When people talk about an energy-efficient home, they often focus on individual things such as insulation, windows, a heat pump or solar panels. In reality, none of these on its own determines how energy-efficient a house will be.

A good result comes from the whole building working together: how little heat the house loses, how airtight it is, how ventilation is handled, how efficiently heating and cooling are produced, and how the architecture responds to sunlight, glazing, orientation and shading.

This also means that an energy-efficient house does not have to mean small windows, low ceilings and the simplest possible architecture. Large areas of glazing and generous spaces can work perfectly well in an energy-efficient home — they simply need to be designed as part of the overall system.

An energy rating is not the same as your electricity bill

An energy performance certificate gives the building an energy rating based on a calculated energy performance figure. The calculation takes into account heating, ventilation, cooling, domestic hot water, lighting and the energy sources used.

The important distinction is that the energy rating describes the building under standardised conditions, not the actual energy use of a particular family.

Two identical energy-efficient homes can have very different electricity bills if, for example:

• one is kept at 21°C in winter and the other at 24°C;
• one household uses considerably more hot water;
• one uses more cooling in summer;
• an electric car is charged at home;
• two people live in one house and six in the other.

How the house is used also matters. Opening the windows for fresh air can be pleasant and sometimes necessary, but frequent airing during cold winter weather lets heat escape without benefiting from the ventilation system’s heat recovery and therefore increases heating demand.

In summer, the same principle works in reverse. If it is cooler outside than inside, evening or night-time ventilation can help cool the house. But keeping windows and doors open during a hot summer day brings warm air inside and increases the need for active cooling.

An A energy rating therefore cannot tell you what your electricity bill will be next January. What it tells you is how strong a starting point the house provides through its architecture, structure and technical systems.

Even finishing materials can affect actual energy use

Perhaps more surprisingly, even a floor finish can have some influence on the real electricity bill.

Happy Home houses use hydronic underfloor heating, and the floor finish affects how efficiently heat from the floor is transferred into the room.

A thin mineral finish such as microcement conducts heat very well. Parquet provides more insulation, which means the underfloor heating system may require a higher water temperature to maintain the same room temperature.

This does not mean that a house with parquet loses more heat through its structure — the heat loss of the house itself remains the same. The difference can arise in the efficiency of the heating system: the lower the temperature at which a heat pump can operate, the more efficiently it generally works and the lower its electricity consumption can be.

The effect naturally depends on the material, its thickness and the area over which it is used. A few square metres of microcement in a bathroom cannot be compared with a microcement floor across an entire ground floor.

This specific effect of the floor finish is not separately taken into account in the standard energy performance calculation.

It is a good example of why an energy rating and the actual energy consumption of a home are not exactly the same thing.

Energy efficiency starts with the building envelope

The most efficient energy is the energy the house does not need to produce in the first place.

Energy efficiency therefore begins with the building envelope: walls, roof, floor, windows, junctions and service penetrations all need to work together as one system.

In Happy Home houses, the load-bearing structure is made from CLT, or cross-laminated timber, with a continuous layer of PIR insulation on the outside.

With PIR, it makes little sense to judge the solution simply by asking how many centimetres of insulation there are in the wall. Different insulation materials have different thermal conductivity, so the thickness required to achieve the same thermal performance can vary considerably.

PIR allows excellent thermal performance to be achieved with a relatively slim wall construction. With the same external dimensions of the house, a thinner wall also means more usable interior floor area.

We therefore look at the thermal performance and construction of the entire building envelope rather than simply the thickness of the insulation layer.

We discuss insulation thickness and the properties of PIR in more detail in a separate insulation article. The typical Happy Home exterior wall build-up is also explained in “Benefits of a CLT house: why we use cross-laminated timber at Happy Home.”

Airtightness is just as important

If warm air can escape uncontrollably through joints in the building envelope, additional heating will be required regardless of how good the insulation material is.

For this reason, the airtightness of the completed house is tested using a blower door test. Measured air leakage rates in Happy Home houses have been 0.13–0.35 m³/(h·m²) at 50 Pa. These results have also been achieved in our houses with large areas of glazing.

An airtight home does not mean a house that “cannot breathe” and therefore needs ventilation.

The building envelope is supposed to be airtight. Fresh air should enter the house in a controlled way through the ventilation system, not unpredictably through gaps in the structure.

Mechanical ventilation with heat recovery is a natural and essential part of a modern energy-efficient new home.

Ventilation: fresh air without wasting heat

Heat-recovery ventilation performs several functions at once: it supplies fresh air, removes stale air and excess moisture, and recovers much of the heat from the outgoing air.

