Through The Looking Glass

Lessons learnt through energy modelling high-performance glass, unmasked.

The architect and client have added a seven metre stacked slider to the North elevation. What’s the impact?
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Given the rest of the house is a polystyrene box, what do you think?
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The idea of the 7m slider was to give em some sugar; There are fantastic views from this lounge slider, and it’s a real statement piece. However, the result was up to 33% melted honey in the form of overheating, depending on the glass used. That’s four months of the year where the temperature inside this house would be constantly in excess of 25 degrees Celsius!
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There was no way of quantifying, nor mitigating this, without energy modelling. If it wasn’t for the energy modelling, this house, when built, would have been an expensive, unliveable oven, over summer.
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Glass
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I’d be happy to argue that glass is the single most important building material in the house that is non-structural.
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Glass gives you views out.

Glass provides free heat in.

Glass can be more insulating than the walls (H1 R2.0)

Glass can block, refract or reflect light.

However, Newton’s 3rd law of motion comes into play; For every action there is an equal and opposite reaction.
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In building there are always trade offs. The thing is to understand what they are and make informed decisions on what is important in each unique situation.
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Which Glass?
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So which glass is the right glass to use?
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Well, it depends!
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What is the primary purpose of the glass? Is it views, is it solar gain, is it insulation? Is it all three?
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Is the glass facing North, South, East or West?
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Is the glass vertical or sloping?
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Case Studies
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My intention is to present some case studies of new homes across the motu, where specifically energy modelling glass types has had an impact on the decision making process and provided positive outcomes for owners.
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1. Nelson
Let’s start with the most dramatic. As you can see, this house has impressive heating demand characteristics. Depending on the glass option chosen, the performance level is within #passivehouse Classic requirements.
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However, using glass with greater levels of Low-E is more expensive and only increases winter heating demand, even though the frequency of overheating is slightly less.
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This is because the silver coating restricts the amount of light that penetrates through the window (G-value), and therefore restricts solar gain.
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Because we were concerned with overheating, we modelled four different glass options from two different suppliers. The results are tabulated below.

At least now, we had some comparison data, on which to make informed decisons.
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Outcome
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Negotiations were breezy. The NK Windows super sized slider wasn’t coming out, and eaves / external shading wasn’t an architectural option, so mechanical cooling in the form of fan coil units were added to balance the Metro Xcel option.
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Adding fan coil units with 300mm diameter ducting simply isn’t something that could have been retrofitted later!
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The small price of the energy modelling fee, saved an unmitigated disaster!
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2. Horowhenua
Uncle Tom is onto his second house and this time we talked him into energy modelling it. Tom understands building physics, from a practical point of view, so always has heaps of questions around different thermal envelope options and the pros and cons, verses price and cost of each.
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Tom needs detailed data!
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Again, pretty impressive performance statistics, suitable for submission for a Passive House Institute Low-Energy Building certification assessment.
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In this case, overheating isn’t an issue, but Tom wanted to know whether he should spend the extra money on triple glazing or stick with just the two panes of argon filled low-e coated glass.
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We modelled product produced by two suppliers; Veridian and Metro Performance Glass, with the results below.

Outcome
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Tom loves tinkering, so we went with Option 7, Lightbridge 2G, with Lightbridge 3G on the south side. We also made a couple of windows fixed, and altered the door layouts, which offset the price of the additional triple glazing.
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The fully insulated slab is down on this healthy home.
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With detailed data, comes informed decision making!
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3. Lauder
Lighting up Lauder, has been a theme this month, with the recent NZSIP Smart Panels big build event. You can read about the design modelling case study here https://www.healthyhome.kiwi/damien/huddleston
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Due to the location and climate, we had to increase the window sizes and reduce shading to increase solar gain. Without energy modelling, this off-grid home would have needed additional solar panels to provide for the additional heating demand required.
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Outcome
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In terms of the glass choices, the STÄRKE joinery was modelled with Viridian LightBridge 1.1 Low-E, Argon-filled triple glazing unit with 14mm spacer, U-Value of 0.95 W/(m2K), and Solar Heat Gain Coefficient (SHGC, or g- value) of 52%. The Low-E coating was a single film on glass surface #5. A double Low-E coating was explored (Low-E films to both surfaces #2 and #5) however this resulted in less solar gain benefits than heat losses, and therefore an increase in the Heating Demand.
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The default position without modelling would have been triple glazing with two low-e coatings. Modelling showed that this wasn’t the best option.
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This saved money in terms of the price of glass and the cost of ongoing heating.
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Again, with detailed data, comes informed decision making!
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4. Clyde
This is a bonnie wee home modelled on a typical last century miners cottage. However, as a rear section, tucked away just downstream of the dam wall, it has some challenges with solar gain.
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A total of fourteen quotations for window and glass combinations were received from five fabricators. We modelled a selection of four different joinery types from across this price/performance continuum. In addition, we include a Building Code minimum variant as well as an imported and local glass variant for the ThermaDura joinery.
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The overall performance for this NZSIP build, absolutely killed the new house next door, built to the old H1 standards, but quite wasn’t good enough for submission as a PHI low-energy building.

Outcome
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This healthy home is still in the pricing stage. The owner’s architect would like to go with the timber window option as that suits the design vernacular. However, the budget is already blown, due to the difficult foundation conditions and Central Otago building prices. There is a significant saving available, by choosing a PVC option, without compromising performance too much, since certification isn’t a target.
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With detailed data, comes the choice to make informed decisions.
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Conclusions and Recommendations
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Ultimately, my advice is that you want all the solar gain you can get in winter, and no or minimal solar gain in summer. So be very careful with your choice of designs and glass types.
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Glass is rarely the solution to overheating issues; external shading does that.
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Please, Please, Please, energy model your house during design, to test the performance before it is built, thus avoiding costly mistakes.
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It’s often too late, once the summer sun is at its peak, and the builders down the beach!
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With detailed data, comes the choice to make informed decisions.
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If you’d like your dream home energy modelled and checked for thermal performance and overheating, before it becomes a nightmare, send your plans to damien@healthyhome.kiwi for a quote.


Note: The Passive House Planning Package #phpp is a single zone energy model, so averages overheating out across the spaces in the whole building. Thus it doesn’t provide room by room overheating analysis.
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