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Wind Washing

Scope

Identify areas of the home likely to be affected by wind washing (e.g., attic soffit vents, cantilevered floors, open porch ceilings adjacent to second-story floor cavities).

  • Install baffles at each rafter bay with a soffit vent to direct air flow and wind above rather than through attic insulation. Air seal top plate-drywall seams at ceiling corners.
  • Air seal floor cavities below attic kneewalls and cantilevered floors.
  • Air seal second-story rim joists.
  • Air seal walls separating porch attic areas from the house or house attic.

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

Wind washing is the movement of unconditioned air around or through buildings in such a way as to diminish or nullify the intended thermal performance of the home. Wind washing can occur in attics that are vented at the soffits. It can also occur in areas of a home where conditioned spaces abut unconditioned space, for example in areas that are cantilevered (i.e., sections that bump out above the foundation) if those areas are not adequately air sealed with complete air barriers to prevent air from flowing through the floor joist cavities connecting the cantilevered area to the rest of the home. Symptoms of wind washing including cold or hot spots along exterior walls, along second-story walls that border attic spaces, over cantilevered floors, and over or along other areas of the home that border unconditioned spaces.

Air can enter a house by means of natural infiltration (driven by wind and temperature differential forces) and by means of mechanical infiltration (driven by the heating, cooling, and ventilation mechanical systems). Natural infiltration comes into the house from outside through a variety of pathways. Mechanically induced infiltration is driven by exhaust fans, dryers and duct leakage, which can create a negative pressure inside the home, drawing unconditioned air inside.

A common example of wind washing is when wind blows through soffit vents and pushes the attic insulation back away from the edges of the ceiling deck, leaving patches of ceiling deck bare along the outer edges of the attic. Where wind washing occurs, rooms will feel hot in the summer and cold in the winter, especially along exterior walls. This kind of wind washing can be prevented by installing baffles, which are cardboard or plastic panels installed in each roof rafter bay with a soffit vent to direct air up past the insulation and along the underside of the roof deck toward the ridge vents. The sides of the fles are stapled to the attic ceiling or rafters; the baffles provide a 1-2-inch gap between the ceiling and the insulation to serve as a chute for ventilation air to flow from the soffit vents to the attic ridge vents. The lower edge of baffle typically angles down and is sealed to the attic floor just inside of the soffit vent; thus, it provides both a dam to keep insulation from falling through the vent and a solid barrier to keep wind from flowing laterally through the insulation. For more on the installation and use of baffles, see the guide Attic Eave Minimum Insulation.

Wind can also enter the attic or cavities between floors from cantilevered floors that aren't properly air sealed and from porches if the wall between the house and the porch attic is not properly air sealed. Wind that is allowed to flow laterally through the attic can get into interstitial floor spaces, such as under attic kneewalls (Figure 1). (For techniques for insulating and air sealing attic kneewalls, see the guide Attic Knee Walls.) 

In cooling-dominated periods or climates, wind blowing into attic vents can push hot attic air into the inter-story floor or knee wall cavity, bypassing the typical thermal boundaries of the building and introducing considerable heat into the house. Even in the absence of wind, thermal buoyancy can create air transfer between a hot attic and the interstitial cavities. Wind washing can also occur when little or no attic is involved, where outdoor air can infiltrate directly into the floor cavities or knee walls. These breaches of the air and thermal boundaries of the house can lead to significant entry of sensible heat and, in humid climates, latent heat (water vapor) into the structure. Wind washing has been identified as a significant issue impacting energy demand, comfort, and humidity in two-story homes. Wind washing can result in substantially increased second-story cooling or heating loads. When the outside air is hotter than the inside conditioned air, first-floor cooling loads can increase due to warmer ceilings and air penetration from the floor cavity through recessed lights and other ceiling pathways (Cummings et al. 2009). Factors contributing to increased energy losses due to wind washing include the size and amount of openings between unconditioned spaces and interstitial spaces, the temperature of the air pushed into the floor cavities from the attic space (attics with tile or light-colored roofs and taller roofs will tend to be cooler in summer than dark, asphalt-shingled, and low roofs); the presence of complimentary holes on the opposite side of the building cavitiy from the prevailing winds, and the prevalence and strength of the wind.

