How To Design An Energy Efficient Home
Thermal comfort is the ultimate benefit of an energy-efficient home. In this article, I will discuss what to consider when designing a thermally efficient home.
What should you consider when thinking about your project brief?
Like all journeys, it's essential to be clear on the destination. Always start with the brief. Think of the big picture and be aspirational. Your project may be a lifetime opportunity to build a dream home and do your bit for the planet. Make sure you are clear on the ultimate goal of your project.
But also, think seriously about what you really need. The larger the home, the more it costs to construct and operate. It is essential to assess the projects brief and room program if the budget is tight.
Our office mantra is "do more with less". We believe that spaces should work hard by being well planned and flexible. We look for opportunities to use residual spaces to build storage that keeps rooms clutter-free and more flexible.
If all the spaces are comfortable and enjoyable, they will get more use, so think about how to maximise the potential of each room.
The initial investment in a low energy home is higher. But the comfort of a well designed, healthy, calm, and quiet house will reward you daily. You'll never want to live in a cold, draughty home again. If you can reduce the size of the home, it will enable you to afford a higher quality building at a higher cost per square meter and benefit in the long run.
What makes a building layout energy-efficient?
A good energy-efficient design will consider the position of rooms and match daylight and window size requirements with the orientation best suited for large or small openings. In principle, an energy-efficient design favours small windows to the north, medium-sized windows to the east and west, and larger shaded windows facing south.
As with any building design, efficient circulation makes more space available for usable rooms. Grouping and stacking services, such as bathrooms, utility rooms etc., will save on installation costs and make energy consumption more efficient.
What are the systems to consider for an energy-efficient home?
Internet research into energy-efficient building systems will reveal endless amounts of information on heat pumps, solar collectors, battery storage devices, etc.
We believe an energy-efficient home should start with the efficiency of the home first. It should prioritise quality design and construction and utilise systems that are easy to install, operate and maintain.
Ventilation MVHR
Ventilation for heavily insulated buildings is crucial. Older buildings are leaky, and the air exchange provides incoming fresh air for occupants and helps keep the building dry. Modern airtight buildings need adequate ventilation.
A unique feature of all Passive House buildings is the type of ventilation system they use. A mechanical ventilation heat recovery system (MVHR) allows heavily insulated super airtight buildings to have good indoor air quality.
MVHR is a whole-house ventilation system that exchanges indoor air with preheated and filtered fresh air. Outgoing air is extracted from wet rooms and spaces deep within the building and filtered before being expelled. A heat exchanger in the unit transfers outgoing heat to fresh, filtered air before being delivered to main rooms around the house. MVHR ensures optimal air quality, creates a calm and quiet home and is energy-efficient.
If a whole-house system is impossible, individual through-wall MVHR extractors in wet areas might be a solution. They are less efficient but more straightforward to implement due to their small size and lack of ductwork. They are an efficient solution for replacing conventional mechanical extractors.
To find out more, check out our article on MVHR and indoor air quality.
Heating and Hot Water
An energy-efficient building will make efficient use of energy. The design should consider grouping bathrooms and kitchens close together to minimise heat loss along long pipes, contributing to internal heat gains and potential overheating.
If the building is well insulated, the heating demand should be minimal. For example, the space heating demand for a Passive House standard single-family home can be provided by a small heat source such as one or two radiators, heated towel rails in bathrooms or a small heating element on the MVHR supply air. Many Passive House owners like the comfort of underfloor heating and opt for an efficient in-floor heating system in some or all rooms.
For an energy-efficient, airtight home, a heat pump is commonly used. Both air source heat pumps and ground source heat pumps are highly compatible with MVHR, working efficiently to provide low-temperature heating to the system that has already recovered heat from the outgoing air.
The system can be as straightforward or complex as you like, embracing solar hot water collection, photovoltaic collection and battery storage.
The more complex a system, the more difficult and costly it will be to install and maintain. So try to keep it simple and embrace the fabric-first principles. Spend more on the building, so the heating system costs less.
Energy analysis
Over recent decades computer software has made it possible to assess a building's energy performance during its design. Energy analysis software enables designers to predict performance, test options, refine the design and avoid costly mistakes.
There are many different software packages available. One example is the passive house planning package (PHPP). Refined over many years using data collected from monitoring all international Passive House buildings, it is one of the most accurate at predicting the actual performance in use.
A PHPP energy model considers detailed information about the building, site, and climate to assess the energy balance. It looks at:
heat loss (walls, roofs, windows, etc.)
shading (trees, other buildings, overhangs, etc.)
solar heat gains (windows, roof lights, etc.)
ventilation
internal heat gains (equipment, occupants, etc.)
heating demand
With an energy-efficient building, overheating is a risk, and it is vital to ensure comfortable temperatures throughout the year. Too much glass or insulation will lead to overheating, as will insufficient shading. Too little insulation increases the heating requirements.
