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Vapor Barriers & Building Condensation - Part 2
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Vapor barriers and condensation in buildings
How various building wall sheathing materials affect building condensation and moisture
Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices

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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.

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Article Two on Vapor Barriers & Building Condensation

"Vapor Barriers, Part II - Vapor Barriers and Condensation, building researchers are helping out with the tricky questions" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article. Our photograph (below) shows an insulation retrofit that jammed fiberglass between rafters over an attic, combined with a foil "radiant barrier" that in our view risked moisture traps or future roof leak traps (and building damage) hidden under the roof decking.

Vapor Barriers & Building Condensation Explained, Part 2 - PDF form, use your browser's back button to return to this page
Vapor Barriers & Building Condensation Explained, Part 2 - PDF form, continued

Along with tables summarizing building moisture research from the National Forest Products Laboratory, this article answers the following building condensation and materials questions:

Do insulating sheathings on building walls cause condensation problems?
Do insulating wall sheathings put the vapor barrier on the wrong side of the wall?
Is polystyrene insulation better than foil faced insulated sheathing for preventing condensation?
Should vent strips be installed when using foil-faced building insulation on walls?
How do stressed-skin panels affect building condensation problems?
How much vapor transmission takes place through foam insulation?
Is it safe to add retrofit building insulation without adding a vapor barrier?
What conditions cause high indoor humidity and condensation?
How can a crawl space be both insulated and ventilated?
Why do we need to vent ceilings if walls do not need venting?

At VAPOR BARRIERS & CONDENSATION in buildings we looked at the fundamentals of moisture condensation in buildings: what causes condensation, how to control condensation, and whether we should worry about moisture condensation in buildings. We concluded that small amounts of moisture condensation can occur and do occur in wall cavities, but that structural damage rarely occurs because the walls dry out before temperatures are warm enough to support wood rotting fungi.

Still, risks of paint-peeing, corrosion of metals, hidden costly mold contamination, and degrading of insulation R-values do exist. A dry wall cavity is certainly preferable to a wet one.

And the most reliable way to achieve a dry wall is by installing a continuous vapor-retarding membrane such as 6-mil polyethylene plastic, paying a lot of attention to joints and penetrations. In fact, the penetrations are usually more important than the main surfaces, since air leaks generally transport a lot more moisture into a wall cavity than does vapor diffusion.

In this article we will examine questions frequently raised about how various materials and applications affect moisture condensation. Even if you have a good handle on the theory, applying it can be trick. Some building materials both insulate and block moisture vapor flow, confusing the issue. And in some applications, moisture vapor flows reverse seasonally, or spaces need both ventilation and sealing. It is enough to make a moisture vapor conscious contractor move to Phoenix (where presumably it is warm and dry enough that not much mold grows).

Question: How about insulating building exterior wall sheathings? Do they cause moisture problems?

Answer: 1980's tests at the Forest Products Laboratory in Madison WI confirmed earlier reports that in a 2x4 wall in a moderately cold climate (7863 degree days), insulating sheathings caused no greater condensation hazard than ordinary sheathings. In fact, in the FPL tests, the insulating sheathings seemed to protect the siding from condensation, probably by slowing the flow of moisture to the siding.

For thicker insulated walls, which will have colder sheathing, or for buildings with more humid interiors (greater than 40 percent RH, which is most buildings with conditioned air in winter) these findings should be applied with caution. See Tables 1 (at the top of this page) and Table 2 (at left) that present some of the FPL findings.

Also see SIDING WOOD, FAILURES OVER FOAM BOARD where we describe wood siding failures when installed over foam insulating building sheathing, and see SHEATHING, FOIL FACED - VENTS - do we need to vent building walls with siding installed over foam board insulating sheathing?

Question: Don't insulating foam board wall sheathing products used on a building exterior put a vapor barrier on the wrong side of the wall?

Answer: A widely accepted rule of thumb holds that the building's exterior wall surface should be 5 to 10 times as moisture-permeable as the interior vapor retarder installed on the inside surface of the building's exterior walls. ("Retarder" not "barrier" is ASHRAE's preferred term). However, since insulating wall sheathings on a building exterior (under the siding) keep the wall cavity warmer and present a warmer face to the wall cavity, higher levels of vapor in the wall can be tolerated before condensation occurs. Hence the ratio of inside to outside permeability may be lower.

Question: How much lower can the ratio of inside to outside wall surface moisture permeability be without a problem on walls with exterior insulating sheathing?

Answer: You can play with the numbers if you're inclined, or hedge your bets by using a lapped and caulked poly vapor barrier with all wall penetrations sealed (our recommendation). This approach also controls air infiltration. Hence the awkward but useful phrase air/vapor barrier.

