Windows in a Passive House – Part II

While the Passive House standard is (for the most part) non-prescriptive, there are a few areas where it makes sense to specify a certain level of performance based on your climate; windows are one such area.  In my last post, I touched on a few aspects of window design, construction and tuning,  as they pertain to a high performance home.  Here, we’ll look into why window details are so vital to the success of a Passive House in terms of behavior and of course, the PHPP.

As the adoption of Passivhaus spread to other regions of the globe, the Passivhaus Institute realized windows designed for a German climate were either insufficient or overkill in many of these differing climates.  You could argue that a window designed for central Europe would work just fine in the Southern US and you’d be right, but in the interest of keeping costs down it isn’t reasonable.  The PHPP modelling software won’t mind a bit if you over-engineer your windows; the problem is it might not mind if you  under-engineer them either.  PHPP will actually let you get away with double-pane windows in a Canadian climate if the rest of the building envelope is extremely well insulated and super-tight.  The software simply does the math; if you meet the monthly or annual heating/cooling load demands (among other things), PHPP will give you the thumbs-up.

If we think about the behavior of a double-pane window on a cold winter day in a Passive House, we’ll quickly realize why the institute has a minimum standard based on climate.  In an average home, there is a heat-source below almost every window; this heat source helps reduce effects of the cold draft and creates air movement to minimize the formation of condensation.  In a Passive House, the walls are thicker, so the interior glass surface is “further away” from room air currents. There is also much less of an air current because the heat demand is so low…  The result? A double pane window (with its cooler inner glass surface temperature) will create a cold draft and promote condensation; two impermissible characteristics in a Passive House.  Kingston is located in the “Cool Temperate” zone, making the minimum glazing requirement triple-pane thermal units with low-e coatings, argon gas fill and non-conductive spacers.  The overall U-value for these windows can be no more than 1.1 W/(m² K)  (lower is better), so the frame and installation details are vital too.  This calibre of window will have a warmer inner glass surface temperature, resulting in a more comfortable space with greatly reduced chance of condensation; It makes perfect sense.

PHPP calculates both heat losses and gains for each window in the house. To do this, it requires every intimate detail of your windows, including orientation, frame thickness, depth and insulating-value.  It also needs the center-of-glass and spacer conductance numbers along with the solar heat gain coefficient (SHGC).  On a different sheet, you’ll enter in the shading characteristics of window frame reveals, decks/balconies, overhangs and neighboring trees and buildings.  The software does the heavy-lifting in terms of calculations, but if you aren’t using Passive House Certified windows, you may have to do some digging to get the necessary performance numbers from your manufacturer.  North-American standards are typically NFRC (National Fenestration Rating Council), which are quite different from the CEN (European Committee for Standards) methods both in terms of what is measured and how.  We were fortunate that our manufacturer let us talk directly with the independent lab that did the testing on their behalf.  They had the numbers we needed for PHPP.

If it interests you, the Lawrence Berkeley National Laboratory offers (for free download) their comprehensive software simulators for insulating glazing units and frame assemblies:  WINDOW and THERM respectively.  They are fascinating bits of code.

On a closing note, I am delighted to be able to say that we (finally) have been issued a permit!  That was an adventure in itself; more on that later…


6 thoughts on “Windows in a Passive House – Part II

    • Reiner, for this project we really want to see what’s possible with Canadian manufactured products. In keeping with that theme, we chose Fibertec. They’re not PH certified, but do have some decent numbers, and at a good price.

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