Roof Dew Point

When a window wall or ceiling is not well insulated and becomes cold enough condensation can occur on the interior surface making that surface wet.

Roof dew point.

Why moisture condenses out of air. This is what you see when you look at your car early in the morning. One method to accomplish this is to graphically illustrate the temperature change. When combining exterior foam insulation with fiberglass cellulose or mineral wool roof insulation the foam layer must be thick enough to keep the sheathing temperature above the dew point.

Let s consider miami and phoenix again as two extremes. In the summer miami s relative humidity can reach 85 at a temperature of 80 f. As it cools below 35 degrees water condenses on it. 2 the car roof cools radiatively.

3 the car roof continues to cool. The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. Warmer air can hold more water than cooler air. Dew point in a house this same effect can occur in a house where the interior temperature and humidity are high but exterior conditions are cold.

First you need to calculate the dew point of the interior air during the winter. Relied on by architects engineers hvac designers and roof consultants alike the dew point is defined by the national roofing contractors association nrca as the temperature at which air becomes saturated with water vapor. As it cools below 32 the water on the roof freezes. It is a roof system designer s goal to make sure the temperature at the vapor retarder membrane is warmer than dew point temperature which in effect causes dew point temperature to occur within the insulation that is installed above the vapor retarder.

Obviously a lot of condensation will form on a chilled beverage glass. The air saturation point is the point at which the air can hold no more water it is fully saturated. So the vapor retarder should be effective in preventing or minimizing condensation from occurring within the roof assembly. In this example the dew point temperature is 49 f i e.

Otherwise water vapor can condense on the back side of the sheathing elevating the moisture content of the materials and possibly resulting in mold and rot. The temperature at which air has a relative humidity rh of 100 percent. Or see dew point calculation for walls for making more precise dew point calculations. The dew point or properly dew point temperature is the temperature to which air will have to cool to reach its saturation point.

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. Definition of dew point.