Blinds Make Better – Keeping Warm

I winter months, we all know to put an extra layer or two on before venturing outside to keep us warm. Blinds and shutters have a similar affect on buildings – they provide an extra thermal layer to a window, typically the thermal weak point of a building. For that reason, some shading products are specifically designed to help reduce heat losses and in this article we will give you some useful pointers so that you can confidently advise your customers on the full benefits blinds and shutters can add to their home this winter.

By Dr Zoe De Grussa

Technical and Sustainability Consultant

British Blind and Shutter Association (BBSA)

Buildings and Heat Loss

When a building is heated the heat is always searching for its nearest exit to reach colder conditions.

There are two main aspects in a home that affect the rate of heat loss – the thermal efficiency of a building and air leakages.

Buildings loose heat through all sides of a building = floors, roofs, walls, doors and windows – often referred to as the building envelope. The thermal efficiency of these elements affects how much heat is lost and is determined by the U-value of these elements. Air leakages are caused by uncontrolled draughts and controlled ventilation routes like bathroom extractors and window trickle vents. It’s important buildings have some way of ventilating as it helps reduce moisture, condensation, and improves the air quality within buildings. However, too much ventilation via air leakages and we lose the warmed air we paid for much faster than is desirable.

Historically government grants like the Energy Company Obligation (ECO) have provided incentives to make homes more thermally efficient by improving the amount of insulation installed in roofs, under floors and within wall.

Similarly, support has been offered for upgrading windows from single-glazed to double glazed, a more energy efficient option that reduces the amount of energy required to heat up building. This is also essential in delaying climate change and whilst swapping out gas boilers for heating and cooling sources that rely on electricity (e.g., air source heat pumps) will help as these systems are more energy efficient, they will only get us part of the way there. These more environmentally friendly methods of heating and cooling buildings are only cost effective and considered energy efficient if the building envelope is thermally efficient.

After all what is the point in having a very energy efficient heating system if it still requires a significant amount of energy to keep the building thermally comfortable and is unaffordable to run? Improving the thermal performance of the building envelope does just that. It helps reduce the amount of heating energy we need to keep our homes and other buildings warm.

Thermal transmittance of window and shading

With improvements to U-values of walls, windows can be the energy loss weak point. Shading products add an extra layer of insulation to the window and therefore always positively affect the U-value of a window when the shading product is extended. The U-value determines the thermal transmittance of a window, or in other words how much heat can pass/escape through it. It identifies the rate of heat transfer through a single material (a window) or a combination of materials (a window with a blind or shutters). The unit of measurement is in watts per meter squared Kelvin (W/m²K). kelvin is another measure used to determine temperature and is equivalent to a one degree increase in Celsius.

Today when a new building is built or renovated there are targets on how the building envelope must thermally perform. Building regulations introduced the concept of energy-conservation measures into home in the 1970s which was triggered by the oil crisis (Davis, 2013). Table 1 shows how the U-value requirements have improved over the last five decades for new homes in England. The biggest change for windows came in 2000 when the regulations required the installation of double glazing where previously single glazing was the industry standard. Original double glazing could only achieve a U-value of 3.1 W/m²K where today it can achieve more than half this (Davies, 2016).

The lower the U-value is, the better the shading material / product is at preventing heat loss and the more thermally efficient a window will be.

Table 1. Improvements in the U-values of building component for new homes (Davies, 2016; MHCLG, 2021).

New buildings built today lose almost a quarter less heat than building built in the 1970s (DECC, 2013). However, there is still room for further improvement particularly if the UK is to meet its target of reaching net-zero emissions by 2050 (gov.uk, 2021).

How a window and shading product is installed can significantly affect the thermal transmittance of a window system. If a window is fitted poorly or creates a thermal bridge then the thermal transmittance may be significantly higher than what was originally intended or what the window manufacturer claims the window can achieve. Similarly, how a blind is fitted can affect the U-value of the window and blind system combined.

Why knowing the U-Value of a shading and glazing system matters?

Knowing the U-value of your window and shading systems means you can estimate the amount of heating energy that could be saved when closing a blind or shutter. To calculate this, you need to know:

• The U-value of the window and blind system
• The area of glazing
• The temperature difference (td) between the inside and outside. BS EN 15022 uses a td of 15°C (5°C outside and 20°C inside).

To work out how much heating energy could be saved we need to determine the heat loss through a window with and without a blind.

In this example we are going to assume we have a house with 20m² of single clear glazing which has a U-value of 5.8 W/m²K. We are also going to base our calculation on a outside temperature of 5°C and inside temperature of 20°C -a temperature difference, td, of 15°C.

To calculate how much heat is lost through the windows in a house without blinds you multiply the area of glazing (in m²) by the U-value of the glazing (W/m²K) by the temperature difference (td) The equation for this is:

Heat loss without blind =

= 20m² x 5.8 W/m²K x 15td

= 1,740 Watts

Therefore,1,740 Watts is the amount of heat energy that is instantaneously lost through the windows.

To determine how much heat is lost through the window in a house with blinds the same formulae is used except the U-value for the window is replaced with the U-value of a window and a blind. This U-value will always be lower.

