Built Expressions Bangalore :: Single Glazing or Double Glazing

Comments (0)

Comments This Article

Single glazing or double glazing - which is better?

The question of single or double glazing to the north is an interesting one. Naturally, double glazing provides better insulation, but it also reduces the amount of solar radiation (heat) coming in to the building, so there is a trade-off.

The amount of glazing in a strine home (designed by Strine Environments, Australia) is maximised on the north elevation, to achieve as close as possible to 100% of the north facade. This is most unusual when compared to standard housing designs. It means that there is a lot more glass in a Strine home. Maximising the glazing to the north takes full advantage of the solar heat gain in winter.

The figures below indicate that single glazing is better than double glazing on the north face of a well-designed passive solar house. They explain why in Strine’s three decades long experience that single glazing (of course with drapes for insulation) to the north works well in the Canberra region climate and doesn’t warrant the extra expense of double glazing. In fact, we always get comments that our homes are lovely and hot in the middle of a sunny winter’s afternoon! That’s when the high thermal mass of a Strine concrete home is essential to store all that free solar heat.

The Solar Heat Gain Coefficient (SHGC) for Canberra winter days is very significant because of the clear skies and sunny days. Around noon Canberra gets approximately 800 watts (that’s 0.8 of a kilowatt) per square metre of northern facade. This is almost equal to a little 1 bar, 1 kW, radiator per square metre, or 2 kW per sliding door on the north. A ‘typical’ Strine home has the equivalent of 50 square metres of north facing glass: that collects a peak of 40 kW of free heat energy coming in to the home on a clear winter’s day. Of course a Strine home incorporates an optimised eaves overhang and other shading devices (particularly pergolas) to keep the high summer sun from shining on these generous northern windows.

However, the heat loss factor (U value) for glass is very poor compared to an insulated wall. Single glazed windows can be 10 to 20 times worse than an insulated wall . Heavy drapes or blinds (close fitting with a box pelmet or closed top and lined or made of an insulating fabric or cellular air traps ) can reduce the heat loss through single glazed windows by more than 50%. Thermal double glazing (different to acoustic double glazing) has a similar effect as heavy drapes and can also reduce heat loss by more than 50% compared to single glazing. Adding heavy drapes to double glazing further reduces the heat loss by ANOTHER 30%, down to 35% of the loss from single glazing.

The Solar Heat Gain Coefficient (SHGC) figures from the WERS (Window Energy Rating Scheme) web site show that double glazing reduces the solar heat gain of 0.72 for single glazing to 0.61 for ‘4/10/4’ double glazing and down to 0.23 for ‘5LowE/10Argon/5’ double glazing. This is a huge reduction of 15% to 68% of the free solar heat (equal to a peak of 6kW to 27kW for a typical Strine home where the drapes or blinds can be reefed to not obscure any of the windows), so if double glazing is used it should have a high SHGC to minimise this reduction. The insulation U value of the best double glazing with conventional aluminium frames is still only 2.7 or 58% better than the 6.4 U values for single glazing. The saving in the quantity of heat lost by using typical aluminium-framed double glazing in the large amount of Strines north glass is only 2.4kW (when there is a 15°C temperature difference across the glass – the average over a typical Canberra winter day). This is a significant benefit over the cold night hours but small when compared to the 6kW to 27kW reduction in free heat from double glazing, over an average of around 4 hours per day.

Wherever possible glazing to all other orientations (east, south and west) should be double glazed with insulating frames and shaded. Double glazed windows should have the lowest U value. Shading is an important component of glazing to these elevations: shading to east and west helps reduce summer sun penetration and overheating in the summer and shading to the south helps reduce radiation heat losses to the black night sky that the Canberra region experiences, more so than the coastal capitals. This black night sky is a perfect absorber of energy and because on the south it is not balanced by daytime winter sun there is most net heat loss from the south facade of a building. This is particularly so through the glazing on that facade, as it has a much lower insulation value than the wall. A verandah to the south facade is ideal to thermally shield the whole wall and any glazing.

Double glazing also helps to eliminate condensation inside a home, as it insulates the warm air inside from the cold outside. Drapes on single glazing need to be well sealed on all edges to prevent the warm air inside getting to the colder glass and condensing in troublesome quantities.

So, single glazing or double glazing – it’s a great question and it really depends on the amount of thermal mass, the quality of the double glazing and of course, budget. If you follow the four golden principles of passive solar design (see our fact sheet), experience has shown us that you do not need double glazing in Canberra.

...................................

Concrete and Embodied Energy - Can using concrete be carbon neutral?

Concrete is the most widely used building material in the world. There is now approximately 2 tonnes of concrete for each person on the planet earth. The small amount of embodied energy (carbon) in one tonne of concrete, when multiplied by the huge amount of concrete used, results in concrete being the material that contains the greatest amount of carbon in the world.

The embodied energy of a material represents the amount of carbon (carbon dioxide) embodied in that material.

Justification for Higher Embodied Energy in Buildings

A higher embodied energy level in buildings can be justified if it contributes to lower operational energy over the life of the building. For example, large amounts of thermal mass, high in embodied energy, can significantly reduce cooling and heating needs in well designed and insulated passive solar buildings, particularly In climates with greater cooling or heating requirements and significant day/night temperature variations (like Canberra and region).

As the operational energy of a building over its life cycle far exceeds its embodied energy, using the high thermal mass of concrete to virtually eliminate heating and cooling energy requirements results in saving lots of energy that creates a carbon neutral outcome over the life of the building.