Windwashing can be prevented at initial construction by ensuring that exterior walls, cantilevered floors, and attic kneewalls are properly air sealed; and by specifiying unvented attics, or by properly designing the attic eave area and installing insulation dams and baffles. The level of effort needed to repair areas impacted by wind washing can vary greatly depending on the difficulty of accessing the repair areas and the total area needing to be sealed. 

 

Wind washing reduces the effectiveness of attic insulation by pushing insulation away from the edges of the attic space

Figure 1. Wind washing reduces the thermal performance of the home by pushing unconditioned air through insulation and into interstitial spaces causing unwanted heat loss or heat gain. Reference

 

How to Prevent Wind Washing in New Construction

  1. In a vented attic, install baffles and insulation dams in each rafter bay with a soffit vent. See the guide Attic Eave Minimum Insulation.
    OR
    Design the home with an unvented attic. See the guide Unvented Attic Insulation.
  2. Air seal cantilevered floors. See the guide Cantilevered Floor.
  3. Air seal attic kneewalls. See the guide Attic Knee Walls.
  4. Air seal walls between porch attics and the house or attic. See the guide Walls Adjoining Porch Roof.

How to Locate Areas of Wind Washing in Existing Construction

  1. Visually inspect potential areas, along attic eaves, under cantilevered floors, and under attic kneewalls to see if the air barrier is incomplete or missing. These spaces may be difficult or impossible to physically access (Figure 2).  Some disassembly or borescope investigations may be needed (Figure 3). 
  2. Use a blower door for floor cavity diagnostic pressure testing to determine the pressure difference between the house and the first-second floor cavity.
    • Place the reference tube inside the cavity of the floor between the first and second stories. This can be done by drilling a small hole through first-floor ceiling into the floor cavity (Figure 4), by pushing the end of the tube into the space with an extension pole from within the attic (Figure 5), or by pushing the tube through a leaky can light or other ceiling fixture located in the floor cavity.
    • Set up the blower door and bring the house to -50 Pa with respect to (wrt) the outdoors; turn off all HVAC.
    • Measure the house wrt to the floor cavity pressure.
    • With the house depressurized by the blower door to -50 Pa, the house pressure is measured wrt the floor cavity. The house wrt floor cavity will be negative (less than zero) as long as the floor cavity has some connection to a vented attic space. (This test could also be done in the pressurization mode if preferred.) If the floor cavity is much more connected to a vented attic space, the reading will be closer to -50 Pa. If the floor cavity is relatively tight to the attic and leakier to the indoor space, the reading will be closer to 0 Pa. Use of an IR camera while the blower door test is underway will help you identify the locations of greatest air flow and heat loss. If the measurement is -34 Pa or more negative, more detailed visual investigation of the floor cavity is warranted.
Limited attic access can make inspections for missing air barriers and insulation challenging.
Figure 2. Limited attic access can make inspections for missing air barriers and insulation challenging.

Limited attic access may make it necessary to use a bore scope when inspecting for missing air barriers and insulation
Figure 3. Limited attic access may make it necessary to use a bore scope when inspecting for missing air barriers and insulation.

Floor cavity air pressure is measured by placing a tube into the floor cavity through a small drilled hole
Figure 4. Floor cavity air pressure is measured by placing a tube into the floor cavity through a small drilled hole.

Floor cavity pressure is measured by inserting a tube into the floor cavity using an extension pole
Figure 5. Floor cavity pressure is measured by inserting a tube into the floor cavity using an extension pole.