With a complete energy model, any aspect can be adjusted, tweaked or tested, such as the building orientation, insulation thicknesses, window sizes, and solar shading, to see the performance and refine the design. For example, the designer can resize a window to exploit a view by compensating elsewhere. Or roof insulation can be increased to make thinner walls.
Ideally, the energy model will be developed as soon as possible, either at the concept or planning stage of design. The model should accompany the developing design through to construction.
If you target a certifiable Passive House standard, PHPP can tell you if the design is on track to pass before the planning stage.
We use PHPP alongside hand sketches and 2D and 3D computer software to optimise homes for our clients and their families. Over many iterations, the design evolves into the healthiest, quietest, most enjoyable home in which to live.
We know from experience, anyone considering an energy-efficient project should commission some form of energy analysis.
What do you need to consider with the exterior "fabric" of the building?
The Passive House methodology may seem complex and technical, but the fundamental concept is simple: fabric-first. It is a concept that applies to all buildings - put more of the budget into the insulation, windows, etc. and less into building systems by prioritising insulation and staying away from fancy tech add ons.
Insulation
It is good to start with 300mm of insulation (not just wall thickness) when planning the initial layout for a Passive House standard building. Later, you can assess options with an energy model, such as offsetting thinner wall insulation with better windows or thicker roof insulation.
The thickness of insulation will vary due to insulation type and method of construction. Natural insulations like sheep's wool are thicker but more sustainable and better for indoor air quality.
Common foam insulations such as polystyrene, cellulose and polyurethane, are more insulating per thickness than their natural counterparts.
Hi-tech installations such as aerogels and vacuum insulated panels can minimise wall and roof thicknesses but come at a premium.
When renovating solid wall buildings, maintaining the breathability of walls is essential to protect the structure and prevent mould growth. We have utilised wood fibre internally to insulate solid brick walls and insulating cork renders both externally and internally.
Airtightness
The technical design and detailing of heavily insulated buildings is essential. The building must be designed with a continuous airtight "wrapping" around the entire building to retain heat and manage moisture movement. Compromises resulting from poorly detailed or installed penetrations for incoming or outgoing services, or junctions around windows, will significantly impact the airtightness. Not only is heat lost, but convection can allow moisture into the structure and cause dampness, mould and rot. This is bad for your health and the health of your building.
To meet Passive House certification, one small slit or hole in the airtightness layer will likely lead to failing the pressure test.
Careful consideration to airtightness must be given during the design and construction.
Thermal bridges
The building must also be designed with a continuous line of insulation around its entirety. Any gaps or structures that cross the insulation line will cause thermal transfer, resulting in cold spots internally where mould can grow, putting your health at risk. Escaping heat follows the lowest path of resistance.
Examples of thermal bridges are poorly detailed junctions between floors and walls, the use of traditional (uninsulated) cavity wall ties, and structural connections for balconies or solar shading, and anchor points for external fixings.
Thermal bridges should be eliminated or managed in the design of the building. These points are often thermally modelled individually to ensure correct detailing.
Windows
The following should be considered when selecting windows and detailing the junctions:
Window (and room) placement should consider the orientation of the building.
The windows should be sized appropriately for the rooms
The u-value of the glazing unit, frame, spacer and installation all impact the energy-efficiency
A poor g-value will impact the solar heat gain and can lead to overheating
A good shading design can help the building keep cool in the summer and warm in the winter.
The installation details should consider insulation covering the frame
The window configuration should minimise the window frame, the least energy-efficient part of an energy-efficient window.
Heat gains
Many homeowners desire open plan living spaces that have a strong connection to the outdoors. Many want spaces that connect to rear gardens with large areas of glazing. Few people consider solar gain. Without assessing the potential for overheating in the design, the first summer could deliver a hot surprise with the future-proofing of the home jeopardised.
When designing the building, the first considerations that impact solar heat gain are the building orientation, room distribution and window size. Large areas of glazing are a big risk for overheating. More detailed considerations concern window specification and shading.
Good energy analysis software such as PHPP will consider anything that has a notable impact on the solar heat gain in the calculations. PHPP models the solar heat gain for the entire year using climate data for the specific location. It accounts for the buildings altitude and average skyline. It will consider shading from trees, adjacent buildings, window frame configuration, frame thickness, and window reveal depths.
It also accounts for internal heat gains generated from warm bodies and household appliances.
We believe that any new home design should be thermally modelled, even as just insurance to future proof your home as global temperatures rise.
Conclusion
A building project is a long process with many challenges along the journey. Perhaps the critical aspect is the clarity of your brief and your ultimate goal. Being clear about your ultimate goal and what this project means to you will help guide decision making and help you overcome obstacles along the way. Be realistic, be positive and reward yourself by building a truly remarkable home.