Question: is polystyrene better than foil-faced foam insulating sheathing boards in preventing moisture condensation problems in building walls?

Answer: Theoretically, yes, because it is more permeable to water vapor; but no, because it has a lower R-value per inch. In the FPL tests,the foil-faced sheathing did slightly better, probably because the wall cavities were slightly warmer. Placing the rigid foam insulating board on the interior of the building wall side-steps the whole problem.

Question: How about using vent strips on exterior walls where foil faced building sheathing is to be installed?

Answer: These are probably not a good idea. Wall vent strips were tried on one wall in the FPL tests, and they actually increased the amount of wall cavity condensation. One possible reason is that the air drawn out of the wall through the vents was replaced with moist indoor air.

Vent strips were used only at the top of the walls. [That wall venting design is similar to the problem of installing a ridge vent on a home with no soffit intake venting. The presence of the high vent and no source of outdoor air leads to the ridge vent acting as a "pump" to draw indoor air out of the building, increasing home heating costs, or in the case of the wall top exit vent, also increasing the movement of indoor moisture into the wall cavity - DJF].

If wall vents are placed at both the top and bottom of the wall to solve this problem, the air movement through the wall cavity may degrade the R-value of the wall.

Our wall vent photo (above) shows a home-made wall vent installed by a building owner who hoped to avoid a moisture problem in the wall and in a raised wood floor over a concrete slab. At this building the ventilation system served only as an entry path for carpenter ants and water.

See SIDING WOOD, FAILURES OVER FOAM BOARD where we describe wood siding failures when installed over foam insulating building sheathing, and see SHEATHING, FOIL FACED - VENTS - do we need to vent building walls with siding installed over foam board insulating sheathing?

Question: What about stress-skin insulated building panels and moisture problems?

Answer: Many stress skin building panels have no vapor retarder on the inside, just drywall, and low permeance sheathing, such as OSB or waferboard on the panel exterior surface. Theoretically, water could condense within the panel, most likely at the foam/sheathing board interface. However, since on a 0 degF winter day, less than quarter of an ounce of water will diffuse through an entire 4x8 foot stress skin panel of foam (3 1/2 inches thick) over 24 hours, I wouldn't lose sleep over this. I have asked around, and have heard of only one problem with moisture (frost under the plywood facing of the insulated stress-skin building panel) and that was under near-arctic conditions.

I would be more concerned about caulking the stress skin building panel joints well so that moist air would not leak out and contact cold surfaces - not to mention lose heat. Nonetheless, a coat of vapor barrier paint wouldn't hurt.

Question: How do you determine the amount of vapor transmission through foam insulating board?

Answer: Moisture permeability ratings are like U-values. So if you can calculate heat transmission you can calculate vapor transmission. Perms measure the grains of water transported per hour per square foot per inch of mercury vapor pressure (the difference between the inside and outside moisture vapor pressures on the surface or material). So, multiply the perm rating times the number of square feet of the wall, times the vapor pressure differences on the two sides of the wall, and you can count the grains of water.

Question: Is it safe to add retrofit insulation without also adding a vapor barrier?

Answer: If you add fibrous insulation to a cavity wall, it will increase the risk of a wall condensation problem and may exacerbate existing problems such as peeling paint. [Both Bliss and Friedman report having inspected buildings whose exterior paint was intact and sound until soon after insulation (without vapor barriers) was blown into previously empty wall cavities of homes in northern climates.-- DJF]

Nonetheless, various field studies in both moderate and cold climates have failed to find serious problems in the walls of retrofitted homes with or without vapor barriers. There are mitigating factors in older homes. Of the ones that were monitored for relative humidity, few were much over 40 percent. Plus, most had highly permeable wood plank exterior wall sheathing, which tends to store and release any moisture condensate.

A reasonable approach would be to seal around moldings, electrical outlets, and other wall penetrations and keep building interior in the 40 percent range. When you redecorate, consider vapor barrier paint on the interior surface of exterior walls.

[In other words, we probably agree that where a newly-insulated older home has had a serious paint failure, there was most likely also a pre-exiting indoor high moisture level and indoor leaks or moisture problems, such as a wet basement or crawl area -- DJF.]

Two detailed articles discussing insulation retrofits, air leaks, moisture problems, and insulation effectiveness are at ENERGY SAVINGS RETROFIT CASE STUDY and ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE.

Question: What conditions create high indoor humidity?

Answer: In a very tight house, the normal moisture generated by human respiration and perspiration, along with cooking, bathing, and cleaning, can cause a moisture buildup. With additional moisture sources (building leaks, wet basements), high moisture levels can build up even in a not-so-tight building.

