Shutter Condensation Calculator Explained
For the given room temperature Tin and
calculator solves the defining
Equation (1) for the Dewpoint
The solution is found numerically using Newton's Method and by initially guessing the room temperature as the dewpoint. The algorithm must be able to evaluate the saturation vapor pressure of water at arbitrary temperatures, and therefore relies on interpolation formulas for this quantity.
At freezing conditions (sub-zero Deg. C temperatures), the Dewpoint Calculator uses the saturation vapor pressure over ice, thus computing the frostpoint.
After finding the dewpoint temperature, the calculator determines the heat flux q W/m2 through the window when its inner surface is at that temperature by using the relationship
q = (Tdp - Tout) x Uw
where Tout is the outside temperature and Uw W/m2K is the U-value of the window. It then determines the U-value Us W/m2K of a shutter that is consistent with that heat flux by using the similar relationship
Us = q / (Tin - Tdp)
To design a shutter, use the actual U-value of your window, the warmest temperature that you normally keep the room, the coldest outdoor temperature and the most humid that the room is likely to be.
According to Greenspec, with the addition of Passivhaus, typical available U-values are:
|1.7||Double-glazing with low-e coating|
|1.3||Double-glazing with low-e coating and Argon filled|
|0.4||Triple-glazing with multiple low-e coatings and Xenon filled|
Note: For a fixed relative humidity, neither dewpoint nor frostpoint depend on the atmospheric pressure. However, if a moist air sample is pressurized at a constant temperature and at a constant absolute humidity, both the relative humidity and the dewpoint will rize.
|Dewpoint Versus Temperature Curve for RH=5%|
Deg.C = Deg.K - 273.15and
Deg.F = 9/5 Deg.C + 32.
Pvs(T). These formulas are of varying accuracy and have different ranges of validity. The Dewpoint Calculator uses the formula put forward in 1997 by IAPWS, the International Association for the Properties of Water and Steam. It is very accurate for
273.15 K < T < 647.096 K, with the relative accuracy never exceeding
0.06%. For details, please consult the source code.
x, Newton's Method computes the iterates
x0. If the problem is not ill-conditioned, iterates will converge quickly to the solution
This calculator is based on the Dewpoint Calculator by Wolfgang Kühn, which does all the hard work.
It was amended for shutter calculations by Dave Howorth - scc at howorth dot org dot uk
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