In a typical system, fresh air is supplied to the living room, bedrooms and other clean living spaces, while extract air is taken from kitchens, bathrooms, WCs and other rooms with higher moisture or odour loads.

This is the basic arrangement. Depending on the project and the client’s preferences, additional supply or extract points can also be added to bedrooms or other rooms.

The incoming and outgoing air streams do not mix. Heat is transferred from one air stream to the other through the heat exchanger, so the heating system does not have to warm all incoming winter air from outdoor temperature up to room temperature from scratch.

In addition to energy efficiency, ventilation plays an equally important role in indoor comfort and humidity control.

People, showers, drying laundry, cooking and everyday life constantly add moisture to the indoor air. Ventilation removes excess moisture and helps keep the indoor climate more stable.

This is also important for the CLT structure. The indoor humidity range that is suitable for timber is broadly similar to the range that is comfortable for people. Excess moisture is undesirable, but so is extremely dry indoor air.

The ventilation units used in Happy Home houses also monitor indoor humidity. If particularly stable humidity is desired — for example to avoid very dry indoor air during winter — a separate humidification unit can be added to the ventilation system.

Filters also reduce the amount of outdoor dust and pollen entering the house, although they require regular maintenance to work properly.

The heating system should not have to compensate for weaknesses in the house

The lower the heat demand of the house, the less work the heating system needs to do.

Happy Home houses use hydronic underfloor heating. One of its major advantages is that it can heat the space evenly using a relatively low water temperature, which works particularly well with a heat pump.

Depending on the house and plot, the heat source may be:

• an air-to-water heat pump;
• ground-source heating with a horizontal ground loop;
• ground-source heating with vertical boreholes.

Which solution is best for a particular house depends on the size of the house, the plot, the client’s preferences and the budget.

Air-to-water heat pumps, ground-source heating and hydronic underfloor heating will be covered in a separate article.

Large windows and high ceilings do not rule out energy efficiency

Large areas of glazing have a greater effect on the energy balance of a house than a conventional insulated wall. In winter, their heat loss must be considered, while in summer the main issue is solar gain.

That does not mean the answer to energy efficiency is simply “fewer windows”.

Daylight is one of the most important qualities of a good living space, and at Happy Home we do not solve energy efficiency by making rooms darker.

With large glazed areas, several things matter at the same time:

• the thermal and solar-control properties of the glass;
• the dimensions of the window;
• its orientation;
• useful solar gain;
• airtight installation between the window and the structure;
• summer shading.

Window size and proportions are not determined by energy efficiency alone. In one location the best solution may be a large glazed wall, while elsewhere a narrower or differently proportioned window may be more appropriate — but the aim is always to bring sufficient natural light into the room.

The window design is therefore the result of daylight, views, room use, energy efficiency and architecture working together.

A high ceiling adds volume, but does not automatically make a house inefficient

A higher ceiling means a greater volume of air to heat, and this needs to be considered when sizing the heating and ventilation systems.

At the same time, underfloor heating delivers warmth from below and the ventilation system is designed according to the required air volumes.

A good energy-efficient design does not begin with the question: “What do we need to give up for the sake of energy efficiency?”

Instead, the desired architecture — large windows, high spaces and openness — needs to be made to work technically. The structure, glazing, heating, ventilation, cooling and shading are designed together with the architecture.

Cooling demand starts with architecture, not air conditioning

An energy-efficient home needs to work well not only during a cold winter, but also on a hot summer day.

A well-insulated and airtight house retains temperature very effectively. In winter this is a major advantage, but in summer it means that just as much attention needs to be paid to preventing excessive solar heat from entering the building.

Cooling demand is influenced from the moment the house and its position on the plot are designed.

The most important factors include:

• the orientation of the house;
• the location of rooms within the house;
• the size and orientation of glazed areas;
• the solar-control properties of the windows;
• eaves, cornices, terrace roofs and other architectural shading;
• existing mature trees and other shading features on the plot.

Orientation and shading can significantly reduce cooling demand

The same area of glazing can behave very differently depending on its orientation.

South-facing glazing can provide useful solar gain in winter. In summer, when the sun is much higher in the sky, a correctly designed cornice or overhang can block much of the direct summer sun while still allowing the lower winter sun to enter the room.

We use this principle in Happy Home architecture as well. The characteristic cornices of our Denmark house are not purely a visual detail — in front of large areas of glazing, they reduce direct solar gain and help lower the summer cooling load.

Low eastern and especially western sun is more difficult to control because a horizontal overhang is less effective at blocking it.

