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Dew Point Data for Building Wall Cavities InspectAPedia®  -     Advice on calculating the dew point inside building wall or ceiling cavities How to determine the dew point inside insulated building walls

Here we discuss how the wall cavity dew point, the point at which moisture condenses out of air onto a surface, is calculated for a building cavity such as inside of an insulated wall. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Readers should read DEW POINT TABLE - CONDENSATION POINT GUIDE. Also see MOISTURE PROBLEMS: CAUSE & CURE and HOUSEWRAP AIR & VAPOR BARRIERS and HUMIDITY CONTROL TO PREVENT MOLD also VENTILATION in BUILDINGS. Our page top photo shows severe moisture condensation on a basement window and window frame.

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How to Predict the Dew Point in a Building Wall or Ceiling Cavity

• Q&A: Condensation in Walls - Calculate the Dew Point in Building Walls - PDF version; use your browser's back button to return to this page

The question-and-answer article about calculating the dew point in walls and a discussion of mathematical models of moisture condensation, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.

Question about how we calculate the dew point in buildings

All of the literature I have read on condensation within building wall cavities warns of the problem of cavity moisture (a potential source of mold contamination, insect attack, or structural rot). But the literature does not provide any clues as to how to predict wall cavity condensation.

Is there a formula which will determine the dew point inside a building wall when both inside and outside relative humilities and temperatures are known? - J.L.B., G4reenfield Center, NY.

Answer about Calculating and Predicting Wall Cavity Dew Point or Condensation

Mathematical models exist for computing the place and accumulation of moisture condensation inside building walls. Their usefulness, however, is limited for a number of reasons.

First, the models are based exclusively on moisture diffusion theory (moisture molecules moving through building materials). In reality, air leaks into and out of wall cavities, rather than moisture diffusion, accounts for the largest portion of moisture transmission in buildings. Because of variations in workmanship, construction details, uses of sealants and caulks, and similar variables, the relative contributions of diffusion and air leakage in building walls and ceilings is unpredictable.

Second, the moisture condensation mathematical models assume that the building wall is continuous (no holes or penetrations) and that the environmental conditions (temperature, moisture, wind, air pressure) are unchanging.

Actually, conditions constantly change inside and outside of buildings, and cold spots occur at leaks to the outdoors, lapses or omissions of insulation at building corners, air leaks occur around openings for doors and windows, and at thermal short circuits are caused by highly conductive materials such as metal, glass and concrete.

These are the places where the problematic wall or ceiling cavity condensation is likely to occur. So you can also see that the occurrence of wall or ceiling cavity is certainly non uniform in space (building walls or ceilings) and time.

Also the prediction of building wall condensation does not necessarily indicate an actual condensation problem.

The length and severity of winter and the ability of building materials to safely store and later expel moisture are important factors in determining whether a building cavity moisture problem will actually occur.

With this in mind, the best defense against building wall or ceiling moisture damage is a good offense: proper air and vapor barriers, caulking, and thermally-broken door and window components.

To do the wall condensation or dew point calculations, you need to know the temperature and vapor pressure gradients through the wall(or ceiling). These are directly proportional to the resistance's of the wall's components to heat flow and moisture vapor flow (and air leaks). At any point where the calculated vapor pressure exceeds the saturation vapor pressure (derived from the temperature at each point), condensation may occur.

Below we provide links to further information on dew point calculation from ASHRAE and the National Bureau of Standards.

For greater accuracy in predicting wall cavity condensation, the vapor pressure curve is recalculated for each plane of condensation in an iterative procedure.

What is the Dew Point?: the dew point (Tdp) is the temperature at which water vapor just starts to condense out of air that is cooling - for example when warm moisture-laden air contacts a cool surface inside of a wall cavity.

Above the dew point the moisture stays in the air. At or below the dew point moisture leaves the air and in buildings, condenses on the cooler surface that the air is contacting.

This also means that if you are measuring the relative humidity in a room, the RH number only has meaning if you measure the room temperature at the same time and location.

In the table at left, the left-most curve, the 100% relative humidity line offers a simple case - that's air that is 100% saturated. So on the chart below, notice that on the left-most curve, the wet bulb temperature equals the dry bulb temperature - that is, when the air is fully saturated at 100% RH, no more air water can be evaporated out of the air.

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• 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.
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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 CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSEWRAP - TYVEK INSTALLATION DETAILS
HUMIDITY LEVEL TARGET
MOISTURE CONTROL in BUILDINGS
MOISTURE PROBLEMS: CAUSE & CURE
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

• ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries
• National Bureau of Standards (NBS) resource on dew point and wall condensation - see NBS Report #BSM63, Moisture Condensation in Building Walls.

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

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• A Rotting Timber Frame, Steven Bliss, Journal of Light Construction, February 1987.
• The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for this article. All rights and contents are ©Journal of Light Construction and may not be reproduced in any form.
• "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."
  • 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", George Tsongas, Doug Burch, Carolyn Roos, Malcom Cunningham; this paper describes software and the prediction of wall moisture contents. - PDF Document from NIS
• Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems, John Bower
• Linric psychrometric tools can be found at www.linric.com
• "Weather-Resistive Barriers", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
• ...

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