Sunday, August 18, 2013

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: ) but we'll look at simple double stud wall. Here's a good drawing by 475 ( ) 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. 

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