Passive House Window Detail: Locate and Seal

When creating a good window detail there are a couple of things beyond weather tight and and aesthetics which we in Maine must adhere to in order to meet Passive House standards or just to keep our efforts in super insulating the shell from falling apart. The first is to make the opening air tight in the end- if it's not air tight then no Passive House standard and heat loss. The other is to avoid thermal bridges- if we are paying for triple glazed windows and again, invest in super insulating, then we must make sure the center of the insulating wall aligns with the center of the insulating window or a sudden change in temperature will lead to moisture problems and heat loss.

There are many superinsulated construction techniques (here are drawings of some: http://www.mn2030.umn.edu/presentations/session4/Arch2030_PRESENTATION-Jacobson.pdf ) but we'll look at simple double stud wall. Here's a good drawing by 475 ( fourseventyfive.com ) who do a great job of informing the public about Passive House construction techniques:

As you can see the air sealing is accomplished through tapes and membranes and that the window is centered in the insulation layer of the wall. If the temperature of the coldest part of window is on the wrong side of the middle temperature of the wall then there is a sharp change in the isotherm lines and thermal bridging and hence moisture can occur. Ideally the isotherms of the wall narrow and shoot up through the window with the center of the temperature change aligned as in this slide from Quantum Builders presentation whose source is the Passive House Institute:

It pays to install superinsulating triple glazed Passivhaus certified windows properly. These windows allow you to be comfortable close to the glass as we only feel comfortable if temperature difference at window compared with rest of room is less than 7 1/2 degrees F. 

In Maine the window and door business is swinging toward proper unassisted shielding from the elements. 

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.

 

Passive House Windows- Energy Balancing

Here in Maine we need to design our buildings to the Passivhaus ( or "Passive House" as sometimes used in English speaking countries) standard in order to reduce our heating needs to almost nothing and to reduce our electrical energy needs as well. Part of this effort involves choosing triple pane well-insulating windows and installing them with thermal bridge free details.



The recommended window insulation value (resistance to heat transfer in hour square foot degrees Fahrenheit per British Thermal Unit or, hr ft2 F/ BTU ) for a window off the shelf is a minimum R 7.1 or an installed minimum of R 6.68. This R6.68 number holds true for the installed value of the glazing components of a Passive House door as well. These numbers are recommendations for Central European climate and thus a higher R value for Maine is certainly preferable. And certified Passive House windows are best.

Now part of a Passive House strategy to achieve low energy use is to gather as much solar heat gain as is practical without overheating. To this end the glazing in windows must be significant on south sides of the building and somewhat east and a bit on west sides with minimal on north. And the glazing in the windows must be shaded with overhangs tailored to each compass orientation to avoid overheating in summer and let lots of sun in, in winter. And this glazing must be designed to allow a certain amount of solar heat gain without reflecting too much so the heat passes into the building. This solar heat gain has a number called the solar heat gain coefficient or SHGC which represents the amount of solar heat allowed in with 1 representing all of it and 0 meaning none. A good SHGC for Maine would be between 60-62% or .60-.62 SHGC. A minimum of 50% solar heat should be allowed or .50.

Shading devices like overhangs or attached fins to keep summer sun out are important as a Passive House cannot have temperatures indoors exceed 77 degrees Fahrenheit for more than 10% of the occupied year (recommendation is 5% as global warming continuously upgrades the average temp).

A window is "energy balanced" if it takes in the same amount of energy in a year as it loses. It is "positive" energy balanced if it takes in more energy in a year than it loses and a Passive House wants to have a positive energy balance for each window without overheating beyond the above limit.The calculation for energy balance involves a calculation for window transmission losses (QT) and one for window solar gains (QS).

The formula for window transmission losses is QT (in kBTU/yr) =  sum of (Uw, installed x Aw x ft  x Gt) where Uw 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.

The formula for window solar gains is QS (in kBTU/yr) = r x g x Aw x G where r is the solar reduction factor plus the glazing fraction so a multiple of 4 factors and is unitless; g is the "g value" or solar transmittance also unitless; Aw is the area of the whole window unit in ft2 (square feet); and G is the global solar irradiation during heating period in kBTU/ft2yr (thousand BTU's per square foot year).

These windows are key whether building to meet new Passive House standards or retrofitting to meet the EnerPHit Passive House renovation standard. I'll talk about the meaning of the formula components and more in future.