Table of Contents
Introduction
Energy efficiency in buildings is about providing the same or better comfort and services for occupants while using less energy. This chapter focuses on how buildings use energy, what influences that use, and which design and operational choices typically improve efficiency. Detailed technologies and policies will be covered in other chapters, so here the emphasis is on overall principles and how they fit together in the building context.
How Buildings Use Energy
In most climates, the largest shares of building energy use are for space heating, space cooling, and hot water. Lighting, appliances, and equipment such as computers or refrigerators also add to the demand. In some buildings, such as offices or hospitals, ventilation and air conditioning systems can dominate.
The mix depends strongly on location and building type. In a cold climate, heating is usually the main use. In a hot climate, cooling and ventilation can dominate. In humid climates, dehumidification also matters. Understanding which services consume the most energy in a particular building is the starting point for any efficiency strategy.
Building Envelope And Heat Flow
The building envelope consists of all the parts that separate indoors from outdoors. This includes walls, roofs, floors, windows, and doors. The envelope controls how heat enters or leaves the building and how air flows through leaks and openings.
Heat moves in three main ways. Conduction is heat passing through solid materials, such as a wall. Convection is heat carried by moving air, such as drafts through gaps. Radiation is heat transfer through electromagnetic waves, such as sunlight entering through windows or heat radiated from warm surfaces.
Insulation reduces conductive heat flow. Thick, continuous insulation in walls, roofs, and sometimes floors keeps indoor temperatures more stable and reduces heating and cooling needs. Good insulation is particularly important in climates with large temperature differences between inside and outside.
Windows are usually weaker than walls in terms of insulation. Their performance is often described by a U-value, which measures how easily heat passes through. A low U-value means better insulation. Glazing can also be designed to admit useful sunlight while limiting unwanted heat in hot periods.
Air tightness affects how much outside air enters unintentionally through gaps and cracks. Uncontrolled air leakage can cause large energy losses. At the same time, buildings need fresh air for health and comfort. Efficient buildings combine good air tightness with controlled ventilation systems.
Internal Loads And Occupant Behavior
Everything inside a building that uses electricity or produces heat affects energy use. People, lights, appliances, and equipment all add heat to indoor spaces. In cold climates, this internal heat can slightly reduce heating needs. In hot climates, it can increase cooling needs.
Occupant behavior has a strong influence on actual consumption. Setpoint temperatures on thermostats, how often windows are opened, how long lights and devices are left on, and patterns of occupancy all change the energy profile of the building.
Two similar buildings can have very different energy use depending on user habits. Efficient buildings manage these internal loads through thoughtful design, user-friendly controls, and information that helps occupants make better choices.
Passive Strategies Before Active Systems
In efficient building design, passive measures are usually considered before active systems. Passive strategies use the building’s form, materials, and orientation to reduce energy needs without relying heavily on mechanical equipment.
Examples include orienting the building to make good use of daylight, using shading to limit direct sun in hot periods, placing windows to capture winter sun in cold climates while reducing heat loss, and designing compact shapes that minimize the external surface area through which heat can escape.
The basic idea is to first reduce the heating and cooling demand through the envelope and passive design. After that, efficient active systems such as heating, cooling, and lighting can be sized more appropriately and run less often.
Heating, Cooling, And Ventilation Efficiency
Once the building’s passive performance has been improved, the focus shifts to the efficiency of active systems. Heating and cooling systems convert energy inputs into useful thermal energy. Their effectiveness is usually measured by efficiencies or performance ratios.
For heating devices such as boilers or furnaces, efficiency is often defined as the ratio of useful heat output to fuel input. If a boiler consumes an energy input $E_{in}$ and delivers $E_{useful}$ as heat, then the efficiency $\eta$ is:
$$\eta = \frac{E_{useful}}{E_{in}}$$
Ventilation systems bring fresh air into the building and remove stale air. Efficient designs can include heat recovery, where outgoing exhaust air preheats or precools incoming fresh air through a heat exchanger. This can significantly reduce heating or cooling loads while maintaining air quality.
