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Vapor Barriers & Building Condensation - Part I InspectAPedia®  -     Solar Age Magazine Article discusses vapor barriers and condensation in buildings. Where to place or locate the vapor barrier in a building wall Are vapor barriers required in building ceilings? Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices Questions & answers about vapor barriers and moisture condensation in buildings

This article discusses vapor barriers and indoor condensation: explaining 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.

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 and FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER.

Because Grade D paper tends to deteriorate under prolonged wetting, the trend in three-coat stucco is to use two layers of 30-minute paper. Because the paper tends to wrinkle, the two layers tend to form a small air space, creating a rain-screen effect.

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Article One on Vapor Barriers & Building Condensation

"Vapor Barriers, Part I - science and common sense point the way to effective strategies" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.

• Vapor Barriers & Building Condensation Explained, Part I - PDF form, use your browser's back button to return to this page
• Vapor Barriers & Building Condensation Explained, Part I - PDF form, continued

This timeless building condensation and vapor barrier placement article explains the fundamentals of condensation in buildings: what causes building condensation, how to control building condensation, and the problems that condensation causes in structures.

Here Steve Bliss discusses the function of building wall and ceiling vapor barriers and the effects of indoor condensation: answering a series of questions about when and why condensation occurs inside buildings, what are the problems caused by excessive indoor condensation, how moisture enters building wall and ceiling cavities, and what are the best approaches to prevention of indoor moisture and condensation problems.

Mr. Bliss also explains where the vapor barrier should be placed in building walls - it may vary by climate, and he answers a much debated question about whether or not vapor barriers are really needed in building ceilings.

I think a lot about vapor barriers and dew points. It's an occupational hazard. For guidance in these matters I pore over arcane volumes of DOE conference proceedings and muddle through the ASHRAE Handbook of Fundamentals. When puzzled, I talk to the experts in person. And to find out what the real world is doing, I talk to builders around the country. At times, it's rather confusing.

Over the phone and at recent conferences (in the 1980's), I've heard builders and designers ask many of the same questions I've wrestled with. Few of these questions have definitive answers. Often good research is lacking or the theory, research, and anecdotes fail to confirm one another. In many of the case studies reported, the full story is not known. What was the relative humidity in the house with the rotting rafters?

But all the research and analysis has not been in vain. For the major issues, consistent findings have emerged. Better news is that there is little cause for alarm. The energy-efficient housing stock does not seem to be rotting beneath our feet. [For an exception, see leak, rot, and mold concerns involving residential installations at SIDING EIFS & STUCCO.] Here are some frequent questions on the fundamentals of vapor barriers and condensation, with attempts at brief answers. In a following discussion, VAPOR CONDENSATION & BUILDING SHEATHING, we look at more specific materials and applications.

Question: When does condensation occur in a building or a building cavity?

Answer: Condensation occurs on or in building surfaces and materials when warm moist air hits a cold surface, or when moisture vapor flow through a wall or ceiling gets dammed-up and sufficiently cooled. To find the dew point (the point at which condensation will occur on a surface) for a given air temperature and relative humidity (RH), you need a chart or graph. See DEW POINT CALCULATION for WALLS and see
DEW POINT TABLE - CONDENSATION POINT GUIDE.

Question: Where does condensation occur on or in buildings?

Answer: Building condensation occurs on cold interior window surfaces and within building walls and roof cavities. In walls, condensation occurs generally on the inside surface of the building's exterior wall sheathing, or on the back side of the exterior siding itself. In summer, with air conditioning, the situation may be reversed in very humid climates such as in Florida, and even further north (New York) when outdoor temperatures and relative humidity are high,m causing condensation to occur on concrete or other masonry floors, walls, and even under wallpaper. It is possible for condensation to drip and collect on wall plates or under windows.

Question: Is condensation in buildings a serious problem?

Answer: Mold and wood-staining fungi grow well on a wetted organic (wood, paint, paper) surface at 60 deg F. and 60 percent RH. For wood destroying fungi to grow, though, wood fibers must be saturated (about 30-percent moisture content) and warm. Most building experts consider wood above 18 to 20 percent moisture-content to be at risk of rot or mold.

These decay causing fungi grow fastest at 50 to 60 degF, but can grow at lower temperatures, as low as 32 degF. depending on the mold genera and species. Indoor mold is both a rot or building damage hazard and a potential indoor air contaminant that can be a serious problem especially for people who are sensitive such as people who are immune-impaired, allergic, asthmatic, or have other medical or respiratory vulnerability.

