Roof Dew Point Calculation

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.

Roof dew point calculation.

4 c is specifically listed as a relative humidity so no linear interpolation is necessary. The national roof contractors association nrca map. In this working example 75 f i e. In this example the dew point temperature is 49 f i e.

22 88 c is the dry bulb interior temperature and 40 f i e. To be conservative assume an indoor relative humidity of 50 percent at 70 f. This design value is essentially the temperature that. The dew point temperature determines what combinations of temperature and rh will be possible in the storage environment.

Let s consider miami and phoenix again as two extremes. First you need to calculate the dew point of the interior air during the winter. 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. Image courtesy professional roofing.

Draw the roof assembly to a practical. Or see dew point calculation for walls for making more precise dew point calculations. Obviously a lot of condensation will form on a chilled beverage glass. Design values can be found in chapter 14 climatic design.

The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. 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. Dew point temperature can be determined by using a simplified version of the ashrae psychrometric chart which is shown in figure 5. Obtain the winter exterior design dry bulb temperature.

The dew point is a specific temperature at a given humidity at which water vapor condenses. At a constant dew point when the temperature goes up the rh goes down and when the temperature goes down the rh goes up. What s a dew point calculation. 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.

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.