5.6 Energy performance of different building types

5.6.1 Energy aspects of daylight

By using daylight to its full potential, the electricity demand for lighting during daytime can be significantly reduced or even eliminated.

The Architectural Energy Corporation has stated (Architectural Energy Corporation, 2006) that “Daylighting can drastically improve the energy efficiency of a space with adequate control of electrical lighting and solar heat gain”. In offices, the electricity demand for lighting can account for as much as 40-50% of the total energy demand (Walitsky, 2002), which can result in significant savings if replaced by daylighting. In order to quantify the energy savings on electric lighting, the number of hours for which daylight is an autonomous light source in the interior must be known. The relevant light levels for residential buildings were discussed in section 1.7.1.

The optimal use of windows in buildings to provide good daylight conditions with good energy performance requires careful selection of the window characteristics τv, g (and Uw). Due to the laws of physics, the g value will always be at least 50% of τv.

The best solution is often a combination of window and solar shading. A window with high g value and high τv value will generally provide a good result. High values of g and τv will perform well in that part of the year with least light; in parts of the year with excessive light, solar shading should be used. It is important that the design of the building and the placement of windows in it are planned as part of a holistic process where the requirements for daylight and energy performance are continuously evaluated and used as design parameters (Moeck, 2006).

The following example illustrates that high light levels are achieved with daylight and that windows are very energy-efficient light sources.


Example: Energy performance of a house with no windows

In a typical house, the light level achieved with daylight is determined for every hour of the year. Four locations are investigated: Berlin, Paris, Rome and Istanbul. High light levels (above 2 000 lux) are achieved achieved all year round, as illustrated in the figure below.

What impact does daylight have on the energy use in a building? To answer this, it has been investigated what would happen if there were no windows in the house and the light levels had to be achieved with electric lighting. As the amount of electric light influences the heating and cooling need, the resulting energy use for lighting, cooling and heating in the building must be evaluated together. The results from VELUX Energy and Indoor Climate Visualizer are shown in the figure.

"Windows are low-energy light sources"

​For each location, the lowest total primary energy demand is achieved by the building with light provided by windows. The energy demand of the building with no windows is approximately five times higher than that of the building with windows if we use electric light to obtain the same light levels. This underlines the fact that windows are low-energy light sources (Foldbjerg, 2010).

"The use of roof windows will result in higher daylight factors"

Example: Impact of roof window area on daylight and energy performance

It was shown in the Daylight chapter that roof windows deliver more daylight than facade windows. For an actual building that means that a specific daylight factor can be achieved with less window area if roof windows are used.

A low energy 1-storey house with an 8 x 18 m footprint located in Berlin has been investigated. The VELUX Daylight Visualizer was used to find combinations of roof and facade windows areas that reach a daylight factor of 4% and 6% respectively.

By increasing the percentage of roof windows, a higher daylight factor can be achieved. A total window area of 25 m² of facade windows only will provide a DF of 4%, while 25 m² with a mix of 64% facade windows and 36% roof windows will provide a DF of 6%, as indicated by the dotted lines on the figure.

Next the VELUX Energy and Indoor Climate Visualizer was used to determine the heating demand for each combination of RW and FW. The results are shown in the figure below.

​Figure 5.6.1 The energy performance is improved by increasing roof window area. With DF = 4%, the heating demand is reduced from 9.1 to 6.3 kWh/m2, and with DF = 6%, from 13.4 to 9.2 kWh/m2. Both reductions correspond to 31%.

"Natural ventilation combined with mechanical ventilation is more energy efficient than mechanical ventilation alone"

5.6.2 Energy aspects of ventilation

Ventilation – and particularly natural ventilation – has an influence on the energy demand for heating, cooling, and electricity for fan operation.

Ventilation and heating

When the outdoor temperature is below the indoor temperature, energy for heating is required to raise the temperature of the fresh air to the desired indoor temperature. The magnitude of the energy demand depends on the ventilation rate and the temperature difference.

Heat recovery units can be used to recover (reuse) most of the heat in the extract air to heat up the fresh outdoor air before it is supplied to the building. Heat recovery systems are generally only available with mechanical ventilation as it requires a physical unit through which both the supply and extract air can be circulated. Up to 90% of the heat can be recovered.

Electricity is used to operate the mechanical ventilation system, but this amount of energy is small compared to the amount of energy that can be recovered when the outdoor temperature is low. So mechanical ventilation with heat recovery is an energy-efficient solution for new, airtight buildings during winter. However, leaky buildings will have less benefit from heat recovery as mentioned in section 2.2.2. Mechanical ventilation also requires maintenance (filter changes, cleaning, etc.), which should be taken into consideration.

When the outdoor temperature is in the range of 14-18°C (depending on the building), there is no need for energy to heat the supply air. In this situation, natural ventilation is more energy efficient than mechanical ventilation, since no electricity is used for fan operation.

The combination of natural and mechanical ventilation is called hybrid ventilation. See section 2.2.3 for an example of the energy savings that can be achieved with hybrid ventilation, and section 2.1.4  for an example of the impact on energy demand of the ventilation rate.
Hybrid ventilation uses no electricity for fan operation during the summertime.

Natural ventilation and cooling

When the outdoor temperature, in combination with solar gains, causes the indoor temperature to rise, there is a risk of overheating. In some buildings this is handled with air conditioning, but natural ventilation is an efficient alternative that saves energy. Natural ventilation can be used during daytime (summer ventilation) to control the temperature, as mentioned in section 2.4.5.

Natural ventilation can also be used at night (night cooling), to cool the building and thus eliminate the need for air conditioning the following day, as mentioned in section 2.4.6.

Night cooling works by cooling the constructions in the house. The effect is larger if the building is “heavy”. Concrete and bricks are “heavy” materials, so a building with concrete or bricks as wall, ceiling or floor materials is heavy”.

5.6.3 Energy aspects of solar shading

Solar shading has an important influence on the energy performance of buildings. The use of solar shading affects both g value and U value, so solar shading can be used both in warm and cold climates to improve the energy performance of buildings. And as solar shading is dynamic – it can be activated when needed – it is an important part of the window system.

External shading prevents solar heat gains more efficiently than internal shadings. External shading is, therefore, the best choice when the purpose of shading is to prevent overheating and reduce the electricity demand for cooling.

Internal shading does, though, provide some reduction of overheating. Internal shading is generally more efficient at increasing the insulation of the window system, which means that the heating demand of the building can be reduced if used correctly. Internal shading also serves the purpose of controlling daylight.

VELUX ACTIVE Climate Control is an example of a dynamic window system in which the use of the solar shading is optimised automatically, with no interaction from the user. It thus reduces the need for heating and cooling, yet improves indoor comfort significantly (Philipson, 2010).

5.6.4 Building energy performance in cold climates

In cold climates, an important design objective is to minimise the heating demand and the electricity demand for lighting. Secondarily, the electricity demand for fan operation (etc.) should be minimised and the building should be designed with no need for cooling. The latter has been shown to be of increasing importance – the over-heating challenge has been overlooked in the design of many low-energy buildings. Windows provide useful solar gains every month of the year, also during the summer months. 

However, solar gains are a double-edged sword and may lead to overheating. The energy evaluation should, therefore, be based on annual calculations for which tools such as the VELUX Energy and Indoor Climate Visualizer can be used. The example in Figure 5.6.2 shows that the useful solar gains in May to August in Denmark are substantial, which means that even though there are cold days and nights in summer as well, heating is usually not needed during the warm months. The importance of solar gains during the summer is illustrated in the following example.

5.6.5 Building energy performance in warm climates

In warm climates, the main design objective is to achieve thermal comfort during the warm part of the year. Secondarily, to minimise the heating demand during the cold part of the year. As seen in the previous sections, the electricity demand for cooling can be minimised, and often eliminated, by using natural cooling technologies. Such technologies include ventilative cooling and solar shading. In combination with intelligent building design that takes into account the shape, thermal mass and orientation of the building, peak cooling loads can be kept low or even eliminated. Automatic control will enable the maximum potential of natural cooling.  It was shown in section 3.3 that thermal comfort in naturally-ventilated buildings can be achieved at indoor temperatures above 26°C due to adaptation. 

