Note that the exterior insulation has increased the temperature of the sheathing to 40.28 F, which is higher than the dew point of 37.2 degrees. R-20 cavity Insulation with R-5 exterior insulation It is still 70 degrees F, 30% RH inside, with a dewpoint of 37.2 degrees F, and it is still 30 degrees outside: This second sample graph below illustrates the same conditions as the previous slide but with R-5 exterior insulation added to the wall. In some cases, liquid water accumulates in the cavity. The moist air travels through the gaps, through the permeable insulation materials and through holes and inconsistencies in the exterior air barrier, sometimes condensing on the backside of the sheathing and increasing wood moisture content above a safe level. – by air pressure differences developed from wind, stack effect, or mechanical equipment. This warm, moist air is driven through holes, cracks or gaps in the walls – such as around electrical boxes, under the base of the drywall at the plate, interior partition intersections, etc. In this example above, we see the condensing surface (Interior of sheathing) is at 33.5 degrees F So, if that moist air hits a surface that is below 37.2 degrees, the moisture will condense into liquid water.
This problem can be avoided by adding exterior insulation in order to keep the wall sheathing warmer than the dew point (the temperature at which water condenses at a particular relative humidity).Īs an example, when the inside of a building is heated to 70 degrees F and the air inside is 30% relative humidity on a day when the outside temperature is 30 degrees F, the dew point (the point when moisture vapor condenses into liquid) is 37.2 degrees. Air leaks from the inside can carry moisture that condenses on the structural sheathing causing an elevated moisture content – and possibly even accumulation of liquid water – that could lead to rot and deterioration. When warm moist air comes in contact with cold surfaces, condensation occurs - often with very serious consequences. R-20 cavity Insulation with no exterior insulation Nearly one third of the year, moisture could be collecting in the wall cavity, condensing and liquid water could be accumulating in the wall cavity. As we can see, from November through March there is potential for condensation. Safe conditions – defined as no danger of condensation forming – are the times when the light blue line is above the pink zone. The dark blue line represents the mean daily temperate in climate zone 5, and the light blue line represents the temperature of the inside surface of the exterior wall sheathing (back side of sheathing). The data: The sample graph below illustrates when the potential for condensation can occur inside a wall cavity with either no or too little exterior insulation.The result: the surface (usually a wall) gets wet, and mold and mildew begin to grow. This means that warm, moist outdoor air can get in, reach a cool surface, and condense. The reality: Many walls have too little insulation and too many air leaks.It will also (hopefully) make the building more comfortable. The perception: When you insulate a wall, it reduces the heating and cooling costs of a building.This example is for a 2×4 wall in climate zone 4* The question then becomes: when does it make sense to add exterior insulation and when can it be skipped? Another available tool is an online Dew Point calculator.Įlevation 0-500 feet (0-300 feet in Alaska), 30 in.Įlevation 501-1,900 feet (301-1,700 feet in Alaska), 29 in.Įlevation 1,901-3,900 feet (1,701-3,600 feet in Alaska), 27 in.Įlevation 3,901-6,100 feet (3,601-5,700 feet in Alaska), 25 in.Įlevation 6,101-8,500 feet (5,701-7,900 feet in Alaska), 23 in.Įlevation 8,501-11,000 feet (above 7,900 feet in Alaska), 21 in.As energy codes get more stringent, adding exterior wall insulation is becoming a reality for more builders.
#Water dew point calculator pdf
These can be calculated automatically using Wildland Fire RH Calculator Apps for iOS and Android available for free.Ĭlick here to obtain a full set of tables in PDF form. The following tables calculate Dew Point and Relative Humidity based on the observed wet bulb and dry bulb temperatures and the elevation at the site of the observation.
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