Both building rot repair and building mold remediation jobs can be very costly where large areas are involved. See ROT, FUNGUS, TERMITES and for an extensive reference on building mold detection, testing, cleanup, and prevention, see MOLD INFORMATION CENTER. Also see MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD, and MOLD PREVENTION GUIDE. If you are already concerned about a mold problem in a building, MOLD EXPERT, WHEN TO HIRE offers help in deciding when it is appropriate to hire a professional.

Question: How does the moisture get into a building wall or ceiling cavity?

Answer: Water vapor is generated in the building from normal human activities (bathing, cooking), or moisture can enter a building from other sources such as plumbing leaks, roof leaks, surface runoff or even roof spillage leaks into the building (see WATER ENTRY in buildings), and on more dangerous occasions from gas-burning appliances. In winter, water vapor moves outside by passing through permeable materials (a process called moisture diffusion), and as research has shown, the most significant moisture movement in buildings occurs as moisture is carried by air leaking around windows, doors, or other gaps in the building shell.

Moisture diffusion is the movement of water vapor (that is, water molecules, not water droplets) from areas of higher moisture level into areas of lower moisture concentration. Diffusion occurs independent of air movement. But the most significant moisture movement into building walls occurs by air leaks at wall openings or penetrations. Two key articles you'll want to read are MOISTURE CONTROL in buildings and MOISTURE PROBLEMS: CAUSE & CURE. Also see VAPOR CONDENSATION & BUILDING SHEATHING, also HOUSEWRAP AIR & VAPOR BARRIERS and Air Bypass Leaks, Thermal Tracking.

Question: How can I predict how and where moisture is going to be a problem in a building?

Answer: It's tricky to predict where moisture problems will occur in a building, although experienced home inspectors and contractors who have seen or perhaps even disassembled and repaired buildings with moisture damage often have an eye for just were problems are most likely to occur. Taking a more technical approach to building moisture, even if you can do the math, many of the moisture variables such as perm ratings and building air leakage rates will not be precisely known. (See BLOWER DOORS & AIR INFILTRATION for determining a building's air leakage rate.)

Still, a simple moisture behavior model is useful for building design purposes. One approach is to plot the temperatures through the wall on a graph and to overlay a plot of the dew point temperatures. Wherever the actual temperature falls below the dew point temperature, condensation may occur. This method is detailed in the ASHRAE Handbook of Fundamentals and in the National Bureau of Standards (NBS/NIST) Report BMS 63 or this more detailed building moisture model article from NIST.

Question: How common is moisture condensation in insulated 2x4 and 2x6 building walls?

Answer: Researches think that small amounts of liquid or frozen condensation (frost) occurs normally in these cavities.

Question: So why hasn't wall moisture caused more buildings to rot away?

Answer: Wood and other porous building materials safely store a lot of the moisture at well below saturation levels until it re-evaporates from daily or seasonal warming.

Question: If building materials can safely store moisture until it re-evaporates, why is moisture a building worry?

Answer: As houses get smaller and tighter, indoor humidity levels are rising, which increases the risk of problem-causing condensation. Also, the more insulation in the wall cavity, the colder the exterior building sheathing - another factor in condensation. Finally, the use of low permeance sheathings has raised many questions. They go against the conventional wisdom of keeping the outside of a wall five to 10 times more permeable than the inside.

An example of a moisture-related indoor problem that was not widely recognized until around 2001 was the development of large reservoirs of potentially airborne toxic or allergenic mold hidden in building insulation (see Mold in Fiberglass Insulation) or on the wall cavity side of drywall in buildings with leaks or moisture problems. (See FIND MOLD in buildings, HOW TO). Sometimes (not always) mold from these reservoirs becomes an air quality and health problem for building occupants and sometimes a costly cleanup is needed.

In other structures, such as homes sided with low permeance EIFS synthetic stucco, trapped moisture from building leaks or from moisture leaks into walls has led to severe rot damage, also leading to costly building repairs. (See SIDING EIFS & STUCCO.)

Question: What is the best approach to avoiding building problems from moisture condensation?

Answer: The safest approach to avoiding building problems from condensation is to install a lapped and sealed 6-mil poly vapor barrier - Saskatchewan style - on the winter warm side of the wall, combined with paying very very meticulous attention to sealing at every wall penetration so that air leaks do not send moisture into the wall cavities. So little moisture will then diffuse or leak into wall cavities that it won't matter what insulation or sheathing material was used. For more details see these articles:
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR SEALING STRATEGIES

Question: How important is it not to puncture the vapor barrier?