How to Reduce the Impact of Embodied Energy

The single most important factor in reducing the impact of embodied energy (EE) is to design long life, durable and adaptable buildings. Buildings should aim to use materials that have lower EE. The choice of construction material should depend on all of the benefits it contributes to optimising the buildings performance over its life cycle.

Sustainability

Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. (Bruntland, UN, 1987). Sustainability includes environmental, social and economic considerations, not just the single issue of greenhouse gas emissions.

Embodied Energy

Embodied energy is the energy consumed by all of the processes associated with the production of a material or an assembly like a building, from the mining and processing of natural resources to manufacturing, transport and product delivery. EE does not include the operation and disposal of the building material, which would be considered in a life cycle approach. EE is the ‘upstream’ or ‘front-end’ component of the life cycle impact of a material or building. It occurs only once when a material or building is produced, and can be 10% to 20% of the energy used in a home. It is difficult to assess the EE of a material and even more difficult for an assembly of materials and estimates can vary by a factor of up to ten. Figures quoted for EE are generally Process Energy Requirements (PER) and are usually in MJ/kg, although some figures quote CO2 emissions per tonne (CO2/T). EE figures can only be broad guidelines. As energy inputs entail greenhouse gas emissions, there is a direct relationship between EE and carbon content.

Operational Energy

Operational energy accumulates over time throughout the life of a building. Operational energy consumption depends on the efficiency of the building envelope and the occupants’ behaviour and can be up to 90% of the energy used in a home.

Life Cycle Assessment

Life cycle assessment (LCA) examines the total environmental impact of a material or assembly over its whole life. It is necessarily complex.

Choosing Sustainable Building Materials

The criteria for choosing sustainable building materials are:

Embodied energy
Resource depletion
Recyclability
Life cycle contribution
Environmental impact

Embodied energy may not be the most significant factor, depending on the location, design and available energy.

Embodied Energy of Common Materials

Embodied energy (EE) content varies greatly with different materials and construction types. Typical figures for some Australian materials are given in the tables that follow. Generally, the more highly processed a material is the higher its embodied energy. However, materials with the lowest EE, such as concrete, bricks and timber, are usually consumed in large quantities compared to materials with high EE. As a result the greatest amount of EE in a building can be from either low EE materials such as concrete or from high EE materials such as steel.

Material PER embodied energy MJ/kg

Kiln dried sawn softwood 3.4

Kiln dried sawn hardwood 2.0

Air dried sawn hardwood 0.5

Particleboard 8.0

MDF 11.3

Plywood 10.4

Laminated veneer lumber 11.0

Plastics – general 90.0

PVC 80.0

Synthetic rubber 110.0

Acrylic paint 61.5

Stabilised earth 0.7

Plasterboard 4.4

Fibre cement 4.8

Cement 5.6

Insitu Concrete 1.9

Precast tilt-up concrete 1.9

Clay bricks 2.5

Concrete blocks 1.5

AAC Hebel 3.6

Glass 12.7

Aluminium 170.0

Copper 100.0

Galvanised steel 38.0

The EE for a component or assembly is more useful than an individual material. For example the PER EE for an elevated timber floor is 293 MJ/m2 compared to 645 MJ/m2 for a 110mm slab on ground.

Carbon Dioxide

Carbon dioxide CO2 is one of several greenhouse gases that cause global warming by trapping the sun’s radiant energy in the atmosphere. The production of energy generates CO2, meaning that EE equates to carbon content.

Green House Gas Emissions and Global Warming

Increased greenhouse gas emissions (GGE) are contributing to global warming. The key contributors to GGE are from energy production, transportation, industry and agriculture.

Cement

Cement is a dry grey powder, one ingredient of concrete used as a binding agent or glue. Normal concrete contains 7.5% to 15% of cement. Of all the material used to make concrete, cement has the highest EE of 5.6MJ/kg. This is still low compared to MDF or glass, and very low compared to plastics, rubber, aluminium, steel and copper. Cement production accounts for 2% to 3% of human generated CO2 production and consumes about 0.5% of total energy consumption. Cement substitutes like flyash or slag can reduce its EE.

Concrete

Concrete (to a common man)is, a liquid that turns solid, a plastic material, that assumes any formed shape and a brittle solid with high compressive strength and almost no tensile strength. Some important ccharacteristics of concrete can be enumerated as;

Very massive, heavy and dense: 2.3 -2.5 Tonne/m3
Brittle, and always shrinks, never grows, which means all concrete has cracks (not defects)
Cheap: say $200/m3 or $20/m2 for 100mm thick
High conductivity, low resistivity means low insulation value
High thermal capacity to absorb energy (heat), slow to lose energy gives a time lag of 4 hours per 100mm thickness (a good thermal fly wheel)
High decrement factor: the slow response of concrete to temperature variations can reduce the internal temperature variation of a building by more than 50%
Very high fire resistance
Very good acoustic insulation
Very strong when combined with steel tensile reinforcement
Composition of concrete by weight
Cement 10%
Water 10%
Air 5%
Coarse aggregates (gravel) 50%
Fine aggregates (sand) 25%

Concrete sets due to a chemical reaction between the cement and water, called hydration. It is rapid at first, then decreases but continues for years. Good hydration results in good concrete strength, and requires water. It also requires heat to start the process. The conditions that concrete is subjected to as it sets are known as its curing conditions. Curing affects the rate and degree of hydration, which affects the concrete strength. Concrete that is properly cured with controlled moisture levels and temperature is 3 times stronger than air cured concrete.

The characteristic or structural design strength of concrete is its 28 day strength. It is measured in Mega Pascals (MPa) eg 25 MPa for basic concrete. Typical concrete strengths: