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How far could more energy efficient windows reduce energy use in existing homes by 2050?

Posted by Federico Seguro on 27 April 2015 at 12:15 pm

There is a common perception that glass is the weakest point of the building fabric, where heat loss is more likely to occur. As a result, there has been a tendency to minimise the transparent envelope of buildings, especially in cold climates where the building energy demand for space heating is usually very high. Whilst this could have been relatively justifiable when poor energy efficient single glazing units were in use, continuous improvements in glazing technologies are causing a re-think.

Reducing heat loss
From single glazing the addition of one or more panes of glass can raise the glazing thermal resistance to heat transfer therefore reducing the loss of energy by two thirds (from 6W/m2K to 2W/m2K). Heat loss can be reduced further by filling the gap between the panes with low-conductivity gases such as Argon. A highly insulating glazing unit can now have a thermal transmittance as low as 0.5 W/m2K which is a performance level not dissimilar to an insulated external wall.

Controlling solar gain
A further aspect worth investigating is overheating in the summer caused by the glass’ solar energy transmittance that is referred to as g-value. Overheating risks are relatively limited in the UK climate, but certainly something that affects conservatories or other rooms with large south facing windows.

Advances in solar control technologies include specialised coatings that minimise solar gain, but also grey body tint glass, able to absorb and reflect solar heat outwards without affecting the light transmission properties of the glass. Glass products that are able to reject up to 82% of the solar gain are now available on the market.

All these factors are leading to building designers changing their perceptions, driven by the improved glass performance and the architectural value associated with glass use in buildings. Over the last years, there has been a change in the same concept of building envelope that is no more conceived with the negative connotation of an enclosure but as a skin, a medium through which the building ‘organism’ opens itself to the surrounding environment.

What is the real impact of all this new efficient glazing? 

The National Energy Foundation (NEF), in partnership with trade bodies representing the UK glass and glazing supply chain in the built environment, established the NEF Glazing Supply Chain Group and has conducted a foresight exercise aimed at predicting the energy savings that could be achieved by upgrading glazing in the UK existing building stock.

Three scenarios were contrasted against 2050, when the UK has committed to reduce its overall carbon dioxide (CO2) emissions by 80%:

  1. A business as usual scenario (BAU) based on the current trends of energy efficient glazing uptake projected to 2050;
  2. An existing technical potential scenario (ETP), an upper estimate of energy saving potentials associated with the best glazing and solar control systems currently available, with no constraints on uptake.
  3. An enhanced uptake scenario (EU) representative of intermediate uptake conditions on the basis of practical constraints in, for example, historic buildings.

The estimated energy savings assessed by stock modelling vary from a minimum 4.2% to a maximum 8.7% across existing domestic buildings – that’s about 5 times the electricity produced by Sizewell B in a year.  A further 4.4% to 6.8% could be saved in non-domestic buildings, like shops and businesses.  Overall, this could add up to energy savings in the region of 4.2 - 8.0%, as shown in Figure 1.

Therefore, the replacement of poorly insulated glazing with more energy efficient products could provide a substantial contribution to the UK and EU energy reduction targets.

Interestingly, as well as contributing to the national energy efficiency targets, in terms of the reduced needs for space heating and reductions in lighting requirements, and the improved aesthetic of the architectural design, there are other reasons behind the revalued role of glass.

There are a number of less tangible benefits related to occupant wellbeing. The World Green Building Council estimates that a worker in an office without windows would sleep, on average, 46 minutes per night less than a worker in an office with improved daylighting levels, due to the relationship that exists between daylighting and the human circadian rhythm. Similarly, improved performance ie productivity, calmness and focus have been scientifically associated with proximity to windows and access to views.

So overall, improving the UK’s window could be a significant win for all of us.

Photo: SuperHomes

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