Cross-laminated timber, or CLT, forms the load-bearing structure of Happy Home houses. We do not use it simply because it is wood or because CLT is a modern building material. For us, its real value lies in the combination of qualities it offers: a strong and rigid mass-timber structure, factory precision, fast installation, considerable architectural freedom, and the ability to create an energy-efficient and long-lasting home.

CLT differs from both conventional timber-frame construction and modular housing. The panels are manufactured in a factory specifically for each individual house, but the house itself is not a standardised module. This allows us to combine the precision of factory production with individual architecture.

What is CLT?

CLT, or cross-laminated timber, is a mass-timber panel made from several layers of timber bonded together at right angles. A panel typically consists of three to seven layers, with the grain direction of each layer running perpendicular to the one beneath it.

This cross-laminated structure is what distinguishes CLT from conventional timber-frame construction. In a timber-frame wall, loads are carried by studs with insulation placed between them. With CLT, the load-bearing structure itself is a solid timber panel that can form a wall, floor or roof element.

For every Happy Home house, the CLT structure, structural calculations and production drawings are designed specifically for that particular project. The factory does not produce predetermined house modules that dictate the architecture. Instead, the structure is manufactured to follow the exact architectural design of the house.

How is a CLT panel made?

CLT production starts with timber boards. The boards are dried to a consistent moisture level and graded according to strength. They are then finger-jointed into longer lamellas, arranged in perpendicular layers and bonded together under high pressure. The result is a large solid timber panel from which the individual elements of the house are manufactured.

The next important stage is CNC machining. Computer-controlled machinery cuts the wall, floor and other elements to the exact dimensions specified in the engineer’s production drawings and creates window and door openings, connections and other cuts included in the design.

Openings and channels for technical systems can also be produced in the factory. At Happy Home, however, we currently make most service-related openings on site. This provides greater flexibility for interior and technical solutions later in the project.

What arrives on site is therefore not simply a large timber panel that needs to be structurally adapted by the builders, but a load-bearing element designed specifically for the house and manufactured to precise dimensions.

The CLT elements can be produced in the factory while the foundation is being built on site. Once the foundation is ready, the elements are transported to the plot and lifted into position by crane.

Why is CLT strong and dimensionally stable?

Wood is a natural material that behaves differently depending on the direction of the grain. It is very strong along the grain, while most expansion and shrinkage takes place across the grain.

In CLT, the perpendicular layers balance one another. The movement of one layer is restrained by the next layer, whose grain runs in a different direction. This makes a large panel rigid and dimensionally stable.

For house construction, this gives CLT two particularly important properties. First, the panel can carry loads in more than one direction. Second, the cross-laminated structure reduces dimensional changes caused by natural variations in moisture and temperature.

Structurally, CLT is comparable to concrete in many applications, while a mass-timber panel can be considerably thinner and lighter. At Happy Home, we also use glulam, or GL, together with CLT where required — primarily as beams where larger spans are needed. Individual posts may also be used in specific areas with higher loads or long cantilevered sections.

Together, CLT and glulam can solve large spans and more complex structural situations where reinforced concrete might otherwise be required. A timber solution can also be more economical while allowing slimmer structural dimensions.

How is a CLT exterior wall built?

CLT forms the load-bearing core of the wall, with insulation and the required façade layers installed outside it.

A typical Happy Home exterior wall is built from the inside out as follows:

• CLT load-bearing panel;
• PIR insulation;
• windproof layer as part of the PIR system;
• façade battens, creating the required ventilated cavity and a fixing base for the exterior finish;
• exterior façade finish.

In our wall construction, PIR provides both thermal insulation and the windproofing function. The battens installed outside it create the required ventilation gap behind the façade and provide a base for fixing the exterior material.

Most Happy Home houses have timber-clad façades, but depending on the architecture and the client’s preferences we also use different façade boards, including fibre-cement boards.

One advantage of a CLT wall is that the load-bearing structure does not consist of studs repeatedly interrupting the insulation layer. The structural layer is a continuous mass-timber panel manufactured to precise dimensions, while the PIR insulation forms a continuous layer on the outside.

PIR insulation makes it possible to achieve excellent thermal performance with a relatively slim wall construction. With the same external dimensions of the house, this means more usable interior floor area.

You can read more about thermal performance, wall thickness and why we use PIR insulation in Happy Home houses in the articles “House insulation thickness: 8 factors that determine the wall thickness of an A-class home” and “PIR insulation or mineral wool: 6 differences in new-build exterior walls.”

In most Happy Home living spaces, we do not leave the CLT surface exposed. Instead, it is covered with plasterboard or another finishing board according to the interior design.

We do, however, often leave CLT visible in garages, technical rooms, storage rooms and pantries, as well as on exposed roof ceilings in pitched-roof houses. Other walls or ceilings can also be left exposed if the client prefers.

Visible CLT surfaces are sanded at the factory and later treated with a suitable translucent protective or finishing product that preserves the natural appearance of the timber.

If a client would like a CLT wall, ceiling or another surface to remain visible in the interior, this needs to be agreed before the elements are manufactured. The factory can then select timber of the appropriate visual quality and allow for the required sanding and finish standard.