A frequent cause of high indoor moisture is the presence of a dirt floor crawl space, even if there is no obvious crawl space flooding. A water table three feet below the soil surface of a dirt floor basement or crawl space can release 12 gallons of water vapor per 1000 square feet in one day.

Covering the soil with a heavy polyethylene plastic cover should reduce this moisture movement into the home by about 80 percent and reduce crawlspace ventilation requirements by a factor of 10.

See these crawl space ventilation and dry-out articles:
CRAWL SPACE DRYOUT PROCEDURES
MOLD ON DIRT FLOORS

Other building moisture sources are un-vented clothes dryers and combustion appliances, drying firewood indoors, and house plants.

See MOISTURE CONTROL in buildings for an extensive list of diagnostic and "how-to" articles on controlling moisture in buildings. Also see HUMIDITY LEVEL TARGET.

Question: How can I both insulate and ventilate a crawl space?

Answer: one option is to insulate the floor above the crawl area with a vapor barrier on the warm side of the insulation (over the joists) and to leave the crawl space vented in all but the coldest weather, perhaps using thermally operated foundation vents (1980's convention). Low permeance rigid foam board insulation is the best product to use here because it will also resist forming a problem reservoir of toxic but hidden mold. (See Mold in Fiberglass Insulation).

Current (2009) best construction practices no longer ventilate crawl spaces; rather we convert the crawl space to an insulated, "conditioned" space, making sure that we keep out rot and mold causing water. That's because experience and field studies indicate that it is just about impossible to control crawl space ventilation to work optimally for all weather and building conditions. -- DJF

Our photo (above left) shows a poly moisture barrier placed over dirt in a crawl space - also notice that radiator in the right of the photo - the owner converted this crawl to a dry, heated space - what may be missing is foundation perimeter insulation, perhaps using foam board, unless that step was already taken outside.

See these crawl space mold, ventilation and dry-out articles:
  CRAWL SPACE DRYOUT PROCEDURES
  CRAWL SPACE SAFETY ADVICE
  CRAWL SPACE VAPOR BARRIER
  CRAWLSPACE MOLD ADVICE
  MOLD CLEANUP by MEDIA BLASTING
  MOLD ON DIRT FLOORS

Question: Why do I have to ventilate an attic or cathedral ceiling if I don't have to ventilate the building wall?

Answer: No vapor-retarding system is perfect. And due to the stack effect (air movement upwards in buildings as warm air rises), a disproportionate amount of moist air will find its way into the ceiling cavity or attic space. Also, attic and roof ventilation help for summer cooling, ice dam prevention, and a cooler attic means a cooler roof deck which means longer roof life. See ROOF VENTING NEEDED? and also see ICE DAM PREVENTION for more details.

Question: How about insulation and vapor barriers for a full basement: where does the vapor barrier go?

Answer: My opinion is that the basement wall should be treated much like the rest of the building shell - waterproofed on the outside (or more important, keep surface runoff and roof spillage away from the building foundation), and vapor-proofed on the inside (if you are finishing the basement interior walls).

Exterior foundation insulation will help keep the foundation wall warmer and less likely to condense water in winter and summer. By the way, if you've got standing water or even occasional wet floors in the basement, vapor barrier placement is a moot point - you need to solve the water problem first.

See WATER ENTRY in buildings, as well as these basement insulation and moisture articles:
  BASEMENT CEILING VAPOR BARRIER
  BASEMENT HEAT LOSS
  BASEMENT LEAKS, INSPECT FOR
  BASEMENT WATERPROOFING
    BASEMENT De-Watering Systems
    EXTERIOR WATER SOURCE ELIMINATION
    FOOTING & FOUNDATION DRAINS
  FOUNDATION DRAINS, INTERIOR
    FOUNDATION WATERPROOFING
    GEOTEXTILES & DRAINAGE MATS
    SEALERS, Basement Floor & Wall Moisture
    SITE DRAINAGE
    SUMP PUMPS GUIDE
    SWEATING (CONDENSATION) on PIPES, TANKS
    WINDOW / DOOR AIR LEAK SEALING HOW TO
  Window Certification
  WINDOW EFFICIENCY Features & Ratings
  Window Flashing & Sealing Guide
  WINDOW HARDWARE PHOTOS
  WINDOW LEAKS INTO BASEMENT
  WINDOW TYPES - Photo Guide
  WET BASEMENT PREVENTION

This discussion of vapor barriers and condensation in buildings continues at VAPOR BARRIERS & AIR SEALING at BAND JOISTS.

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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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
- 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.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
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.
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