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Spray Foam Insulation Applied to Existing Attic Floor

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    Closed-cell spray foam insulation covers the attic floor to provide a continuous air control layer.
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    Insulate an attic in an existing home by installing spray foam (typically closed-cell) on top of the ceiling plane, i.e., on the floor of the attic, as follows:

    • Before any retrofit work is done, inspect the roof and attic; repair any leaks, remove active knob and tube wiring, and remediate any hazardous materials.
    • If existing bath fans vent into the attic, these must be vented to the outside.
    • Remove any existing insulation, debris, and dust, and prepare the attic floor for spray foam insulation.
    • Verify that all attic floor penetrations can be adequately sealed with spray foam insulation.  Provide additional air sealing measures such as sealants or rigid foam blocking of large holes as needed.
    • Verify that proper ventilation of the attic is provided with soffit vents (preferred) or gable and ridge vents. Verify that there is a baffle at each soffit vent extending from the top plate to above the height of the insulation to provide a pathway for ventilation air while keeping insulation out of the soffit vents.
    • Install a minimum of 1 inch of spray foam insulation over the attic floor to provide a continuous air control layer. Install the spray foam to meet or exceed code-required R-values or add additional loose-fill or batt fibrous insulation over the 1 inch of spray foam insulation to levels that meet or exceed the current adopted building energy codes.

    This approach is suitable for homes with a vented attic where the thermal boundary is established at the attic floor/ceiling plane. This approach requires that there be sufficient height between the underside of the roof deck and the top plate at the eave to install the full code-required amount of insulation while maintaining an air gap for ventilation above the insulation. Installing HVAC ducts and air handlers in vented attics is not recommended.

    For more guidance on applying spray foam insulation to attic ceilings, see the U.S. Department of Energy’s Standard Work Specifications (SWS).

    See the Compliance Tab for links to related codes and standards and voluntary federal energy-efficiency program requirements.

    Description
    Description

    Many older homes have little or no attic insulation and the attic floor/ceiling plane often has numerous air leaks from unsealed drywall-top plate seams, recessed can lights, gaps around vent pipes and wiring, open soffits, etc., that allow conditioned air to escape into the attic.

    Gaps around HVAC flue pipes allow conditioned air to leak through blown fiberglass into the attic.
    Figure 1. Gaps around HVAC flue pipes allow conditioned air to leak through blown fiberglass into the attic. An IR camera image shows gaps around HVAC flue pipes allow conditioned air to leak through blown fiberglass into the attic. (Source: Building Science Corporation.)

     

    One technique that can both increase attic insulation and air seal the attic is to apply a layer of spray foam to the attic floor, then to install additional blown-in or batt insulation to meet or exceed insulation R-value requirements.

    Closed-cell spray foam (ccSPF) is the preferred product for this air barrier application.  Open-cell spray foam (ocSPF) has a rapid and large volume of expansion during installation. This expansion can make application of a continuous uninterrupted layer of foam difficult due to the “billows” and voids created during ocSPF application. In contrast, ccSPF can be more easily applied in a 1-inch layer of consistent thickness.

    The ceiling plane or attic floor should be thoroughly air sealed prior to installing insulation. Robust and continuous air control is critical to a high-performance thermal enclosure. The existing insulation must be removed to provide access to the ceiling plane (for air sealing).

    If there are significant amounts of debris or dust on the floor of the attic, remove the debris and sweep or vacuum the surface before applying the spray foam. Old insulation, dust, and bits of plaster or mortar will interfere with spray foam adhesion, making an effective air seal difficult. Attempting to encapsulate any significant amount of debris with spray foam insulation is not recommended. If the debris contains hazardous materials, it should be removed or remediated regardless of whether the attic is to be outside of conditioned space or included within the thermal enclosure.  For instance, loose-fill vermiculite insulation may contain asbestos. Metal blocking and collars can be constructed around hot flues to keep insulation from touching them. Covers can be purchased or constructed to cover, air seal, and insulate recessed can lights that are not insulated ceiling airtight (ICAT) rated. (See Attic Air Sealing Guide and Details)

    The presence of flooring above the ceiling joists in an attic will complicate implementation of air sealing at the ceiling plane (attic floor). If the air control is to be established at the ceiling plane, then any attic flooring installed above that must be removed to provide full access to the ceiling surface.  Alternately, it may be possible to transition the air barrier to the top of the existing attic floor. To successfully execute this option, sufficient insulation is required outboard (on top of) the floor to control condensation, and a robust air control connection must be made from the top of exterior walls to the perimeter of the attic flooring. 