The main target should, therefore, be to design the building without a cooling system and instead use solar shading and natural ventilation to avoid unnecessary energy use.

​Figure 5.6.2 Example of useful solar gains in an existing building in Denmark. 
Windows provide solar gain all year round – not just in the wintertime. The solar heat gain through windows is the main reason why we can often turn off the heating during summer, even in cold climates.

"The energy performance of an existing house could be worsened if the windows were removed"

Example:  Energy performance of a house with no windows

The heating energy performance of a building with windows is compared to a building with no windows. The building is located in Berlin. The table below shows the results for four different construction periods. The calculations were performed in BSim.
For a new or future building, the energy performance of the house with no windows is of the same magnitude as the house with windows, which means that the solar gains of the windows are of the same magnitude as the additional heat loss.

For existing buildings, the house with windows performs better than the house with no windows.

5.6.6 Consequences of future requirements for better energy performance

Current trends in European and national legislation point towards a continued focus on energy in building legislation, which means that the minimum requirements for the energy performance of new buildings as well as refurbishments will be tightened.

Cold climates

As seen in example above, the energy balance of windows depends on the building where they are installed. In Figure 5.6.2, there was an example of how much of the annual solar gains can be utilised in an existing building in northern Europe. In a high-performing building the heat loss is low, so less solar gain can be used. In high-performing buildings, the focus of windows will be on low Uw value rather than high g value.

The example shows that the relative saving by using 3-layered glazing is largest for low energy buildings, while only small savings are seen for existing buildings.

Example: Solar shading and natural ventilation provide good energy performance and thermal comfort in warm climates.

The performance of a typical building in four cities in warm climates was investigated with the VELUX Energy and Indoor Climate Visualizer. Different combinations of solar shading and natural ventilation were investigated and compared to an air-conditioned house. The investigated cities were Athens, Istanbul, Malaga and Palermo (Asmussen, 2010). The energy performance of the building with air conditioning was in the range of 150 – 160 kWh/m2, which is 3 to 10 times worse than the buildings without air conditioning.

The houses without air conditioning also achieve acceptable thermal comfort. The graph below represents results from Athens and shows that acceptable thermal comfort can be achieved for 98-99% of the time with automatic control of natural ventilation, solar shading and night cooling.
​Figure 5.6.3 The figure shows both energy performance and thermal comfort for Athens and shows that the thermal comfort achieved with automatic control is as good as with mechanical cooling. 
Example:  Relevance of 3-layered glazing in high-performing buildings

The previous example showed the impact of using 2-layered vs 3-layered glazing in Berlin in a typical house of four different construction periods. In the table below, the relative reductions by using a 3-layered pane compared to a 2-layered pane are shown.
For high-performing buildings, the window U value is becoming increasingly important compared to the g value, because less solar gain can be used in low-energy buildings
Architectural Energy Corporation (2006) Daylighting Metric Development Using Daylight Autonomy Calculations In the Sensor Placement Optimization Tool – Development Report and Case Studies, CHPS Daylighting Committee
Asmussen, T. F., Foldbjerg, P. (2010) Efficient passive cooling of residential buildings in warm climates, Submitted for PALENC 2010.
Foldbjerg P., Asmussen, T. F., Duer K. (2010) Hybrid ventilation as a cost-effective ventilation solution for low-energy residential buildings, Proceedings of Clima2010.
Moeck, Yoon, Bahnfleth, et al. (2006) How Much Energy Do Different Toplighting Strategies Save?, Lighting Research Center, Rensselaer Polytechnic Institute. Philipson, B. H., Foldbjerg, P. (2010) Energy Savings by Intelligent Solar Shading, Submitted for PALENC 2010. Walitsky, P. (2002) Sustainable lighting products, Philips.