We will assume an internal free-hanging roller blind has been installed that has 0% openness and has been fixed within the window recess. These types of products are considered to have an ‘average air permeability’ and are estimated to have a U-value of around 3.5 W/m²K when installed with a single glazed window.

The equation for the house with blinds is as follows:

Heat loss without a blind =

= 20m² x 3.5 W/m²K x 15td

= 1,050 Watts

The two results suggest that the installation of blinds can help save 690 Watts in heating energy – just over half a kilowatt and equivalent to a 40% heat loss reduction. This additional energy would be needed to maintain a stable temperature of 20°C in the home if blinds were not installed and deployed. 690 Watts is also equivalent to the amount of power needed to operate a laptop for a day.

To work out how much energy in Kilowatts hours, kwh, you could save when the blinds are extended, we need to assume a length of time that the temperature difference between the inside and outside stays the same i.e., at 15°. For example, if the blinds were closed for 2 hours in the morning and 5 hours in the evening when there was a 15°C difference for seven days of the week. You can multiply the heat losses saved by the number of days and the number of hours the blinds were extended for – 690 Watts x 7 days x 7 hours – to provide the amount of energy in Watt/hours. This comes to 33,810 W/h which is equivalent to approx. 34 kWh. To convert this into a monetary value you multiply 34 kWh by the cost of electricity for 1 kWh. Assuming 1 kWh of electricity is 15p. in this example the blinds would provide an energy saving of around £5 for the week.

This is a simplistic example to show how the energy savings with shading can be calculated. Building simulation tools use annual weather data to calculate the amount of heat lost hour by hour to provide the accumulated savings over a year.

In this example the heat losses through a window have been simplified as in reality the U-values provided by glazing manufacturers are rarely the same as the U-value of a complete window in a building when it is finally constructed. Often the U-value provided for windows are only representative of the U-value of the glazing at the centre of the pane of glass and do not consider the frames or thermal breaks in a window system.

The true U-value of a complete window system needs to consider the:

• Area of the glazing and frame,
• Thermal transmittance (or U-value) of the window frame,
• Thermal transmittance around the perimeter of the window (e.g., considering the properties of any thermal breaks),
• U-value of the glazing.

Heat losses vary significantly throughout a day depending on the temperature differences between the inside and outside. This makes accurate heating energy savings complex to calculate and is why dynamic building simulation models are used to predict the total heat losses throughout a year and the amount of energy that can be saved from improving he thermal efficiency of a building. ES-SO’s Early-Stage Building Optimisation (ESBO) tool can do exactly this – with a few clicks of a button you can determine the amount of heating energy that could be saved by comparing the heating required for a room with and without a blind. For more details see – www.es-so.com/tools/esbo.

Glazing and Shading U-values

In existing UK buildings. We commonly see single, double clear, or double low emissivity (e) windows. Reference U-values for these glazing systems are given in BS EN 14501 and are provided in Table 2. The table also provides the G-value for each of the glazing types which represents the total amount of solar energy that can enter the building through the glazing. You will notice that as the U-values improves (lowers) the G-value worsens (increases) meaning less solar energy can enter when selecting a more insulating glazing type.

Table 2: Reference thermal performance values of glazing according to BS EN 14501:2021.

In new domestic and non-domestic buildings, it is recommended through building regulations that the U-value of windows should be no higher than 1.4 W/m²K and it is suggested that in 2022 building regulations will require this to be lowered to 1.2 W/m²K. Current consultations on what the performance of windows will need to be in 2025 suggest that the threshold will be lowered to a U-value of 0.8 W/m²K. A U-value of 0.8 W/m²K means triple glazing is more likely to be installed unless the glazing industry can improve the performance of double-glazed units further.

Historic Scotland and English Heritage both commissioned research at Glasgow Caledonian University (Baker, 2009) to investigate how the thermal performance of single glazed traditional windows can be improved. In the study they installed differing shading products to a single glazed sash window and measured the amount of heat lost through the window, and calculated the U-value of the shading and window system.

In many cases buildings that still have single glazing installed are unable to have their window upgraded because they are classed as heritage buildings. Therefore, for some heritage buildings installing an internal shading product is the only way the thermal performance of the window can be improved and is one of the methods recommended by English Heritage (2017).

Figure 2 presents the findings from Bakers study which shows the reduction in heat loss relative to the amount of heat loss through the window. It shows that approximately 40% less heat is lost through the single glazed window when it has a roller blind covering it than when there is no blind present. With a honeycomb blind that had a metallised inner coating this rose to 60%, almost the equivalent to replacing the existing glazing with low-e secondary glazing. Traditional solid bi-fold shutters and a roller blind with a low-e coating were most effective and reduced heat loss more than installing secondary low-e glazing.

This this study did not make a comparison to the  installation of double glazing however the U-value for double clear glazing from Table 2 of 2.9 W/m²K suggests that more heat would be saved when either a honeycomb (2.1 W/m²K), shutter (2.0 W/m²K)or roller blind with low-e coating (1.8 Wm²K) is installed with single glazing than replacing the window with clear double glazing (2.9 W/m²K). This is because the measured U-values were lower for the shading combined with single glazing than the reference U-values for double clear glazing.