Vapor Barrier Details for Sealing at the Band Joist InspectAPedia® -
Vapor barriers and condensation in buildings
Construction & sealing details to provide a continuous vapor barrier in buildings at the rim joist or band joist
Caulk & rigid insulation details protect the integrity of the building's thermal shell
Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
Questions & answers about vapor barrier installation details on buildings
This article describes how to install vapor barriers and how to avoid problems with building indoor condensation: we provide details for caulking, sealing, & insulating at the building band joist or rim joist will protect the integrity of the building's thermal shell.
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Multiple approaches to building vapor barrier sealing details are provided to address this air and moisture leak problem: the caulk and block method to seal the rim joist is effective but time consuming; offsetting the band joist to permit exterior rigid foam insulation is described; details are provided for sealing building overhangs such as found on a raised-ranch home; other building air leak sealing methods developed by researchers at the University of Toronto are described. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
This discussion of vapor barriers and condensation in buildings in this article series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems), continues with part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems) followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS. Readers should also see VAPOR BARRIERS & HOUSEWRAP.
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Article Three on Vapor Barriers & Building Condensation - Sealing the Band Joist at Building Floors
"Dealing with Band Joists. Don't let them sabotage your thermal shell" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.
Traditional Methods for Sealing the Building Rim Joist or Band Joist
Part 1 of this article on building details to seal air and vapor leaks at the band joist or rim joist describes two traditional, effective, but labor-intensive methods to stop air, moisture, and heat leaks around the perimeter of building floors and overhangs.
Figure 1 (page top sketch and shown in more detail here), describes the caulk and block method of band joist insulation and sealing: compressible pre-cut foam blocks or custom-cut rigid foam insulation blocks are placed over fiberglass-insulation between the floor joists at the building perimeter, sealed on all sides with caulk. The author suggests a few construction tricks that can reduce the labor associated with this building air, heat, and moisture vapor leak sealing method.
Figure 2 (at left just above) shows a physically easier method for sealing the building floor perimeter by offsetting the band joist inwards and routing of the poly vapor barrier inside of the building floor frame. This method will require a different than usual construction sequence, applying part of the vapor barrier around the floor structure early in the building framing sequence.
Better Methods for Sealing the Building Band Joist
Figure 3 (left) demonstrates how floor overhangs such as on a raised ranch may be sealed using caulking and blocking combined with insulation of the floor overhang soffit space. Notice that in addition to insulating the floor overhang soffit, rigid foam insulation blocks are cut and caulked between each joist pair to seal the building interior against air, heat, and moisture leaks into the soffit space.
Using this method for sealing floor overhangs, the poly vapor barrier is run down the wall and under the subfloor to extend into the interior past the overhanging section of the floor.
If the plans call for gluing wood parquet flooring directly to the subflooring, the poly is omitted in that section, relying on the glue coating to also serve as a vapor barrier.
The author, Mr. Bliss, points out that building heat, air, and moisture sealing is concerned with leakage of air and moisture into building cavities, not diffusion of moisture through building materials. That is because moisture diffusion through materials has been shown to be miniscule in import compared with air and moisture leaks at openings.
Figure 4 (left) shows details of air, moisture, and heat loss sealing details at the walls, attic knee wall floor, knee walls, and ceiling of a typical cape design building. The author points out that in a Cape Cod design, convective coupling of eaves and ceiling/floor joists can "spell trouble".
Air leaks through floor framing into the space behind the knee walls of a cape, ultimately into the outdoors, as well as air leaks upwards through the knee walls into the attic space above are all encouraged by natural convective currents in the building, especially if the floors and walls are not adequately sealed and fully insulated.
"The solution is the same - blocking between the joists to continue the air/vapor barrier on the warm side of the insulation. [See detail in the illustration at left.] This is often overlooked, resulting in air passage between the cold eaves and the floor/ceiling cavity. Since this cavity is between two heated spaces, it is a heated space also. If it is convectively coupled to the outdoors, you'll be heating the outdoors."
Figure 5 (left) demonstrates building sealing methods where the poly vapor barrier is omitted. The author cites a radical Canadian splinter group who argues that the polyethylene air/vapor barrier should be omitted entirely, claiming that the vapor barrier will oxidize, become brittle and fail over time, that under peak wind loading or pressurization testing the poly will tear and leak around staples [this is doubtful if if the poly is adequately secured and protected from movement by its covering building materials], and finally "... to do it the right way with the right materials is just too expensive for the tract builder to consider."
The Canadian alternative (described in the sketch at left and proposed by Joseph W. Lstiburek, P.E. in Toronto) focuses on controlling air leakage rather than vapor diffusion by using compressible foam gaskets in five locations around the floor perimeter combined with installation of 2 1/2" rigid foam insulating blocks, finishing by painting drywall with two coats of vapor retarder paint.
OPINION: Our field experience [DJF] demonstrates that poly vapor barriers remain intact for decades, certainly 20 to 30 years based on our own direct experience in construction using poly vapor barriers and subsequent invasive inspection of the same buildings decades later.
Provided that the vapor barrier polyethylene is protected by having been covered by other building materials, it has a good life. We prefer to use 6-mil rather than 4-mil polyethylene, however, because we've found that the thicker material is less likely to be damaged during construction and because production control does not maintain full material thickness. --DJF
OPINION-DF: While we fully agree that building scientists, including Dr. Lstiburek, have shown compelling evidence that air and moisture leaks in buildings are far more significant in building impact and energy losses than vapor diffusion through building materials, our field work as both a builder and a building inspector suggest that Dr. Lstiburek's non-poly building sealing methods involve a larger number of components and materials (foam gaskets, foam blocks, vapor retarder paint in two coats) that must be accurately placed than the use of polyurethane vapor barriers and caulk - materials familiar to most builders. This approach was reported in May 1984 in Solar Age Magazine - we look forward to seeing the results of subsequent research and field experience.
For details about use of acoustical sealants or tapes to seal polyethylene vapor barriers, see ACOUSTICAL SEALANT CHOICES.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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Questions & answers about vapor barrier installation details on buildings
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in buildings, and other topics.
ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
"2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
"The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
The National Institute of Standards and Technology, NIST (nee National Bureau of Standards NBS) is a US government agency - see www.nist.gov
"A Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "Moist" Transient Heat and Moisture Transfer Model [copy on file as/interiors/MOIST_Model_NIST_b95074.pdf ] - ", George Tsongas, Doug Burch, Carolyn Roos, Malcom Cunningham; this paper describes software and the prediction of wall moisture contents. - PDF Document from NIST
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
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