    In climate zones 6 or higher, a Class I vapor barrier (e.g., polyethylene) or Class II vapor retarder should be installed above the ceiling drywall as required by code. Class I vapor barriers should be avoided in climate zones 5 and below, as condensation can occur on the attic side of the vapor barrier in summer months during air conditioning periods. (See BSD 102 Attic Ventilation.)  Note that 2 inches of ccSPF is typically a Class II vapor retarder.

    One benefit of a vented (rather than unvented) attic is that interior-sourced moisture will be vented out of the attic space before causing damage to the roof sheathing. Another advantage of vented attics in cold climates is that they help to reduce the chances of ice damming on the roof.  Ice dams occur when heat leaks from the conditioned space (through holes in the ceiling plane, insufficient insulation, or heat loss from ductwork) and melts the snow on the roof. This melted snow travels down to the edges of the roof where it refreezes, creating icicles and ice dams. Attic ventilation helps to “flush away” this heat before it can melt the snow. (See BSI-046: Dam Ice Dam and BSD-135: Ice Dams.)

    The vented attic approach requires that there be sufficient height at the attic eaves for code-level required amounts of insulation. In mixed- and cold climates (zones 4 and above), inadequate insulation raises risks of wintertime condensation at the top plate due to cold surfaces. Because spray foam has a higher R-value per inch than blown or batt insulation, it should provide adequate insulation R-value, even with low pitch roofs. Spray foam has the added advantage of air sealing the top plate-to-drywall seams and the baffle-to-top plate seam. Another option is to convert the attic to a sealed, insulated space by sealing the soffit vents and insulating along the underside of the ceiling deck with closed-cell spray foam or above the roof deck with rigid foam. In new home construction, a common solution to increase the roof height for insulation above the top plate is to use raised heel trusses. Older existing roofs are unlikely to have raised heel trusses and raised heel trusses are not likely to be installed as a retrofit measure unless a complete reconstruction of the roof is required, for example if a second or third story is being added.

    The vented attic approach requires that there be sufficient height at the eaves to maintain an air gap above the insulation for ventilation air traveling from the soffit vents to the ridge vents. The vent space should be at least 2 inches high and the width of the framing bay at each soffit vent (Lstiburek 2006). Baffles should be installed at each soffit vent to provide this pathway for ventilation air to move up past the insulation along the underside of the roof deck to ridge vents or mushroom cap vents located near the ridge. The lower edge of the baffle can be sealed to the attic floor at the outside edge of the top plate with spray foam, which also air seals the top plate-to-dry wall seams. The baffles also serve to prevent insulation from covering the soffit vents and they can prevent “wind washing,” i.e., when wind blows in the soffit vents and pushes the insulation back from the eaves. 

    If the attic does not have soffit vents, attic ventilation can be provided by installing other types of vents such as gable or eyebrow vents down low near the attic floor, along with ridge vents or mushroom cap vents located up near the roof ridge.

    Powered attic ventilators (thermostatically controlled fans that exhaust attic air during hot weather) can result in increased infiltration (and associated higher cooling loads).  Studies have shown that powered ventilators have higher electricity consumption than their associated air conditioner savings (FSEC 2005/ Literature Review of the Impact and Need for Attic Ventilation in Florida Homes).

