Roof Dew Point Calculation

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

Roof dew point calculation.

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. 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. 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. 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. 4 c is specifically listed as a relative humidity so no linear interpolation is necessary. Draw the roof assembly to a practical. The dew point is a specific temperature at a given humidity at which water vapor condenses.

Image courtesy professional roofing. In the summer miami s relative humidity can reach 85 at a temperature of 80 f. 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.

Dew point temperature can be determined by using a simplified version of the ashrae psychrometric chart which is shown in figure 5. The national roof contractors association nrca map. The dew point temperature occurs at the intersection of the design dry bulb temperature column and the rh row. 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.

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. First you need to calculate the dew point of the interior air during the winter. Design values can be found in chapter 14 climatic design.

The dew point temperature determines what combinations of temperature and rh will be possible in the storage environment. This design value is essentially the temperature that. To be conservative assume an indoor relative humidity of 50 percent at 70 f. At a constant dew point when the temperature goes up the rh goes down and when the temperature goes down the rh goes up.

In this working example 75 f i e. 22 88 c is the dry bulb interior temperature and 40 f i e.