Thermal performance: The other half of energy efficiency
How to master the two components making up the thermal performance of a building – the building envelope and air leakage – is shared by MAURICE BEINAT, chief technical officer at ecoMaster.
Much has been written about energy efficiency measures for commercial buildings and there are many engineering-based companies that can propose and delivery energy efficiency projects. In fact, what they focus on is the energy services side of the equation. That is, the lighting systems, air-conditioning plant, appliances and other energy consumption systems. What is usually missed is the thermal performance of a building.
Recognising that the thermal performance of a building directly affects the energy required to condition the building, as well as having an impact on the comfort of occupants, is an important step towards maximising energy efficiency efforts.
Occupants that are not comfortable in their space will often undermine energy efficiency measures. A case in point is the ubiquitous under-desk blow heater, which churns out hot air at the rate of 2.4 kilowatts to warm cold toes while the air-conditioning plant is busy cooling the space. In one recently refurbished office, individual departments were, astonishingly, given budgets for the purchase of under-desk heaters for all staff. And, they used them to the max!
The thermal performance of the building can be thought of as comprising two distinct parts: the thermal qualities of the building envelope and air leakage. While heat gain or loss through the building envelope will drive air-conditioning harder, so too will excessive air changes. Having to air-condition the entire volume of air in a building three or four times an hour poses a load far in excess of that required for fresh air.
A simple guiding principle that you may find helpful when considering these issues is to ask yourself the question: would I do this at home? If the answer is no, then you are probably gazing at a problem.
In single- and two-storey buildings the ceiling area is significant. If you are looking at a ceiling that is uninsulated then, just like at home, you are looking at a problem and a source of energy inefficiency. Insulating a ceiling is not rocket science, but it does need to be done with appropriate materials in a way that still allows easy access for maintenance works and uses materials that are friendly to maintenance staff.
Many office buildings have a great deal of glazing. Improving the performance of the glazing usually has a two-fold benefit. The obvious one is to improve the energy efficiency of the building. The less obvious one is to greatly improve the comfort of the occupants who happen to sit near the glazing. Adding a secondary glazing product to existing glazing is usually the most cost-effective solution for both thermal performance and comfort. In extreme weather areas, triple secondary glazing can bring greater benefits.
So, what are the benefits of improving the thermal properties of the building envelope?
IMPROVED AIRCON EFFICIENCY
The heat flow through the building envelope is dependent upon the temperature difference from inside to outside and the insulating properties of the building element. This means that you get maximum heat flow at times of temperature extremes. This coincides with the lowest operating efficiency of the air-conditioning system. For example, on a 42-degree day, if you are air-conditioning the internal space to 20 degrees, you have a 22-degree temperature differential across a wall and potentially upwards of a 40-degree temperature difference across a ceiling. At this time, an air-conditioning system will be operating at about half of its normal efficiency, because it has trouble dissipating the heat out of the condenser.
Even under milder operating conditions, a better building envelope enables the air-conditioning to be switched on later in the mornings because a more constant temperature can be maintained overnight.
Shading of glass from the direct rays of summer sun is also important in minimising air-conditioning load and maximising occupant comfort. Internal shading arrangements are rarely as effective as external fixed or adjustable shading, although it is recognised that this is not possible in high-rise buildings.
PREVENTING AIR LEAKAGE
This brings us to air leakage, the other major factor contributing to the thermal performance, or lack thereof, of a building. Again, the principle of ‘would I do this at home?’ applies.
One such example is the practice of the continuous exhaust of air through office toilets. This is undertaken by high power, high volume exhaust fans running continuously and often on a 24/7 basis. The basic principle is to prevent toilet odours from entering the office space. Operating these with a timer and a motion detector has the capacity to not only reduce direct electrical energy consumption, but also to greatly reduce the volume of air expelled and, therefore, the volume of air to be air-conditioned.
Entrance air locks are an admirable idea, but they are rarely effective. In most cases, the inner and outer doors are too close together to provide an effective air lock and, in almost all cases, the automatic doors are so poorly fitted that even when closed they are effectively 20 percent open.
These are all examples of designed-in flaws, but, often, occupant-instigated air leakage is just as significant. For example, areas where flexible barriers have been provided between a conditioned packing area and an unconditioned factory area, for instance, are permanently defeated by work practices. This causes the air-conditioning in the packing area to effectively try to condition an entire factory.
A second example is occupants circumventing professionally draught-proofed commercial premises. In one instance, I witnessed a strategically placed river stone keeping a lunchroom door open all day on a 38-degree day, effectively air-conditioning the local suburb.
To maximise your retrofit dollar, when undergoing an energy efficiency retrofit, ensure you include thermal performance measures along with energy services upgrades.