Roof Dew Point Calculation

Let s consider miami and phoenix again as two extremes.

Roof dew point calculation.

22 88 c is the dry bulb interior temperature and 40 f i e. 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. Obtain the winter exterior design dry bulb temperature. Determine the winter interior design dry bulb temperature.

First you need to calculate the dew point of the interior air during the winter. In the summer miami s relative humidity can reach 85 at a temperature of 80 f. Draw the roof assembly to a practical. What s a dew point calculation.

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

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. At a constant dew point when the temperature goes up the rh goes down and when the temperature goes down the rh goes up. Image courtesy professional roofing.

The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. Design values can be found in chapter 14 climatic design. 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. The dew point temperature determines what combinations of temperature and rh will be possible in the storage environment.

In this working example 75 f i e. The national roof contractors association nrca map. 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. 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.

4 c is specifically listed as a relative humidity so no linear interpolation is necessary. That gives you a dew point of about 50 f according to the chart at above left.