There is evidence to show that triple glazed windows aren’t always the most effective option due to the environmental impact of materials and production processes. Consequently, the resulting embodied carbon can make the choice of a triple glazed unit less favourable and this should be taken into account when calculating the performance and impact of a building.
There is an understandable perception that low U-value is better, but if the embodied carbon in manufacturing triple glazing outweighs the savings from lower emissions when the window is in use, then it is not necessarily the right solution.
Not since the energy crisis of the 1970’s has the debate on the number of panes in a window been so relevant and so important.
A window’s energy balance is the difference between the amount of heat from sunlight the window transfers to your home (g-value) and the amount of heat that escapes through the window (u-value).
Using only the U-value of a window ignores the benefits from other glazing properties and gives a biased view of the value of triple glazing.
With good energy balance, solar gain will help to heat the inside space, reduce heating costs, effectively reduce CO2 emissions and increase climate payback potential. With good daylighting properties, this will reduce reliance on artificial lighting and consequently reduce energy demand for lighting.
Recent evidence has shown that the average operational carbon saving of a ‘typical’ (1770 x 1200mm) timber triple glazed window over a double glazed window is around 2.6kg CO2/year. As it takes on average, an extra 51kg of CO2 to manufacture triple glazing, this would result in a climate payback period of around 20 years. As glazing units currently have a life expectancy of 20 to 30 years, this hardly seems appropriate and is certainly not appropriate for renovation projects in a building fabric which generally will have less energy efficiency than a new build.
The cost of triple glazing is higher than double glazing for obvious reasons - more glass, extra layer of gas (and type of gas), higher cost of manufacture in handling more components etc. For a solution to be given due consideration, it must be affordable from the point of capital outlay, throughout its operational life and ultimately, its disposal.
High performing double glazing has the potential to tick all the right boxes and when we consider that VELUX provide a double glazed window using an insulated collar in a recessed
application with an installed U-value of 1.1 W/m2K, this makes for a more cost effective solution.
As a comparison, the PassivHaus specification requires roof windows to achieve an installed U-value of minimum 1.0 W/m2k. But what real difference does that 0.1 U-value actually make?
Fabric heat loss calculation
Calculating the heat loss through a building element requires 3 values to consider:
1. Area of element (m²)
2. U-value of element (W/m²K)
3. Temp difference inside and outside of building (?t)
This gives the following calculation:
Area (m²) x U-value (W/m²K) x Difference in Temp (?t) = Fabric Heat Loss (W)
Therefore, for 1 m² of glass x 0.1 of U-value x 11° temperature difference (using met office data on 2012 mean UK temperature of 9° outside and assumed constant 20° inside):
1 x 0.1 x 11 = 1.1W (per hour)
Then 1.1W x 24 hours = 26.4W (per day)
Then 26.4W x 365 days = 9636W (per year)
This shows that for every 0.1 U-value difference in 1m² of glass, the heat loss changes by 9636W (or 9.64kWh) per year.
Using this calculation, we are able to determine the energy lost and the CO2 created by the energy loss per 1m² of roof window glazing for a 0.1 U-value difference between high performing double glazing and triple glazing.
Most homes in the UK are heated using gas, Therefore:
9.64 kWh (energy loss per 0.1 U-value per m² per year) x 0.20435kg (CO2 per kWh unit of gas) = 2kg (additional CO2 output)
We can also calculate the extra cost associated with increased energy loss using high performing double glazing instead of triple glazing.
Space heating using gas (4.5p per kWh - January 2014):
9.64kWh x 4.5p = 43.38p - say 43.4p per 1m² of glazing per year.
If we assume that the average loft conversion/new build house in the UK has 3m² of roof window glazing, then this is roughly equivalent to 3 x VELUX PK10 windows (942 x 1600mm).
If you purchase 3 x high performing double glazed VELUX GGL PK10's (--60 pane), this will cost £1452 (full list price Feb 2014).
If you purchase 3 x triple glazed GGL PK10's (--66 pane), this will cost £1812 (full list price Feb 2014).
Therefore, 3 x triple glazed windows cost £360 more than 3 x high performing double glazed windows of the same size.
Currently, glazing is considered to have a maximum life expectancy of 20 to 30 years.
Over 20 years therefore, the 3 x triple glazed windows will cost an extra £18 per year (20 x £18 = £360).
The energy performance calculation shows that with triple glazing, you only save £1.30 per year for all 3 windows in energy costs at today’s energy rates over double glazing. That is a difference of £16.70 out of pocket per year.
If energy prices double every 5 years for the next 20 years, you will save a total of £98, which averages out at £4.90 per year. That is £13.10 out of pocket per year.
If the glazing should last for 30 years, the 3 x triple glazed windows will cost an extra £12 per year (30 x £12 = £360). If gas energy prices double every 5 years for 30 years, you will save a total of £13.70 per year with triple glazing. This example at best provides payback at £1.70 per year for all 3 windows (57p per
window per year).
Embodied carbon calculations
VELUX carry out Environmental Product Assessments on their roof windows which assumes a life of 40 years for the frame & sash and 20 years for the insulating glass unit. The assumption being that the glazing will be replaced once in the windows lifetime.
When using energy balance, the impact of the VELUX roof window on the environment over a 40 year period is greatly reduced.
At the time of writing, assessments have not been completed on the new generation of –60 and –66 pane variants referred to above. However, the following is an example of the global warming potential for a standard double glazed roof window of a similar size.
Test window is a VELUX GGU SK08 0050:
Centre-pivot, white finish, 1.14m x 1.40m, U-value 1.3, g-value 0.66
Global warming potential from raw materials to installation and end of life treatment: +146 kg CO2-equiv. Global warming potential with average use (East/West orientation): -424 kg CO2-equiv. Global warming potential with best use (South orientation): -1660 kg CO2-equiv.
The energy balance of the window is assessed using methodology based on ISO 18292:2011  - energy performance of fenestration systems for residential buildings calculation procedure - and made for replacement of existing windows in a typical single family house located in Würzburg, Germany.
The assessments do not take into account the reduced energy need for artificial lighting where good daylight design is employed and so the figures provided above indicate the minimum potential climate payback of the test window.
The challenge is to influence key decision makers in order to ensure that designers and builders do not take a ‘tick the box’ approach in order to create the right results in software simulations and calculations. If we can encourage project teams to use the Life Cycle Analyses of products and materials to develop the most appropriate solution for a project, then this supports the need to move away from the ‘one size fits all’ mentality and creates a more flexible template for future design solutions, with the opportunity to put pro-active yet flexible design principles such as Active House at the heart of the design process.
Sustainability & Design Manager
VELUX Company Ltd
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