    Installing HVAC ducts and air handlers in vented attics is not recommended. Locating ductwork and/or air handling equipment in a vented attic can contribute to energy losses, performance issues, and ice dam formation in snowy climates, especially if the ducts and air handlers are leaky or poorly insulated.  One exception is if the ductwork can be encapsulated in spray foam and buried beneath the attic floor spray foam insulation. The ducts must be tightly air sealed and covered with a sufficient amount of spray foam insulation to minimize the risk of condensation forming on the outside of the ducts. See Ducts Buried in Attic Insulation.  However, because mechanical equipment such as air handlers cannot be adequately air sealed, locating them in a vented attic is not recommended.

    If ductwork and air handling equipment are installed in the attic, options include temporarily removing the equipment (to provide access to the floor below for spray foaming) or converting the attic to an unvented attic that is sealed along the roof line, combined with retrofit duct sealing and testing. 

    Closed-cell spray foam is installed to air seal the floor of the existing vented attic then covered with additional loose-fill fibrous insulation.
    Figure 2. Closed-cell spray foam is installed to air seal the floor of the existing vented attic then covered with additional loose-fill fibrous insulation.

     

    How to Insulate Attic Floor with Spray Foam Insulation

    1. Before any retrofit work is done, inspect the work area for active knob and tube wiring, bathroom fans venting into attics, roof leaks, and other hazardous conditions. Repair these conditions prior to adding insulation. Inspect for and remove or remediate hazardous materials like asbestos-containing vermiculite insulation.
    2. Remove any existing insulation, debris, and dust from the floor of the attic, and clean the work area to allow for adequate adhesion of the spray foam.

      Clean the attic floor of debris prior to installing new attic insulation. Use baffles to provide a path for ventilation air entering the attic from the soffit vents.
      Figure 3. Clean the attic floor of debris prior to installing new attic insulation. Use baffles to provide a path for ventilation air entering the attic from the soffit vents.
    3. Verify that all attic floor penetrations can be adequately sealed with spray foam insulation.  Provide additional solid blocking for large holes and open soffits, install metal blocking and collars around hot flues, and install covers over non-ICAT-rated recessed can lights. Don’t block intentional openings such as combustion air ducts and soffit, ridge, and gable vents.
    4. Verify that proper ventilation of the attic is provided with soffit and ridge vents or eyebrow, gable end, mushroom cap, and/or other vents. (See Figure 4.) If soffit vents exist, inspect existing baffles and install new baffles if needed. (See Figure 3.)

      Four attic venting approaches for vented attics that don’t have soffit vents - vents are installed low for incoming air and high for outgoing air.
      Figure 4. Four attic venting approaches for vented attics that don’t have soffit vents - vents are installed low for incoming air and high for outgoing air.
    5. Install a minimum of 1 inch of spray foam insulation over the attic floor.  Cover the entire ceiling including up to and over the exterior wall top plates and seal all penetrations to create a continuous air control layer. Install loose-fill fibrous insulation over the spray foam insulation to levels that meet or exceed thermal insulation levels for the current adopted building and energy codes. If roof is low over attic eaves, completely fill those areas with spray foam to baffles to ensure code-minimum or higher insulation levels over the top plates.

      Spray foam insulation is installed to cover the entire attic floor and the juncture of the attic floor, the top plates, and the base of the baffles.
      Figure 5. Spray foam insulation is installed to cover the entire attic floor and the juncture of the attic floor, the top plates, and the base of the baffles.
      Loose-fill fibrous insulation is installed over the spray foam on the attic floor and up against the baffles on top of spray foam insulation to full required insulation height.
      Figure 6. Loose-fill fibrous insulation is installed over the spray foam on the attic floor and up against the baffles on top of spray foam insulation to full required insulation height.

     

    Success
    Ensuring Success

    Remediate any hazardous conditions that will be affected (e.g., exposed or aggravated) by the planned work. Examples of hazardous materials that may be found in attic/roof assemblies of existing structures include (but are not limited to) lead, asbestos, mold, animal dropping/remains, etc. Follow applicable laws and industry procedures for mitigation of hazardous materials. Engage the services of a qualified professional when needed.

    For more information on vermiculite, see the EPA guide: Protect Your Family from Asbestos – Contaminated Vermiculite Insulation.

