Condensation at a basement window (C) Daniel Friedman 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
  • POST a QUESTION or READ FAQs about how to calculate bulding moisture levels & moisture contributions from various sources.

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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.

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

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.

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.



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