The position of the house, room layout, glazing, views and movement of the sun therefore need to be considered together.

The plot itself can also have a major effect on actual cooling demand

In a wooded setting, existing mature trees can provide strong natural shade in summer and significantly reduce the real cooling demand of the house.

The shading effect of trees and mature vegetation is not taken into account in the standard energy performance calculation, even though in real life it can have a major effect on summer indoor temperatures and cooling demand.

This is another good example of why the calculated energy rating and the actual energy use of a house are not always identical.

When designing a house, we therefore look beyond the calculation itself and consider the actual plot: which trees will remain, where the strongest sun comes from, and which rooms can be better protected through the position and architecture of the house.

First reduce cooling demand, then choose the cooling system

The energy performance calculation also includes a check of summer indoor temperatures and identifies the rooms with the greatest risk of overheating.

This gives us information already during the design phase about where active cooling needs to be provided.

In Happy Home houses, this is often the living room and one or more bedrooms. Depending on the project, a room such as the sauna lounge may also require cooling because of higher solar loads or simply for comfort.

A client may also choose to add cooling to rooms where the calculation does not show a critical overheating risk but where cooling would still improve everyday comfort — for example in all bedrooms.

Where active cooling is part of the project, its energy use is also included in the building’s energy calculation.

The principle is simple: first we reduce the cooling load through orientation, architecture, shading and the opportunities offered by the plot. Only then do we size the active cooling system.

Solar panels are part of the energy design, not a random addition later

Solar panels can improve the calculated energy performance of the house and reduce the amount of electricity purchased from the grid, but they do not replace a good building envelope, airtightness, ventilation, a well-designed cooling strategy or an efficient heating system.

Solar panels need to be considered during the design phase.

The project needs to establish:

• whether the energy calculation for the particular house requires solar panels;
• what capacity is needed;
• where the panels should be located;
• how roof orientation and possible shading affect them;
• how the necessary cabling and technical connections will be arranged.

For a small residential building with more than 220 m² of heated floor area, nearly zero-energy building requirements apply. Where a photovoltaic system is needed to achieve this level, it must be included if the solution is technically feasible and economically justified.

Solar panels may also form part of the required energy solution for a smaller house. The question is therefore not simply whether panels could be “added later” — their need, capacity and location should be considered during design.

If solar panels are included in the energy performance calculation, they also need to be physically installed by the time the occupancy permit is applied for.

Happy Home does not sell or install solar panels itself. The client orders the system directly from the manufacturer or installer so that both the product and installation warranties remain directly between the client and the specialist supplier.

How do you know that energy efficiency did not remain only on paper?

The energy calculation is carried out during the design phase. The quality of construction can also be checked once the house has actually been built.

One of the most important checks is the airtightness test. This shows whether the airtight building envelope designed on paper has also been achieved in practice.

Once the ventilation system has been installed, the airflows are measured and adjusted according to the design.

This is not merely a formality. If supply and extract airflows are not correctly balanced, unwanted pressure differences can develop inside the house. Air may begin moving through places where it was never intended to move, and with greater pressure differences even opening doors or windows can become less comfortable.

It is therefore not enough for a house simply to “have ventilation”. The system needs to be correctly designed, installed, measured and balanced.

Happy Home’s energy-efficiency solutions have also received external recognition: in 2025, a Happy Home house received an energy-efficiency award in Estonia’s Factory House of the Year competition.

That recognition did not come from one heat pump, thicker insulation or solar panels.

Energy efficiency is a property of the house as a whole. It comes from the way the structure, insulation, windows, airtightness, ventilation, heating, cooling and architecture work together.

What should you ask a builder about an energy-efficient home?

An A energy rating alone tells you less than how that result is achieved.

Before choosing a construction partner, it is worth asking:

• What structural and insulation solution is used?
• Is the airtightness of the completed house actually tested?
• What airtightness results has the builder achieved in previous houses?
• What ventilation system is included, and is it measured and balanced after installation?
• Which heating system is used as the basis for the energy calculation?
• How has the orientation and architecture of the house been used to reduce summer cooling demand?
• Which rooms are at risk of overheating according to the calculation, and where is cooling planned?
• How is large glazing shaded?
• Are solar panels required to achieve the energy performance of this particular house?
• Which solutions work together to achieve the intended energy performance?

At Happy Home, the CLT structure, insulation, airtightness, windows, heat-recovery ventilation, low-temperature heating, cooling, orientation, shading and architecture are designed together.

This means you do not have to choose between good architecture, daylight, spaciousness, a comfortable indoor climate and energy efficiency. In a well-designed home, they support one another.