Homes don’t need to be dark to be energy efficient

©Lund+Slaatto

Does daylight necessarily come with overheating?

At regular intervals, the energy requirements for new homes are tightened. For a long time, the legislator has focused on insulation and u-values. The Swedish window industry has followed along well. Ever since the turn of the millennium, the u-values have been top class. This has allowed fairly large window areas that can meet relatively high requirements for daylight.

Overtemperatures are now increasingly recognized as a problem in homes. The latest referral from the Housing Authority on future building regulations emphasizes that summer heat must be taken into account, but does not set indoor temperature limits. Are we heading for a darker future with small windows to deal with the problem? We don't think so. This article wants to suggest an adjustment in the window industry. More focus on thermal comfort in summer can reduce problems with excess temperatures without sacrificing daylight. Windows and facades are obviously not the whole solution to the problem, we need to look at different solutions for passive cooling in homes. But windows are a very important part of the equation. And it is our part.

Take light transmission as a starting point

There have been discussions about whether and how we should handle daylight in buildings. From the industry, we are clear that we do not know if a building is good or bad without a proper daylight analysis. And these are almost always based on 68% light transmission in the glass. Why then and how can we work with thermal comfort within the 68 movement??

The window market structure of good cop bad cop

The window market in Sweden is divided between approximately 40% 2+1 windows and 60% triple-glazed windows.
2+1 windows or linked windows are unusual outside of Sweden. They have the advantage that you can install an intermediate blind. We mainly think of the blind as a glare protection, but if you close the blind completely, more than 80% of the solar energy is blocked. 2+1 with an intermediate blind thus provides effective protection against excessive temperatures. 2+1 also provides very good noise protection even in thin constructions.

The disadvantage of 2+1 is that it must be openable for cleaning between the glasses, which reduces the glass measurement and increases the embodied CO₂ footprint. And it can be more difficult to reach good u-values with a connected construction. As a rule, the outer single glass must be hard-coated low e glass, which lowers the light transmission - to around 68%.

The conclusion is that the success of 2+1 windows leads to daylight calculations being made with 2+1 windows as a basis. And that these connected windows provide great opportunities to avoid overheating if they are used correctly.

"Triple-glazed low e argon argon" has been what we have been talking about when it comes to windows since Elitfönster launched such a window at Nordbygg in 1998. Triple-glazed windows have the great advantage that they do not need to be openable, fixed windows are almost always triple-glazed. It provides a lower embodiedCO2 footprint, a larger glass surface in relation to the hole in the wall and a longer technical life. And higher light transmission, always at least 70%.
The disadvantage is the solar energy transmission. If you put a blind inside this window, you get good glare reduction. But no help at all against excessive temperatures, the blind can rather act as a convector that enhances the heating effect.

TGU low e argon thus gives the industry a sweaty reputation. Is there an easy fix? Thanks to the development of solar contol coatings on glass, there actually is. Otherwise, we would hardly have written these lines. Instead, let's join the 68 movement:
We can lower the light transmission in triple-glazed windows to just under 70% without having to make changes in how daylight analyzes are carried out. But it has lacked significance because available solar control glass, often referred to as "70/40" are still too dark. Because "70" is the light transmission in double glazing, triple glazing lands at just over 60%. In recent years, there has been a new generation of solar control glass that strives for 70% light transmission, even in triple glazing. There are several products that can handle 68% light transmission in triple glazing, in some cases without a large price premium compared to the "70/37" glass that exists on the window side today.

If the window industry changes the standard to "TGU solar control argon", we believe that the problem with excessive temperatures in homes will be greatly reduced. Especially if you change strategy on the inside. Replace the pleated blinds with an old-style translucent white roller blind (did Carl Larsson have those at Sundborn perhaps?) and you get increased efficiency against excess temperatures. You can also go "all-in" with aluminized and motorized inner fabrics for maximum comfort.
The conclusion is that we can distinguish between discussions about daylight and excess temperatures, if we calibrate all windows against 68% light transmission.
How do the alternatives compare to each other in the simulation tools ESBO and IDA-ICE? They are surprisingly similar. In order to obtain the correct g-total (energy transmission for the combination of glass and shading device), we need to start from exact products. But all manufacturers are in the database, so easy to shift and make your own calculations:
 

* By view is meant the blind angled 45 degrees or roller blind lowered to 2/3

By putting a little more thought into the glass, we create the conditions for a clear improvement in thermal comfort indoors in the summer.
Both options have a disadvantage in that the solar control is not automated, which lowers the real efficiency. But we want to make as little interventions as possible in the status quo to get a comparable cost structure.

The alternative with triple glazing and internal roller blinds has a major disadvantage in that white blinds do not provide good glare reduction from the sun. A thin white fabric can feel almost as blinding as the sun. But contractors often get nervous if they have to take responsibility for something hanging inside an apartment, what happens if it breaks? And thermal comfort is the responsibility of the builder but glare reduction is the responsibility of the tenant.

Here we have a relatively simple and cheap solution for 60% of Sweden's windows. If you pre-drill the window frames with the correct measurements, it also allows for an easy installation of roller blinds just before tenant entry. And the tenant can choose a glare-reducing and more efficient fabric with a good view at his/her own risk.