Cooling efficiency is commonly characterized by performance indicators that relate cooling output to electrical input. While detailed metrics appear in other chapters, the general principle remains the same. More cooling per unit of electricity means higher efficiency.
Proper sizing and design are crucial. Oversized systems tend to operate inefficiently and may cycle on and off frequently. Carefully matched systems, designed for the reduced loads of an efficient envelope, typically perform better.
Lighting And Daylighting
Lighting is another major use of energy in buildings. Modern efficient lighting uses less electricity to provide the same level of illumination. The effectiveness of a light source can be described by its luminous efficacy, which is the amount of visible light produced per unit of power, usually in lumens per watt.
Natural light from the sun offers a free alternative during daytime. Daylighting design integrates window placement, building orientation, interior finishes, and shading devices to maximize useful daylight while avoiding glare and overheating. When daylighting is done well, electric lights can be dimmed or switched off for many hours.
Lighting controls, such as occupancy sensors and daylight sensors, help ensure that lights operate only when and as much as needed. Combined with efficient light sources, these strategies lower energy use without sacrificing visual comfort or safety.
Controls, Automation, And Smart Operation
Controls and automation systems manage how building equipment operates over time. Simple thermostats, timers, and manual switches already influence energy use. More advanced building automation systems can coordinate heating, cooling, ventilation, and lighting according to occupancy schedules and external conditions.
Smart controls can, for example, reduce heating when spaces are unoccupied, pre-cool or pre-heat when energy is cheaper or when renewable generation is abundant, and adjust ventilation rates based on indoor air quality sensors. For users, intuitive interfaces are important so that energy saving features are not overridden out of confusion or discomfort.
In efficient buildings, technology supports good performance but does not replace basic design choices. Controls are most effective when loads are already reduced through envelope and passive measures.
Refurbishment And Existing Buildings
Most of the buildings that will be in use for decades already exist. Improving efficiency in these buildings is often called retrofitting or refurbishment. It includes measures such as adding insulation, replacing windows with more efficient models, sealing air leaks, upgrading heating or cooling equipment, and improving controls.
The order of improvement usually mirrors that of new design. First reduce demand through the envelope, then install efficient systems, and finally use smart controls. Even simple actions such as sealing major air leaks or adjusting control settings can yield noticeable savings with modest cost.
Deep renovations combine several measures at once to achieve large energy reductions. While they can require higher upfront investment, they also provide long term benefits through lower bills and improved comfort.
Measuring And Comparing Building Efficiency
To understand and improve building efficiency, energy use must be measured and related to the size or function of the building. A common indicator is annual energy consumption per unit of floor area, for example kilowatt hours per square meter per year. This allows comparison between buildings of different sizes.
Many regions use building energy labels or certificates that grade buildings based on calculated or measured performance. These labels help owners and occupants see how efficient a building is compared to typical practice or best practice.
A key principle is that energy efficiency in buildings is always evaluated as the ratio of useful services provided to energy consumed. Lower energy use for the same comfort, light, and function means higher efficiency. If either comfort or function is compromised, then energy savings do not reflect true efficiency.
For building planners and managers, monitoring actual consumption over time is essential. Meters and submeters help to identify which systems use the most energy and where improvements are most effective.
Comfort, Health, And Efficiency Together
Efficient buildings do not only aim at lower energy bills. They also aim at stable indoor temperatures, good air quality, and sufficient light. When the envelope, systems, and controls are thoughtfully designed, efficiency and comfort usually reinforce one another instead of conflicting.
Well insulated and airtight envelopes reduce drafts and cold surfaces, which improves perceived comfort. Proper ventilation controls moisture and pollutants. Daylighting connects occupants with the outdoor environment and can improve well being. In this sense, building efficiency is a key part of sustainable, healthy living and working spaces.
This integrated view recognizes that buildings are more than their energy consumption. They are places where people live, work, and learn. Energy efficiency in buildings, when done well, supports all these functions while using fewer resources.