Passive House Windows- U Value

As stated in last PH post, the formula for window transmission losses is:

QT (in kBTU/yr) =  ∑(Uw, installed x Aw x ft  x Gt)  

Where Uw, installed is the U value (U= 1/R) of the window installed in BTU/hr ft2 F; Aw is the total window area in ft2 (square feet); ft is the temperature correction factor usually 1.0 and is unitless; and Gt is heating degree hours in kFhr/yr (thousand degree Fahrenheit hours per year) for your area.

Here's a good explanation of Passive House U value info:

http://www.passivhaustagung.de/Passive_House_E/window_U.htm

Looking at the U value installed formula: 

Uw, installed= [(Ug x Ag) + (Uf x Af) + (Ψg x Lg) + (Ψinstall x Linstall)] / Aw 

We see that this is the formula for the U value of a window-

Uw= [(Ug x Ag) + (Uf x Af) + (Ψg x Lg)] / Aw
 
-plus the linear thermal bridge coefficient of the installation edge around the window added before dividing by overall area of window (Aw). So to get the installed U value if we get an accurate uninstalled window U value which is possible when using a Passivhaus Certified window from the window label, we multiply the uninstalled window U value by area of window and add the product of the installation linear thermal bridge coefficient and the perimeter of window installation length and then divide all by area of window again.


The length of the installation, Linstall, is just the perimeter of the window. The linear thermal bridge coefficient, Ψinstall, of the installation is another matter. The definition of thermal bridge free construction is to have a linear thermal bridge loss coefficient of < 0.006 BTU/hr ft2 F.

You must model the whole window installation using THERM to get the coefficient. You subtract the separate ULdT calc of the window and of the wall from the calc of whole assembly to get Psi installed (one explanation ).

It's best to use a window installation detail that is either Passivhaus certified or closely resembles one. While it is possible to determine the U value of the window after gathering information from the manufacturer (explanation in this link ), it is generally much easier/safer to pick a certified Passivhaus window to begin with and just enter that data in your PHPP. 

As you might have guessed, it's not just necessary to buy good triple glazed Passive House windows but the detail for installation is crucial. If not installed with a thermal bridge free detail, you lose the effectiveness of the great windows. One person explained the relationship between all the insulation areas around a building by pointing out the doubling of insulation equals half the heat loss. Therefore, when you make a heat loss place, you have to massively increase your insulation everywhere else.

Next we'll look at how to approach the window details.