How to Find the Dew Point for Any Temperature & Relative Humidity in a Building
Insulating foam sheathing is becoming fairly common in cold climates.
But in most cases, it violates the rule of thumb that the
exterior of a building should be five to ten times more permeable than the interior. This is particularly true with foil-faced sheathings.
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Some people argue that it’s all right to use insulating sheathing, because it warms up the wall cavity enough to prevent condensation
problems. This is supported by tests (based on 40 percent relative humidity indoors) done at the U.S. Forest Products Laboratory in
As shown in our illustration above, To find the dew point for any temperature and relative humidity: Start with the temperature, move up to the current relative-humidity line.
Move left to the saturation curve, and down to find the dew point, as shown.
How can you gauge the dew point for
First, you need to calculate the dew point of the interior air during the winter. To be conservative, assume an indoor
relative humidity of 50 percent at 70°F.
That gives you a dew point of about 50°F, according to the chart at above left.
Next, calculate how cold it must be outside before the inside surface of the sheathing will fall below the dew point. The inside surface of the sheathing is where condensation is most likely to occur.
You can calculate the temperature at any point in the wall if you know the total R-value inside and outside that point. The temperature rise through the wall and the R-values are in direct proportion.
For example, in Wall A, when it’s 32°F outside, the temperature at the sheathing surface is 7/18 of the way from 32 to 70, or 47°F, which is below the dew point of the interior air. In Wall B, with two inches of foam, the temperature at the sheathing surface is 53°F — safely above the dew point.
Since the average winter temperature in most of central and northern New England is at or below 32°F, Wall A appears risky unless you use one of the more
permeable rigid insulations (headboard or rigid fiberglass), or have a perfect air/vapor barrier. That’s not a bet you should make.
How to Read a Psychometric Chart to Find the Building Surface Dew Point
Psychometric charts look intimidating: in building school my [DF] instructor called these "psychotic charts". But really a psychometric chart is not so crazy or difficult as it crowded lines appear.
The chart shows the relationship between dry bulb temperature (which is what your normal thermometer reads), vapor pressure (how much humidity or water is in the air), and wet bulb temperature (what you would measure as temperature with a wet bulb thermometer that uses evaporation off of the sensor. The chart is tellins us the point at which water will condense out of the air onto a cooler surface.
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Several detailed psychrometric charts useful for determining the dew point are shown and linked-to just below. There moisture is expressed in vapor pressure in mmHg (millimeters of mercury).
The curved relative-humidity lines intersect with the diagonal lines to show the dew point for various temperatures and levels of relative humidity.
Dry bulb (Tdb) temperature (chart bottom or "x" axis) is just the measured indoor temperature measured using a standard thermometer.
Relative humidity (RH) is the amount of water in the air. We use the term "relative" humidity because the amount of water that a given volume of air can hold decreases as temperatures drop. (Think of it as cold air being more dense, squeezing water molecules out of suspension in the air.)
So RH is the ratio of the actual water vapor pressure in the air to the water vapor pressure in air that is fully saturated (can't hold any more water) at that same temperature.
Wet bulb temperature (Twb) measures the amount of water that can be taken out of the air (by evaporation) - the old "sling psychrometer" used a simple mercury thermometer with a wet cloth over the sensing bulb to measure Twb by swinging the thermometer around at the end of a string - or properly: a sling psychrometer. Really.
Also see Tools for Measuring Humidity This article describes alternative methods for measuring indoor temperature, humidity, relative humidity.
See the left-most curve, the 100% relative humidity line for 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.
Define Dew Point: Now the good part: 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.
That's why, for example, when measuring basement humidity we will get different RH measurements in the center of the room than we will find right against a cool foundation wall - we discuss this in more detail at HUMIDITY LEVEL TARGET.
Dew Point Example: in the chart below, if the room temperature (Tdb) is 43 degC and the relative humidity (RH) is 20% (the curved line reading up from 43 degC) then the dew point is 15 degC (reading horizontally across to the left-most curved line and noting the dew point temperature scale set along that curve).
Dew Point Example 2: in the psychrometric chart given below, read up from 50 degC dry bulb temperature to the 20% RH curve, then follow the horizontal line from that point to the left to the outermost curve on the chart. If you don't go blind following this chart (click to enlarge it) you'll see that the Dew Point (Tdp) and also the wet bulb temperature (Twb) at the end of that line is 21 degC. Lots of variations of psychometric charts are available; they will all work about like this.
To convert temperatures from Fahrenheit to Celsius use: Tc = (5/9)*(Tf-32) where Tc= the Celsius temperature and Tf= the Fahrenheit temperature.
"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 NIS
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