Moisture Calculations for buildings
Heat Loss due to Exhaust Ventilation
MOISTURE CALCULATIONS - CONTENTS: How much moisture do people generate in a building?Heat loss calculated due to exhaust air ventilation in buildings. Causes of high indoor moisture. How to eliminate sources of un-wanted moisture & condensation indoors. Role of vapor barriers, infiltration barriers, building condensation. Sources of building mold, rot, paint failures. Major vehicle of moisture movement out of homes. What to do about high indoor moisture levels
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Building moisture level & moisture source calculations:
This article explains methods for calculation of indoor moisture levels produced by building occupants - how much moisture does a person generate in a building. This website discusses how to inspect, diagnose problems in, and install or repair building insulation & ventilation systems including heat loss, moisture, & interior stains.
Our page top photo shows extreme condensation at the header of a basement window in a home exposed to interior leaks. High indoor moisture levels can lead to costly mold contamination problems as well as insect attack and rot on buildings.
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Realistic Estimate of Moisture Produced by Building Occupants;
Building Heat Loss Caused by Exhaust Ventilation
Question 1 - Human Moisture Contribution in buildings:
The otherwise fine article on indoor moisture (MOISTURE PROBLEMS: CAUSE & CURE Solar Age, 1/84) contains an error concerning moisture generation in buildings. It suggests that three people produce about 16 pounds of water per day from respiration and perspiration. A more realistic estimate would be 3.9 - 6.6 pounds. -- David A. Herbert, Seal Beach, CA
Author Anton TenWolde used the standard values listed in the 1979 Equipment volume of the ASHRAE Handbook. These are 0.2 pounds of water per hour for a person at rest, 0.6 pounds of water per hour for a person at hard work, and 0.4 pounds of water per hour produced by a person indoors on average.
He assumed a typical building occupancy schedule. Environmental conditions, clothing, and level of activity all affect the level of moisture produced by humans indoors. The values used in the article were conservative.
The National Bureau of Standards says that a family of four typically produces two to three gallons of water vapor a day. Additional moisture migrates up through the building from basements and crawl spaces, particularly where no vapor barrier was placed on or below those floors or surfaces. Once in the house, water vapor enters wall and ceiling cavities by two primary means: diffusion and convection.
Question 2 - Impact on Heat Loss from Building Air Exhaust Ventilation
Wouldn't exhausting 30 cfm of air cause an additional heat loss per day of 31,000 BTUs (over 0.2 ACH infiltration) rather than 11,000 stated in the article? -- Bill Stuble, Green River WY
Running the exhaust fan reduces the rate of natural air infiltration by tending to dominate the air exchange. Mathematically, the effect of combining natural and forced air ventilation in buildings can be approximated by taking the square root of the sum of the squares of the two air exchange rates. Compared this way, the lower figure makes sense.
[DJF Comment: running an exhaust fan in a building with no air-to-air heat exchanger in place will increase the outside air infiltration rate into the building by the same openings that were present before the fan was turned on.]
This article is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
The text below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
-- Adapted with permission, from original material appearing in Solar Age Magazine and written by Steven Bliss.
The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.
"Moisture Calculations", Q&A article, Solar Age, April 1984 - PDF version, Use your browser's back button to return to this page
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Questions & answers or comments about how to calculate bulding moisture levels & moisture contributions from various sources.
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
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"Moisture Problems: Causes and Cures. Understanding moisture problems can steer you free of trouble", Steve Bliss: Building it Right, Solar Age, March 1983 p. 37, 38. -- Adapted with permission, from original material to form this web page article.
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."
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
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