Answer: ASHRAE lists a typical foil vapor barrier at 0.02 perm if un perforated, and 0.08 to 0.16 perm if it has "a few holes larger than pinholes per square foot. After monitoring a number of test walls for two years, researcher Gerald Sherwood of the Forest Products Laboratory (FPL) in Madison WI concluded: "Puncturing the vapor retarder, as with an electrical outlet, can completely change the moisture patterns in the wall," and that once punctured, 6-mil poly performed no better than paper.

Question: Is it really necessary to make the vapor barrier continuous around band joists (rim joists) at floor structures?

Answer: This seems like the preferable way when feasible. Many builders, though, prefer to caulk pieces of foil-faced rigid-foam insulation board between the band joists and caulk or tape these to the wall vapor barrier. See FRAMING DETAILS for BETTER INSULATION and also FRAMING DETAILS for DOUBLE WALL HOUSES. This seems adequate. Non-hardening acoustical sealant remains the best bet for polyethylene patchwork. See ACOUSTICAL SEALANT CHOICES.

Question: How about putting the vapor barrier a third of the way into the wall cavity?

Answer: This approach was developed by the National Research Council of Canada for use in 10,000 degree-day climates. So it should be all right in milder climates. If it is 70 degF. indoors and 0 degF. outdoors, the vapor barrier temperature will be 70 - (1/3x 70) = 46.7 degF. If the indoor RH is above 45 percent at 70 deg. F. this could be a problem. And since many homeowners feel more comfortable with the indoor RH at 50 to 55 percent, that condition is likely in many homes. Consequently I would not recommend this approach if you are anticipating similar conditions for prolonged periods.

Question: Are there problems with using multiple vapor barriers?

Answer: There is no theoretical reason why this should be a problem as long as neither of the vapor barriers falls below the dew point.

For example, using foil-faced insulation plus poly on the wall inside surface should pose no problem.

Watch out: But watch out for multiple vapor barriers that are spaced apart and exposed to potential leakage. DJ Friedman reports finding severe rot to floor joists in a 10-year old home built over a wet crawl space that had a concrete floor. Kraft-faced insulation had been installed with the vapor barrier "up" towards the warm side or building floor underside (correctly) but an owner, hoping to avoid a problem from the wet crawl area, had loosely stapled poly over the underside of the floor joists.

Condensation and ultimately so much water had accumulated on the upper side of this poly that it was visible as pools or stains on the plastic. The lower few inches of floor joists over this area were so badly rotted, after just ten years, that the inspector (Friedman) was able to tear off the bottom portions of rotted floor joists with his bare hand.

Question: Do you need a vapor barrier in the building ceiling?

Answer: In mild climates, some researchers claim you can safely omit the ceiling vapor barrier if you have good attic (under-roof) ventilation. Exactly how mild and how much ventilation is not clear. I would not leave the vapor barrier out of a cathedral-type ceiling where there is little space for moisture vapor to disperse.

OPINION-DJF: and it seems to me a foolish "economy" to save the small cost of installing a poly ceiling vapor barrier as well as careful sealing against air leaks around ceiling penetrations for lights or plumbing, in view of the frequency with which home inspectors find severe attic condensation problems.

For details about use of acoustical sealants or tapes to seal polyethylene vapor barriers, see ACOUSTICAL SEALANT CHOICES.

This discussion of vapor barriers and condensation in buildings continues at VAPOR CONDENSATION & BUILDING SHEATHING.

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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VAPOR BARRIERS & CONDENSATION in buildings
  VAPOR BARRIERS & AIR SEALING at BAND JOISTS
  VAPOR BARRIERS & CONDENSATION in buildings
  VAPOR BARRIERS & HOUSEWRAP
    Leaks into vinyl-sided building
    Select & Use House Wrap
    Code Requirements for Building Wrap
    Sheathing Wrap Performance Measures
    Water Resistance of Housewraps
    Air Infiltration of Housewraps
    Performance Table for Housewraps
    Can the Vapor Barrier be Omitted?
  VAPOR CONDENSATION & BUILDING SHEATHING
  WOOD SIDING FLASHING DETAILS
  WATER BARRIERS, EXTERIOR

• 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.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• 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
  • 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-0910110969
    
• 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
• The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
• "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 Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
• "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 Maintenance and Repair", U.S. DOE
• "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.
• "Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
• www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
• "Active Solar Heating Systems", U.S. Department of Energy, including
• "Radiant Heating Systems" U.S. DOE
• "Absorption Heat Pumps & Coolers", U.S. DOE
• "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.