Roof Dew Point Calculation

The dew point temperature determines what combinations of temperature and rh will be possible in the storage environment.

Roof dew point calculation.

Determine the winter interior design dry bulb temperature. That gives you a dew point of about 50 f according to the chart at above left. Furthermore the higher the relative humidity the closer the dew point to the current air temperature with 100 relative humidity meaning that dew point is equivalent to the current temperature. Draw the roof assembly to a practical.

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. Image courtesy professional roofing. In the summer miami s relative humidity can reach 85 at a temperature of 80 f. The dew point is a specific temperature at a given humidity at which water vapor condenses.

What s a dew point calculation. The national roof contractors association nrca map. This design value is essentially the temperature that. Dew point temperature can be determined by using a simplified version of the ashrae psychrometric chart which is shown in figure 5.

4 c is specifically listed as a relative humidity so no linear interpolation is necessary. The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. At a constant dew point when the temperature goes up the rh goes down and when the temperature goes down the rh goes up. 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.

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. Let s consider miami and phoenix again as two extremes. Obviously a lot of condensation will form on a chilled beverage glass. In this example the dew point temperature is 49 f i e.

Design values can be found in chapter 14 climatic design. In this working example 75 f i e. 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.

Or see dew point calculation for walls for making more precise dew point calculations. 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.