Roof Dew Point Calculation

Determine the winter interior design dry bulb temperature.

Roof dew point calculation.

The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. What s a dew point calculation. Draw the roof assembly to a practical. Builders or designers perform dew point calculations to determine whether a certain component of a wall ceiling or roof in most cases the sheathing will stay warm enough during the winter to avoid condensation problems.

Obviously a lot of condensation will form on a chilled beverage glass. To be conservative assume an indoor relative humidity of 50 percent at 70 f. In this working example 75 f i e. This design value is essentially the temperature that.

According to nrca dew point calculations are necessary whenever the outside average january temperature is below 4 c 40 f and when the expected interior winter rh is 45 percent or greater. That gives you a dew point of about 50 f according to the chart at above left. Dew point varies depending on the amount of water vapor present in the air with more humid air resulting in a higher dew point than dry air. At a constant dew point when the temperature goes up the rh goes down and when the temperature goes down the rh goes up.

4 c is specifically listed as a relative humidity so no linear interpolation is necessary. First you need to calculate the dew point of the interior air during the winter. How to calculate the dew point more precisely e0 0 611 kpa l rv 5423 k in kelvin over a flat surface of water t0 273 k kelvin t temperature in kelvin td the dew point temperature in kelvin. In the summer miami s relative humidity can reach 85 at a temperature of 80 f.

Or see dew point calculation for walls for making more precise dew point calculations. In this example the dew point temperature is 49 f i e. The national roof contractors association nrca map. The dew point temperature determines what combinations of temperature and rh will be possible in the storage environment.

Image courtesy professional roofing. 22 88 c is the dry bulb interior temperature and 40 f i e. Dew point temperature can be determined by using a simplified version of the ashrae psychrometric chart which is shown in figure 5. As previously mentioned the interior design conditions are 70 f for design dry bulb temperature design inside temperature and 65 percent relative humidity design relative humidity.

Obtain the winter exterior design dry bulb temperature. Let s consider miami and phoenix again as two extremes.