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June 11, 2018
Regardless of what you’re building, in high-performance construction or renovation windows are always going to be your biggest challenge. They are a thermal bridge that you can’t eliminate - they are going to mess with your heat loss or your heat gain. Beyond the straightforward energy issue, they have the potential to cause comfort problems: overheating, glare, asymmetrical radiation...
I have worked for a long while (nearly 30 years) designing and consulting with homeowners on passive solar design for houses. When you carry out solar gain studies, you have a 3D puzzle to play with. You have to balance the glazing area with the type of construction (low mass, medium mass, high mass) and the floor area that is exposed to the solar gain, then balance that out with how the solar gain gets stored, and circulated. ‘Do not exceed’ ratios of south-facing floor to glazing area are standard issue for anyone designing passive solar over the last few decades:
Percent of glazing S-facing floor area
Conventional design & window placement
Conventional design & better window placement
Up to 15%
High-mass passive solar design with recirculation and heat storage
Good passive solar design, creating maximum comfort with minimal over or under heating has always been a dance between window size, placement and glazing type. Other factors include orientation of windows and major elevations, and exterior seasonal shading (overhangs or fast-growing/die-back plantings on trellises and pergolas, or brise-soleils) to minimize western sun in the summer but allow it in the winter. Window orientation is often dictated by views and factors other than optimal solar gain.
As a rule of thumb, low mass construction -- typical stick framing -- shouldn’t have any more than 8% glazing to exposed floor area to avoid overheating from solar gain. Going to a higher south-facing glazing to floor area requires more mass to absorb the excess solar gain (we’ll dive into passive solar design in a future article series).
Size Matters, Sort of...
In the past, cold climate passive solar homes have required much larger windows to gather up as much solar gain as possible. However, this also means there is as much heat loss at night as possible, especially on those deep winter nights when the clear sky acts like a perfect black body and sucks heat out of all exposed surfaces.
Passive solar design requires that the whole package - envelope and glazing - be balanced to provide additional energy to the house. Even though extra sunlight can be harnessed with more glazing, the reality is, low thermal performance glazing will cause much more energy to be lost than gained. With high-performance windows, the game changed. A home with a large WWR ratio can have comparable energy use to one with a small WWR, depending on the glazing, coatings and fill chosen. Passive solar homes don’t need those 2-storey banks of glazing to capture enough solar gain.
While I’ve pointed out the passive solar rules of thumb above, most metrics on window performance that we glean from energy modelling software are not based on glazing to floor area, but on the Window to Wall Ratio (WWR). In the last 15 years, the average amount of glass in residential housing has nearly doubled, going from 8% before 2000 to nearly 17% in 2016, according to an article by Lisa Bergeron, Government Relations Manager for JELD-WEN of Canada.
Hot2000 and other modelling tools don’t model passive solar gain very well, mainly because the collection, storage and distribution of passive solar is predicated on the dynamic performance house-as-a-system. Rapid changes in available sunlight, exterior temperatures, wind speeds, and time of day make modelling useful passive solar gain a theoretical exercise at best. However, we can model assemblies and window types against each other on an annual basis, and they usually point to the need for balance between the wall assembly and the window size and choice. Here are some of the results of a modelling study that JELD-WEN carried out with Building Knowledge Canada.
Modelled Window to Wall Ratios
The study modelled four archetypes in three climate zones with six different WWRs. Each house type was modelled with code-compliant windows and then with triple-glazed units in four different wall assemblies.
- Here are some highlights from the +1400 Hot2000 runs:
- Above 13% WWR, a high-performance triple-glazed window had a greater positive impact on the effective R-value of a wall assembly with R22 batts than simply adding R5 insulated sheathing.
- Above 20% WWR, a high-performance triple-glazed window had a greater positive impact on the effective R-value of a wall assembly with R22 batts than simply adding R10 insulated sheathing.
- High-performance Low Solar Glass triple-glazed windows could reduce air conditioning loads by up to 50%+
- High-performance triple-glazed windows improved the total effective R-value of above grade wall assemblies by: 16% at 15% WWR, 22% at 20% WWR, and 24% at 25% WWR.
Modelling in a cold climate design guideby the Efficient Windows Collaborative backs this up: the impact of window size on the amount of useful solar gain for cold climates compared to window type was modelled for three different window types:
- Double, Clear, Non-metal Frame
- Double, High Solar Gain Low-E, Non-metal Thermally Improved Frame
- Triple, High Solar Gain Low-E, Non-metal Thermally Improved Frame
The modelling showed that while window area impacts heating energy use when clear, double-glazed windows are used, the delta decreases with double- and triple-glazed high-solar gain low-E windows. You can see from the graph that high performance windows offer the benefit of more passive solar gain and lower energy annual energy costs without a penalty for glazing area. On any project requiring new windows, to really dial in the performance of individual windows, energy modelling can be used to determine the best U-factor, low-e coating, fill and SHGC by orientation.
This does not account for the wall assembly, just the window performance.
Where Oh, Where, Does My Window Go?
Orientation - what about that?
In the same Cold Climate Design Guide from the Efficient Windows Collaborative, that gives some good examples of what happens to window performance at different orientations for those three different window types modelled in Minneapolis. Of course, south-oriented windows are the best performers, regardless of window type. Interestingly, the delta in the performance per orientation is greater with clear double-glazed windows but flattens out with as units with higher performance and better U-values are modelled.
Again, the modelling only accounts for the window performance and no other differences in the building envelope.
Over the years, I’ve also done several hundred sets of energy modelling on house archetypes, locations, and envelope optimization for both new construction and retrofits.
Once you do a deep dive into this world, there’s no getting past two fundamental conclusions: The house works as a system. The variables are endless in searching for high performance.