Roof Dew Point

The dew point is a specific temperature at a given humidity at which water vapor condenses.

Roof dew point.

First you need to calculate the dew point of the interior air during the winter. The temperature at which air has a relative humidity rh of 100 percent. The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. Or see dew point calculation for walls for making more precise dew point calculations.

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. 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. To be conservative assume an indoor relative humidity of 50 percent at 70 f. In the summer miami s relative humidity can reach 85 at a temperature of 80 f.

Definition of dew point. So the vapor retarder should be effective in preventing or minimizing condensation from occurring within the roof assembly. 2 the car roof cools radiatively. This is what you see when you look at your car early in the morning.

Warmer air can hold more water than cooler air. Let s consider miami and phoenix again as two extremes. As it cools below 35 degrees water condenses on it. Why moisture condenses out of air.

In this example the dew point temperature is 49 f i e. 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. If the dew point temperature falls within the insulation that is above the vapor retarder there is sufficient roof insulation above the vapor retarder. As it cools below 32 the water on the roof freezes.

That gives you a dew point of about 50 f according to the chart at above left. Obviously a lot of condensation will form on a chilled beverage glass. 1 initially the air and the car are at temperature 37 f and the dew point is 35. The air saturation point is the point at which the air can hold no more water it is fully saturated.

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. 3 the car roof continues to cool. 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.