How to Repair Attic Areas Affected by Wind Washing

  1. Inspect for locations of wind washing using visual inspection, blower door testing, and infrared thermography as described above.  Locate open floor joist cavities under second stories and other uninsulated, un-airsealed areas that connect to ventilated unconditioned space. Figure 6 shows an attic kneewall that is insulated but the floor joist cavities underneath this second-story room are completely open to the vented attic, allowing unconditioned air to flow through the floor space. Figure 7 shows the same view with an infrared camera. The dark purple and blue areas show cold conditioned air pouring into the hot attic from the interstitial floor space.
  2. Air seal the open floor joists with spray foam, rigid foam or another solid sheet good that is cut to fit and sealed in place over the open joist cavities with caulk or foam. The space could also be filled with rolled fiberglass batt insulation that is stuffed into each cavity and covered with spray foam to air seal.
  3. Add insulation to the attic knee wall if the wall is currently uninsulated, under insulated, or poorly insulated. The wall should be airsealed by caulking all holes and seams prior to insulating. If it is not desirable to remove the existing insulation to air seal the wall, than the wall can be airsealed by spray foaming (or installing a rigid air barrier that is sealed at all seams) over the fibrous insulation. The wall and floor joist cavities can be sealed and insulated in one step by using spray foam as shown in Figure 8.
  4. Conduct a post-retrofit inspection including a blower door test and infrared thermography to determine the effectiveness of the repair, as shown in Figure 8.
  5. Install baffles at soffit vents as described in the guide Attic Eave Minimum Insulation to minimize lateral air flow in the attic while maintaining desired ventilation along the underside of the roof deck.
The floor cavities under this attic kneewall are completely open to the unconditioned attic space and a prime target for wind washing
Figure 6. The floor cavities under this attic kneewall are completely open to the unconditioned attic space and a prime target for wind washing.

Infrared imaging shows cold conditioned air pouring out of the open floor cavities under this attic kneewall into the hot unconditioned attic
Figure 7. Infrared imaging shows cold conditioned air pouring out of the open floor cavities under this attic kneewall into the hot unconditioned attic.

The attic kneewall and the open floor cavities under kneewall are both sealed and insulated in one step with spray foam insulation
Figure 8. The attic kneewall and the open floor cavities under kneewall are both sealed and insulated in one step with spray foam insulation.

Infrared photography shows how effectively the spray foam insulated and air sealed the attic kneewall and the floor cavities under the kneewall almost completely blocking air flow and heat loss under the kneewall
Figure 9. Infrared photography shows how effectively the spray foam insulated and air sealed the attic kneewall and the floor cavities under the kneewall almost completely blocking air flow and heat loss under the kneewall.

 

Ensuring Success

Air seal exterior walls and walls separating porch attics from the home, air seal floor joist cavities under attic knee walls and cantilever floors, and install baffles and insulation dams in vented attics. Use blower door testing in combination with visual inspection and infrared thermography to identify areas of wind washing and to verify repair.

Climate

Wind washing occurs in all climates. In humid, cooling-dominated climates, ensure that ducts in vented attics are insulated to at least R-8 and covered with a continuous vapor barrier before adding insulation that may come in contact with the ducts. Condensation can form on the exterior of cold air supply ducts that lack adequate insulation and a continuous vapor barrier.

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 (Version 3/3.1, Revision 08), Rater Field Checklist

Thermal Enclosure System:

2. Fully-Aligned Air Barriers.5 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).6

Footnotes:

(5) For purposes of this Checklist, an air barrier is defined as any durable solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage. EPA recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 in. or 1.5 in., respectively, to qualify as an air barrier unless the manufacturer indicates otherwise. If flexible air barriers such as house wrap are used, they shall be fully sealed at all seams and edges and supported using fasteners with caps or heads ≥ 1 in. diameter unless otherwise indicated by the manufacturer. Flexible air barriers shall not be made of kraft paper, paper-based products, or other materials that are easily torn. If polyethylene is used, its thickness shall be ≥ 6 mil.

(6) All insulated ceiling surfaces, regardless of slope (e.g., cathedral ceilings, tray ceilings, conditioned attic roof decks, flat ceilings, sloped ceilings), must meet the requirements for ceilings.

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

DOE Zero Energy Ready Home

DOE Zero Energy Ready Home (Rev 05) Exhibit 1: Mandatory Requirements: Item 1, Homes must be certified under ENERGY STAR Qualified Homes Version 3. Item 2, Ceiling, wall, floor, and slab insulation shall meet or exceed 2012 or 2015 IECC levels.

