air-permeable insulation.  The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.5 for condensation control.  The air-permeable insulation shall be installed directly under the air-impermeable insulation.
    4. Where preformed insulation board is used as the air impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.

The IRC for Climate Zones 1, 2, 3, or 4 requires that a Class I vapor control layer not be installed on the interior side of the assembly.  This is to prevent inward-driven moisture from being trapped in the wall assembly.  Installing a low-permeance vapor control layer on the interior in a cooling dominated climate can quickly deteriorate the assembly. 

Table N1102.1.1 of the 2012 IRC lists the thermal insulation requirements for each assembly.  A summary of the requirements combining Table R806.5 and Table N1102.1.1 from the 2009 and 2012 IRC editions is shown in Table 1.

2012 IRC Table R806.5 and Table N1102.1.1 R-Values

IRC insulation values

In cold climates it is important to note the ratio of vapor impermeable to vapor permeable R-values.  For cold climates, the air impermeable insulation is maintained at 50% or more of the total R-value of the roof system. This is for condensation control.  When building high-R value roof systems Building Science Corporation recommends that this ratio be maintained or exceeded.  If an R-80 cathedral ceiling or cathedralized attic is to be constructed in a cold climate, it is recommended that a minimum of R-40 (50%) be air impermeable insulation installed and layered according to Section R806.5 of the 2012 IRC.

2009 and 2012 IECC

Total ceiling insulation values are shown by climate zone in Table R402.1.1; excerpts are shown in Table 1 below.

Ceiling Insulation R Values, Excerpted from Table 402.1.1 of the 2009 and 2012 IECC

Ceiling insulation R-values

2009 IECC Sec 402.2.1 and 2012 IECC Sec. R 402.2.1 note that where the table specifies R-38, R-30 will be sufficient if the full height of R-30 extends uncompressed over the wall top plate at the eaves. Similarly R-38 will be sufficient where R-49 is required if a full thickness of R-38 extends over the top plates. Sec 402.2.2 notes that in parts of ceiling where the attic design does not allow space to install more than R-30 of insulation, R-30 will suffice; however, this exception is limited to no more than 500 square feet of total ceiling area.

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.

The DOE Zero Energy Ready Home National Program Requirements also specify as a mandatory requirement (Exhibit 1, #3) that ducts are located within the home’s thermal and air barrier boundary.

(16) Exceptions and alternative compliance paths to locating 100% of forced-air ducts in home’s thermal and air barrier boundary are:

  1. Up to 10’ of total duct length is permitted to be outside of the home’s thermal and air barrier boundary.
  2. Ducts are located in an unvented attic, regardless of whether this space is conditioned with a supply register.
  3. Ducts are located in a vented attic with all of the following characteristics:
    1. In Moist climates (Zones 1A, 2A, 3A, 4A, 5A, 6A and 7A per 2012 IECC Figure R301.1) and Marine climates (all “C” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation with an additional minimum 1.5” of closed-cell spray foam insulation encapsulating the ducts; total duct leakage ≤ 3 CFM25 per 100 ft2 of conditioned floor area; and ductwork buried under at least 2” of blown-in insulation.
    2. In Dry climates (all “B” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation; total duct leakage ≤ 3 CFM25 per 100 ft2 of conditioned floor area; and ductwork buried under at least 3.5” of blown-in insulation. Note that in either of these designs the HVAC equipment must still be located within the home’s thermal and air barrier boundary.
  4. Jump ducts which do not directly deliver conditioned air from the HVAC unit may be located in attics if all joints, including boot-to-drywall, are fully air sealed with mastic or foam, and the jump duct is fully buried under the attic insulation.
  5. Ducts are located within an unvented crawl space.\
  6. Ducts are located in a basement which is within the home’s thermal boundary
  7. Ductless HVAC system is used.

ENERGY STAR Certified Homes

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) and achieve Grade 1 installation per RESNET Standards (see 2009 IECC Code Level Insulation – ENERGY STAR Requirements and Insulation Installation (RESNET Grade 1).

See the interactive map of 2009 IECC insulation levels by climate zone.

See the guide 2009 IECC Code Level Insulation - ENERGY STAR Requirements for more details.

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): 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): ICC
    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.

  2. Author(s): Grin, Smegal, Lstiburek
    Organization(s): BSC
    Publication Date: December, 2013
    Report covering moisture and leakage issues associated with spray foam insulation application to the underside of roof sheathing.
  3. Author(s): Lstiburek, Grin
    Organization(s): BSC
    Publication Date: November, 2010
    Report investigating implementation of advanced framing in both production and prototype built homes built in a variety of climate regions across the USA.
  4. Author(s): Straube, Smegal
    Organization(s): BSC
    Publication Date: March, 2009
    Report considers a number of promising wall systems that can meet the requirement for better thermal control.
  5. Author(s): Baker
    Organization(s): National Research Council Canada
    Publication Date: March, 1969
    Article discussing the causes of wood decay and moisture issues.
  6. Author(s): Lstiburek
    Organization(s): BSC
    Publication Date: January, 2010
    Fact sheet providing detailed information about air sealing attics.
  7. Author(s): Straube
    Organization(s): BSC
    Publication Date: October, 2006
    Article outlining the casuses and solutions for ice dam problems.
  8. Author(s): Lstiburek
    Organization(s): BSC
    Publication Date: December, 2006
    Report discussing the importance of durability and energy efficiency in building construction.
  9. Author(s): Lepage, Schumacher, Lukachko
    Organization(s): BSC
    Publication Date: November, 2013
    Report explaining moisture-related concerns for high R-value wall assemblies and discusses past Building America research work that informs this study.
  10. Author(s): Straube, Smegal, Smith
    Organization(s): University of Waterloo, BSC
    Publication Date: April, 2010
    Document describing a hygrothermal modeling study, including all of the US climate zones, a range of interior humidity levels and numerous arrangements and types of insulation.
  11. Author(s): ASHRAE
    Organization(s): ASHRAE
    Publication Date: September, 2008
    Standard providing guidance on how to best design buildings with adequate moisture control features.

Contributors to this Guide

None Available

Last Updated: 07/09/2014

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