Indoor climate not dependent on just insulation values

The first variant to be introduced at the BouwBeurs is UltraGlass. A glass composition with a very low U-value of (0.5 W/m²K), which is achieved by using solar control glass. The second variant, called CoolGlass, is a unique composition on the Dutch market. CoolGlass has a built-in sun-resistant blackout foil, which is switched on and off electrically (switchable glass). Why is switchable glass an important development? With this blog I will create clarity between the different types of glazing in the field of insulation and sun protection.

Light Entry & Solar Heat Entry

Light and heat that shines from the sun through the atmosphere on the earth has a radiation wavelength expressed in nanometers. UV light, visible light, infrared and heat radiation.

The visible light between the wavelength of 380 and 780 nm consists of different colors. We see them in a rainbow, for example, when the light splits. However, we normally perceive that as “white” light. Infrared, the heat radiation from the sun is a short wave radiation and extends up to about 2500 nm. Infrared in the long wave area is what we ourselves generate in terms of heat: Our own body that radiates heat at about 36°C, radiators and so many other heat sources in indoor spaces.

Until about the 1980s, little was possible in glass. At that time we already had sun-resistant glass by coloring the glass through and through, so that the heat is absorbed. We also had the pyrolytic coatings, whereby after the glass came out of the oven, a metal oxide layer was applied to the warm glass, which reflects both the light and the heat radiation. No spectacular solutions and often highly reflective glazing. Playing with light and heat was only possible to a limited extent. With the reduction of solar heat, a lot of light was often lost. From the 1980s, new coatings were developed, which we know today as the so-called microwave coatings, or also called selective coatings. The process is an offline coater (CVD process) that applies metal or metal oxide particles to the surface in a vacuum. This is also known as a condensation process.

A 'microwave' coater (CVD process)

We can achieve much more with this process, because we stack different layers on top of each other, each with different properties. We can 'play' with this, so that with sufficient light entry we still keep a lot of heat from the sun. These glasses have different names per manufacturer; from Stopray to Sunguard, from Cool-lite to Ipasol. With type designations values of which the first number indicates the percentage of light entry and the second the sun entry. For example type 67/37. These values are precisely determined in accordance with European standard EN 410 and expressed in t-value (light) and g-value (solar heat), the amount of light and heat that is directly and indirectly (first absorbed by the glass and emitted indirectly) by the glass enters.

The different glazing types give different graphs in the spectrum of light and solar heat transmission. Modern glazing provides a high level of light in addition to good sun protection, often with little reflection and neutral in colour.

Glass is a very bad insulator

A pane actually has very poor insulating properties, because glass has a high emissivity and heat transmission. Single glass still achieves a little insulation value because there is a layer of air on either side of the pane that insulates (the so-called transition resistances). Insulating values are expressed in U-value in W/m².K. Single glass has a U-value of 5.8 W/m²K, insulating glass 3.0, and the current HR++ panes approx. 1.1 W/m².K. It actually shows the loss in Watts per m², per degree difference (indoor and outdoor temperature).

These low U-values are achieved by the cavity, the insulating pane and a coating that changes the emissivity of the glass. Due to the EPC standard from the Building Decree, HR++ windows are often used in residential construction. However, this is only heat-resistant glass, without sun-resistant properties. In non-residential construction, summer conditions are taken into account, in particular to optimize cooling in the summer and energy consumption. One degree of heating costs less than one degree of cooling in the summer. In addition, in residential construction the daylight openings are often determined by the minimum light entry requirement in the room. This while complete facades of glass are used in non-residential construction, and the glass determines the optimization of the indoor climate. The capacity of the cooling is then determined on the basis of the solar control properties. A building is optimized in the summer in such a way that the indoor level does not exceed 23°C, but that is where things often go wrong. Or because one calculates with an outside temperature that is too low (there are cities that have had an average outside temperature of 5°C higher in recent years than is used in the calculation) or the glazing does not achieve the desired sun protection.

Selective coatings

With modern European solar control glass types, both the winter and the summer situation are taken into account. That is why the pane has sun-resistant properties with a good insulation value of around U=value 1.0 W/m²K. But to be able to really assess this glazing in terms of insulation value, you should not only look at the heat loss through the pane (U-value) but also what the glass produces in energy (g-value). How much energy is left through the glass? This is expressed in the equivalent U-value: Ueq = U-value – (g-value*f).

The desired temperature

From research by Prof. dr. dr. Hauser from Kassel shows that the desired indoor temperature is not achieved in the summer and that the indoor temperatures are too high for a considerable number of hours. This despite the use of solar control glazing. Below you will find the results of his research, which were carried out with heat-resistant glass with a g-value of 60% and adjustable blinds, solar control glass with a g-value of 40% and switchable glass.

This shows that solar control glass is actually insufficient to stay under the maximum 500 hours and that solar control glass, with 90% of the facade share of the glass, can run up to 2000 hours compared to sun control glass in combination with adjustable blinds and switchable glass.

STAKA has reasoned that what applies to non-residential buildings should also apply to housing. Especially if the space under the roof hatch is heated up too much in the summer. That is why the switchable glass is offered as an option. This keeps the indoor climate in better control than only solar control glazing or glazing with external sun protection. In addition, external sun protection on the roof is vulnerable to wind, is heavily polluted and is less durable because it has to be replaced sooner. Switchable glass is the best solution. For good advice, visit STAKA at its stand A.028 in HAL 11 at the BouwBeurs in Utrecht.