Exhibit 2: Design the home to meet the HERS index of the DOE Zero Energy Ready Home Target Home, for which insulation levels must meet the 2012 IECC and achieve Grade 1 installation, per RESNET standards.
Exhibit 2:  Infiltration: Climate Zones 1-2: 3 ACH 50; Zones 3-4: 2.5 ACH50; Zones 5-7: 2 ACH50; Zone 8: 1.5 ACH50. Envelope leakage shall be determined by an approved verifier using a RESNET-approved testing protocol.

2009 IECC

Table 402.4.2, Air barrier and thermal barrier: Exterior wall insulation is installed in substantial contact and continuous alignment with the air barrier. Air permeable insulation is not used as a sealing material.

2012 IECC

The IECC 2012 requires new construction to have a tested air tightness with whole house air leakage not to exceed 5 ACH50 for Climate Zones 1 and 2. An even tighter restriction of 3 ACH50 is mandatory for Climate Zones 3 through 8. IECC 2012 has language in Table R402.4.1.1 that may be interpreted that floor cavities are required to be insulated and air sealed from unconditioned spaces, however floor cavities are not specifically mentioned. Without more specific language addressing floor cavities, code enforcement is more likely to be missed. Florida code has adopted more specific language that requires an “Air barrier on perimeter of floor cavities between floors” for multi-story homes.

Table R402.4.1.1, Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material.

2015 and 2018 IECC

Table R402.4.1.1 requires that a continuous air barrier be installed in the building envelope including at the junction of the top plate and the tops of exterior walls, all kneewalls, all rim joists, and at any exposed edges of insulation in floors.

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 IRC

Table N1102.4.2, Air barrier and thermal barrier: Exterior wall insulation is installed in substantial contact and continuous alignment with the air barrier. Air permeable insulation is not used as a sealing material.

2012 IRC

Table N1102.4.1.1, Air barrier and thermal barrier: A continuous air barrier is installed in the building envelope including rim joists and exposed edges of insulation. Breaks or joints in the air barrier are sealed. Air permeable insulation is not used as a sealing material.

2015 and 2018 IRC

N1102.4.1.1 Air Barrier and Insulation Installation, Ceiling/attic: Air barrier in any dropped ceiling/soffit is substantially aligned with insulation and any gaps are sealed. General requirements: A continuous air barrier is installed in the building envelope; breaks and joints in the air barrier are sealed. Air-permeable insulation is not used as an air-sealing material.

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): Florida Solar Energy Center
    Organization(s): BA-PIRC
    Publication Date: May, 2015

    Case study describing investigations of wind washing in 56 homes; recommendations for cost-effective retrofits and tips for new construction.

References and Resources*

  1. Author(s): Lstiburek
    Organization(s): Building Science Corporation

    The Builder's Guide provides the building industry with the latest and best practical information on how to apply building science principles to structures as systems in hot-humid climate regions.

  2. Author(s): Southface Energy Institute, Oak Ridge National Laboratory
    Organization(s): Southface Energy Institute, ORNL
    Publication Date: February, 2000

    Information sheet with information about insulating and ventilating attics.

  3. Author(s): Department of Energy
    Organization(s): DOE
    Publication Date: April, 2017

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  4. Author(s): U.S. Environmental Protection Agency
    Organization(s): EPA
    Publication Date: December, 2015

    Webpage with links to Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 and 3.1  (Rev. 08).

  5. Author(s): Withers
    Organization(s): FSEC
    Publication Date: April, 2015
  6. Author(s): Cummings, Withers, LaHiff
    Organization(s): FSEC
    Publication Date: December, 2009
    Research study describing field testing performed in 32 two-story Florida homes to characterize wind washing failures of the house air and thermal boundary.
  7. Author(s): Withers, Cummings
    Organization(s): FSEC
    Publication Date: September, 2010
    Report about evaluation methods used for analyzing wind washing and results of upgrades made to homes to increase energy efficiency.
  8. Author(s): Withers, Cummings
    Organization(s): FSEC
    Publication Date: September, 2010
    Research study finding significant wind washing potential in 40% of homes in the study area.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Pacific Northwest National Laboratory

Building America Partnership for Improved Residential Construction/Florida Solar Energy Center

Last Updated: 05/27/2017