    Guidance for mitigating asbestos-related issues from HUD, EPA, NIH, OSHA and FEMA is available in Homeowner's and Renter's Guide to Asbestos Cleanup After Disasters and Asbestos: Worker and Employer Guide to Hazards and Recommended Controls

    For guidance on handling knob and tube wiring, see Weatherization Assistance Program guide: Retrofitting Insulation in Cavities with Knob-and-Tube Wiring an Investigation into Codes, Safety, and Current Practices.

    Seal all large penetrations with rigid blocking to provide a continuous air control layer at the attic floor. Visual inspection, infrared thermography, and blower door testing are all tools that can be used to verify air barrier continuity.

    Apply spray foam insulation to a debris- and dust-free surface to provide adequate thermal resistance and to meet or exceed the levels specified in the current adopted building and energy codes.

    Follow the spray foam manufacturer’s instructions and guidelines for installation and 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.

    Electrical junction boxes accessible from the attic will be buried during this insulation retrofit.  The NEC (Article 314.29, 2005 NEC) requires junction boxes to be accessible "without removing any part of the building."  Burying the junction box within the ccSPF layer would clearly violate this requirement, and should not be done.  The ideal approach would elevate the junction box above the insulation level; alternately, a flag marker could be placed to indicate junction box location.  A local code official could provide further guidance.

    All roofs must be vented according to the applicable current adopted building code.  If the roof is not vented according to the applicable building code then a qualified contractor in the state should be engaged to install the necessary venting.

    Climate
    Climate

    The ceiling/attic floor assembly 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. 

    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. 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 ceiling/attic floor assemblies.

    Attic Insulation Requirements per the 2009 and 2012 IECC.

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

     

    Training
    Right and Wrong Images
    Image
    Right - Spray foam insulation air-seals the ceiling-to-drywall seams at the wall top plate.
    Right - Spray foam insulation air-seals the ceiling-to-drywall seams at the wall top plate.
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    Right - Closed-cell spray foam covers the ceiling and joists to insulate and air-seal the ceiling deck.
    Right - Closed-cell spray foam covers the ceiling and joists to insulate and air-seal the ceiling deck.
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    Right – Open-cell spray foam fills the attic floor joists.
    Right – Open-cell spray foam fills the attic floor joists.
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    Author(s)
    Neuhauser Ken,
    Gates Cathy,
    Pettit Betsy
    Organization(s)
    Building Science Corporation,
    BSC
    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.
    Author(s)
    Lstiburek Joseph W
    Organization(s)
    Building Science Corporation,
    BSC
    Publication Date
    Description
    Document providing background and approach for the prep work necessary prior to adding attic insulation - focusing on combustion safety, ventilation for indoor air quality, and attic ventilation for durability.
    Author(s)
    Lstiburek Joseph W
    Organization(s)
    Building Science Corporation,
    BSC
    Publication Date
    Description
    BSD-102 report providing guidance about whether to construct a vented or unvented attic based on hygro-thermal zone.
    Author(s)
    Centers for Disease Control and Prevention,
    U.S. Environmental Protection Agency,
    Federal Emergency Management Agency,
    U.S. Department of Housing and Urban Development,
    National Institutes of Health,
    Hurricane Sandy Rebuilding Task Force
    Organization(s)
    FEMA,
    EPA,
    HUD,
    NIH,
    CDC
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    Description
    Guidance from EPA, FEMA, HUD, and NIH on how to deal with asbestos in homes following natural disasters.
    Author(s)
    National Institution of Health,
    U.S. Environmental Protection Agency,
    U.S. Department of Housing and Urban Development,
    Occupational Safety and Health Administration
    Organization(s)
    NIH,
    OSHA,
    EPA,
    HUD
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    Description
    Guidance from HUD, EPA, NIH, and FEMA for workers who deal directly with asbestos.
    *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

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    Building Science-to-Sales Translator

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

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    Technical Description

    There are two levels of attic 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 Attic Insulation
    Sales Message

    High-efficiency attic 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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