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Moisture Calculations for buildings; Heat Loss due to Exhaust Ventilation InspectAPedia®  -     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

This article explains the 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.

Readers should see MOISTURE PROBLEMS: CAUSE & CURE, and also see ROT, TIMBER FRAME which demonstrates severe damage due to moisture, insulation, and vapor barrier defects, and see INDOOR AIR QUALITY IMPROVEMENT GUIDE which includes details about whole house ventilation systems. Also see DEW POINT TABLE - CONDENSATION POINT GUIDE for an explanation of dew points and indoor humidity in buildings, HOUSEWRAP AIR & VAPOR BARRIERS and VENTILATION in buildings. Also see WATER ENTRY in buildings where we describe the cause and prevention of water leakage into buildings.

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Q&As: Amount of Moisture Generated by People in buildings; Building Heat Loss due to Exhaust Ventilation

The link to the original Q&A article in PDF form immediately below is followed 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

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article, (see links just above) 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

Answer:

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

Answer:

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, (see links just above) 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.

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Technical Reviewers & References

• InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
• 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. His recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) is available from the publisher, J. Wiley & Sons, and also at Amazon.com. See our book review of that publication.
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• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR SEALING STRATEGIES
DEW POINT TABLE - CONDENSATION POINT GUIDE
MOISTURE CONTROL in buildings
  ATTIC CONDENSATION CAUSE & CURE
  Attic Moisture or Mold Sources
  BASEMENT CEILING VAPOR BARRIER
  BASEMENT HEAT LOSS
  BASEMENT LEAKS, INSPECT FOR
  Chimney Leaks
  CONDENSATION or SWEATING PIPES, TANKS
  CRAWL SPACE DRYOUT PROCEDURES
  DEW POINT CALCULATION for WALLS
  DEW POINT TABLE - CONDENSATION POINT GUIDE
  DRYER VENTING
  DRYWELL DESIGN & USES
  EFFLORESCENCE, Salts & White / Brown Deposits
  FLOOD Damage Assessment & Repairs
  FLOOD DAMAGED FOUNDATIONS
  FLOOD VENTS & FLOOD PORTS
  FLOODS IN buildings-priorities
  FOUNDATION WATERPROOFING
  GRADING & SITE WORK, EXTERIOR
  GUTTERS & DOWNSPOUTS
  HUMIDITY CONTROL & TARGETS INDOORS
  HOUSEWRAP AIR & VAPOR BARRIERS
  MOISTURE in BUILDING WALLS, EFFECTS
  MOISTURE in CELLULOSE INSULATION
  Mold in Fiberglass Insulation
  Mold on Books, Book Conservation
  Mold on Fiberboard Insulating Sheathing
  MOLD PREVENTION AFTER FLOODING
  MOLD RESISTANT CONSTRUCTION
  MOISTURE CALCULATIONS
  MOISTURE METER STUDY
  MOISTURE PROBLEMS: CAUSE & CURE
  MOLD in buildings
  ROOF VENTILATION SPECIFICATIONS
  Soffit Intake Vents & Attic Condensation
  WATER ENTRY in buildings
  VENTILATION, WHOLE HOUSE STRATEGIES
MOISTURE PROBLEMS: CAUSE & CURE
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

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.
• "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.
• "Moisture Calculations", Q&A article, Solar Age, April 1984
• 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
• Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems, John Bower
• "Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
• Ice Dam Leaks in building attics and roof cavities, how to inspect for evidence of leaks, identify causes, and correct bad attic ventilation, improper roof venting, and these causes of attic mold or roof structure damage
• Mobile Home Inspections common defects unique to factory built housing, inspection methods
• Mold Cleanup, How to clean or remove mold in buildings
• Mold Resistant Drywall is mold-resistant or "mold proof" Sheetrock^R from US Gypsum or Georgia Pacific worth it?
• Mold Prevention: Avoiding Mold Problems in buildings by Using Mold-resistant Construction Products & Practices
• Mold Information Center: What to Do About Mold, mold action plan, valid and invalid test methods, mold remediation
• Rot, fungus, termites, carpenter ants, powder post beetles, & other wood destroying organisms
• Stairways: A Checklist for Stair, Railing, Landing Safety Inspections
• Links to our list of additional information sources: Interior Defects in buildings
• ...