The most effective way to make a prefab more energy-efficient and save money on utility bills is by integrating all the energy-efficiency features into its design. The most critical design element of an energy-efficient prefab is its exterior envelope since, being the separation between the indoor and outdoor spaces, it should be adequately insulated and reduce heat loss as much as possible. Still, two additional actions will highly improve the house’s energy efficiency: orientating the house with an understanding of the natural climatic conditions of the area and using only energy-efficient equipment.
What is energy efficiency?
Energy efficiency is the quality of a house to consume less energy while providing a comfortable living environment. Energy-efficient homes offer a warm environment during the winter months, stay cool during the hotter seasons, and still, their utility bills are highly reduced.
Placement and orientation of an energy-efficient prefab house
To achieve an energy-efficient prefab, it is essential to understand the climate conditions affecting the plot where the house will be placed. We should consider the temperature, the solar path, and the wind patterns throughout the year. This information tells us where the house should face and offers us a first approach to how it should be designed.
For example, if we are building a prefab house in a colder climate, such as the UK or northern Europe, we want to let as much sun in as possible to heat the indoor space naturally with the sun’s rays. By choosing a prefab model with big windows in one facade and orienting that facade toward the south, we create a greenhouse effect that will nicely heat the indoor space. In this case, it is also essential to avoid placing too many windows in those areas where the wind is strong, or the sun doesn’t reach them since they will contribute to heat loss.
The opposite applies to houses placed in warmer areas like the Mediterranean countries. In these cases, we use the understanding of the solar path to avoid the summer sun rays directly impacting the house’s windows while welcoming them during the winter months. Given that the sun is higher during the summer months, this can be achieved by orienting the window facade towards the south and placing a pergola structure in front to protect it from the summer rays.
A bioclimatic understanding of the house can help us choose an energy-efficient design and guide us towards orienting it. While the way we built the house is essential, other elements such as trees, mountains, or other buildings should also be considered since they create shadows and protect from the wind.
Design an energy-efficient thermal envelope
The house’s envelope is defined by its exterior covering: the walls, the windows, the roof, and the floor. Given that the house’s envelope is the barrier separating the comfortable indoor space from the exterior environment, it is a key element to consider when designing an energy-efficient house.
An energy-efficient prefab should be severely insulated, covering all elements conforming to the envelope, including the roof and the floor. Elements such as the insulation materials, the types of windows, and the airtightness of the house are essential to consider throughout the entire envelope.
Insulation requirements
Proper house insulation should consider the walls and never forget the floor and roof in warm and cold climates. There is a wide variety of insulation materials to choose from, but below is a list of the most common insulation materials used in prefab houses:
| Insulation material | Brief description | U-Value at 100mm (W/m2K) [1][2] |
| Structural insulated panels (SIP) | This material is easy to install since it comes with the structural material and the insulation together. Generally, there are three different insulation materials used in SIPs: expanded polystyrene (EPS), extruded polystyrene (XPS), or polyurethane foam (PUR). | EPS = 3.52XPS = 3PUR = 4.50 |
| Sheep’s wool | This material is natural, sustainable, and very efficient. It can be applied on both walls and roofs, and despite being organic it has a longevity of 60 years, it is fire resistant and keeps insects away. | 2.63 |
| Glass wool | While this material isn’t of natural origin, it offers a positive environmental impact since it is built with recycled glass bottles. | 2.85 |
| Rock wool | Rock wool is a very efficient material since it offers good thermal insulation and sound reduction and fire safety. | 2.85 |
| Rock wool | While this material isn’t generally eco-friendly, it has some advantages since being applied as a foam it is quite efficient in reducing draughts. | 1.08 – 1.26 |
The thickness of insulation varies depending on the material and the requirements of each location. While the amount of insulation required in every part of the house’s envelope should be adequately calculated by an architect or an engineer, you can get an idea of the thickness needed for each with a rough understanding of the concept of thermal transmittance.
Every insulation material has an associated thermal transmittance (U-value) which defines the ability of that material to transmit heat. By looking at this value, we know how thicker each material should be in relation to the others. Those materials with a lower U-value will require less material to achieve the same insulation results. Still, the proper calculation will consider other aspects, such as the other materials conforming to the wall.
Energy-Efficient windows
The windows are an essential element to consider when choosing an energy-efficient prefab since regular houses lose 10% of their indoor temperature through these elements [3]. In Europe and the UK, to know if a window is energy efficient, we can look at its labels:
EU Energy label: Offers information about how energy efficient a product is. Every product is rated on a scale of 7, with the least energy-efficient product ranked with a red D and the most energy efficient with a dark green A+++.
British Fenestration Rating Council (BFRC): Applicable only in the UK and with a similar format to the EU Energy label. In this case, there is a scale of 8 values, with the least energy-efficient window being ranked with a brown G and the most energy efficient with a purple A+
The importance of airtightness and ventilation
To ensure an energy-efficient prefab, it isn’t enough to choose the right windows and insulation materials; it should all be properly airtight. The main goal of air tightening is to eliminate unwanted air leakage and draughts through the house’s envelope. As a result, the comfortable indoor temperature can be easily maintained with less energy production.
To avoid air leakage, it is essential to pay special attention to the different joints in the house, the most important ones being those between windows and walls, walls and roofs, or walls and floors. Still, the materials used to build the envelope are essential to define an airtight space.
Airtightness can help save a good amount of money on energy bills. However, air tightening means no air exchange, which can have health consequences. Therefore, an airtight house should always provide a mechanical ventilation system, ideally with heat recovery, to reduce heat loss as much as possible.
Choose energy-efficient equipment
Once we have chosen the proper orientation for the house and designed an effective exterior envelope, it is time to choose energy-efficient heating and cooling equipment. Even if energy-efficient homes highly reduce the need for heating and cooling, using energy-efficient equipment helps reach the desired indoor temperature. We can opt for energy-efficient appliances to further reduce energy consumption and bills cost.
When choosing an energy-efficient house, we must ensure that the amount of energy used by the different equipment is kept low, maintaining the energy bills to a minimum. To identify those more energy-efficient systems, we should look at their EU Energy label, which works just as we have described in the window section: with a red D being the least energy efficient and a dark green A+++ representing the most energy efficient system.
Are prefab and modular homes more energy efficient?
Prefab houses aren’t energy efficient by definition, though the process of prefabrication eases achieving these parameters. Below are some of the most noticeable advantages offered by off-site construction:
- Quality control: The centralisation of the construction works grants further quality control inspections throughout the process, ensuring better thermal envelope performance.
- Industrialised building environment: Being built in a factory-like setting, prefabs have access to sophisticated tools and machinery, which eases a finer work quality reducing the chance of leaving open spaces that will cause drafts.
Given the advantage of building off-site, a good selection of manufacturers offers the opportunity to build up to energy efficiency standards. Choosing an energy-efficient house increases the construction cost, though it will reduce the utility bill cost enormously afterward. According to the BBC, an average home in the UK can save up to £555 on energy bills by upgrading to energy efficiency standards [4].
References:
[1]Insulation materials and their thermal properties Greenspec https://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/
[2]Sprayfoam gives a unique combination of great U Values and air permeability resistance Fusion Insulation http://www.fusioninsulation.com/thermal-insulation.html
[3]Windows: Heat loss & Heat gain Greenspec https://www.greenspec.co.uk/building-design/windows/
[4]Stallard, E. (2022 October 7) Home insulation: How can it cut energy bills? BBC News https://www.bbc.com/news/explainers-60289396