Windows make the Karuna House.  They are central to the home’s aesthetics, its HVAC system, its wall assembly, its airtightness, its comfort, and its energy performance.

In high performance green building like Passive House and Minergie-P, it’s difficult to overstate the importance of fenestration.  Partly that’s because windows and doors represent big holes poked through a meticulously engineered and constructed wall assembly, opening potential weak points in the system for the transfer of heat, air and moisture.  So you’ve got to execute high performance installation of your high performance windows in a project like Karuna (as described in the Karuna Wall Assembly page.)  No leaks allowed in the hull of this ship:

But equally important is the opportunity that high performance windows offer:

Opportunity #1: Energy Performance.

The windows at this green home building project are triple-glazed, high solar heat gain, R-8 windows made by Optiwin in Austria.  (See our own Skylar Swinford’s post about his tour of the Optiwin factory.)  These windows have a very thin frame (the weakest part, thermally-speaking, of a window) so that the whole frame can be over-insulated and fully embedded into the wall assembly.  While it’s true that even at R-8 the windows are several times less insulative than the surrounding super-insulated wall, they are net-positive in energy performance because they capture so much solar gain.  And in a very low energy load building like Karuna, energy performance gains like these can make a big difference – it’s not just a drop in the bucket.

High performance windows are not some newfangled luxury item, either.  Triple-glazed windows have been standard in Scandinavia since the 1970s, and are now code in Germany.  More and more high performance window options are becoming available in the US, a welcome development among designers and builders of high performance buildings.

The same is true of high performance doors.  The exterior doors at Karuna are also Optiwin, with the exception of the cellar door that Hammer & Hand built.  These doors look a bit like vault doors, thick with insulation, gasketed and airtight, and equipped with multipoint locking mechanisms to ensure a tight seal.  See Hammer & Hand master jointer Dan Palmer describe the construction of the Hammer & Hand Passive House door here:

Opportunity #2: Aesthetic

High quality windows open design options for the architect, allowing high performance projects to break out of the shoebox mold and become a medium for high design as ripe with possibility as anything else out there.  Holst Architecture’s design of Karuna is proof positive.

Because high performance windows can be so good at capturing solar heat gain, shading becomes important, especially in the shoulder seasons when sun angle brings lots of rays into the home but outdoor temperatures are still mild.  On modernist homes, with their clean lines free of traditional, shade-providing overhangs, the potential for overheating is more acute.  Exterior shade systems like the one at Karuna make modernism in Passive House feasible, without sacrificing comfort.

Exterior Shading System Pt 1

Exterior Shading System Pt 2

Incidentally, the same glass attributes that make a window good at capturing solar heat gain also make the windows very clear, so clear that you’d never guess that they’re triple-paned.  So you get better daylighting and better views with high performance windows, two key elements of the architectural experience.

Opportunity #3: Comfort

One of the biggest benefits of a high performance window is its warm inner surface.  A conventional window, by contrast, has a very cold interior surface in the winter, setting up an uncomfortable convection current as inside air cools and falls along the window interior, then warms and rises, only to be cooled again.  The constant draft this creates can render unlivable big areas of the house near conventional windows.

The human body is a tremendously sensitive gauge of heat and cold, so this convection can be very unpleasant.  Thermostat settings creep up – a typical house might be set as high as 74 degrees.  But in a high performance house with high performance windows like Karuna, a thermostat setting of 68 degrees can deliver comparable thermal comfort, and makes the spaces immediately adjacent to windows cozy and draft-free.

Opportunity #4: Simplified Mechanical Systems

In high performance design and construction of projects like Karuna we view windows as part of the HVAC system because they are so connected to the design of the home’s mechanicals.  Above we touched upon how the warm interior temperatures of high performance windows lead to lower thermostat settings by occupants.  This increased comfort combined with the overall increase in energy performance brought by high performance windows means that heating and cooling equipment can be significantly downsized and simplified.  Windows have no moving parts (apart from those used to open and close), so the focus on high performance windows over complex mechanical systems brings simplicity, comfort and energy performance.

Part of the HVAC strategy to reach net-zero energy use at Karuna is to use a low-temperature radiant system throughout the floors of the home, harnessing the heat transfer power of water to balance temperatures between rooms.  This strategy would be impossible with draft-inducing, energy-losing conventional windows.  High performance windows are key.