CLT provides architectural freedom

The structural capabilities of CLT give the architect considerable freedom when designing the spatial character of a house.

In Happy Home architecture, this allows us to create large glazed areas, open spaces, high ceilings, long spans and cantilevered solutions without making the structure unnecessarily heavy or bulky.

CLT does not mean a restricted standard design or rooms determined by module dimensions. Our house models are architectural starting points that can be adapted according to the family’s needs, the plot, views and the movement of the sun.

Factory production does not restrict the architecture. Instead, it enables an individual design to be realised with a high level of precision. For Happy Home, this is one of CLT’s most important advantages: factory precision and individual architecture can work together rather than at the expense of one another.

The CLT structure is manufactured precisely to the project

One of the strengths of CLT is that the load-bearing structure is manufactured in a controlled factory environment precisely according to the individual project.

For the client, this means that the main structure arrives on site as a clearly defined and highly accurate solution. It provides a precise starting point for the construction stages that follow and reduces the need to adapt the load-bearing structure on site.

The value of factory production therefore lies not only in speed, but also in the ability to control the quality and geometry of the structure before it reaches the building site.

Over the years, Happy Home’s CLT houses have also received several recognitions in Estonia’s Factory House of the Year competition, including awards for the best detached house, technical solutions, lighting design and energy efficiency.

The CLT structure can be erected very quickly

The installation of a CLT house is one of the most impressive stages of the construction process. Within a matter of days, a completed foundation becomes a three-dimensional house: walls, floors and the roof structure appear, and the proportions and spaces of the building suddenly become tangible.

Happy Home’s CLT elements are produced in the factory while the foundation is being completed on site. Once the foundation is ready, the elements are delivered and installed by crane. The main structure of the house can be erected in approximately one week.

Fast installation allows the following construction stages to begin quickly and helps make the overall process easier to plan. A shorter construction period can also reduce the time during which the client may be paying simultaneously for rent, an existing home loan or other temporary living arrangements.

A common question with timber construction is what happens if it rains during installation or before the building is completely weather-tight. This is taken into account in the construction process. The roof elements are covered from above with a waterproof protective layer, while the lower edges of the wall elements are protected with waterproof tape.

Once the building is weather-tight, it is thoroughly dried using mechanical drying equipment before interior works begin. The moisture levels of the structures and materials are checked to ensure they are suitable for the next stages of construction.

Timber and moisture: finding the right balance

Wood is hygroscopic, meaning that it absorbs moisture from the surrounding air and releases it again. Its moisture content therefore naturally changes together with the indoor climate even after the house has been completed.

This also means that timber can help buffer variations in indoor humidity to some extent. When humidity is higher, the wood absorbs some moisture, and when the air becomes drier, it releases some of it again. Together with a well-functioning ventilation system, this helps create a more stable indoor environment.

With CLT, the aim is not to make the structure as dry as possible. The indoor climate that is comfortable for timber is also very similar to the one that is comfortable for people. Excess moisture is not the only concern — prolonged very dry air is also undesirable. Excessive dryness can cause mass timber to shrink and visible surfaces to develop cracks, while also making the indoor environment uncomfortable for occupants.

Mechanical ventilation with heat recovery is a mandatory part of a new home. The ventilation system used in Happy Home houses also monitors indoor humidity and helps keep it within a suitable range. If more precise humidity control is desired, a separate humidification system can be added.

In this way, the structure and the building services work together to maintain a stable indoor climate that is suitable for both the occupants and the timber structure.

Can moisture or mould become trapped inside a CLT house?

This is a question clients ask us relatively often, frequently because of previous experiences with other types of construction or technical systems where excessive moisture, mould or poorly functioning ventilation have caused problems.

Mould does not occur simply because the structure is made of wood. The risk arises when a material remains excessively damp for a prolonged period and the moisture cannot dry out.

For this reason, it is important during construction to avoid trapping long-term moisture within the structure. Once the building has been closed to the weather, it is dried using mechanical drying equipment, and before structures are closed and interior finishing begins, the moisture levels of the materials are checked to make sure they are suitable for the following works.

In the completed house, the ventilation system removes excess indoor moisture and helps maintain a balanced indoor climate. At the same time, the entire building envelope must be correctly designed and constructed so that moisture cannot become trapped within the structure.

CLT helps create a highly energy-efficient home

CLT provides an excellent basis for an airtight and energy-efficient house, although the final result always depends on the building as a whole.

Energy performance is influenced by the load-bearing structure, insulation, windows, junctions, penetrations, heating system, ventilation and architecture.

The CLT panel forms a continuous structural layer, while the joints between factory-made elements can be carefully designed and executed. Together with the continuous layer of PIR insulation on the outside, this provides an excellent basis for an airtight building envelope.

Measured air leakage rates in Happy Home houses have been 0.13–0.35 m³/(h·m²) at 50 Pa, demonstrating a very high level of airtightness. The result comes from the entire construction system working together — the continuous structural panel and insulation layer, carefully designed junctions and accurate workmanship.

Energy efficiency itself is much broader than airtightness. It is determined by the complete building envelope, insulation, windows, technical systems, ventilation and architecture working together.

A Happy Home house has also received an award for energy efficiency.

CLT has excellent fire resistance

When discussing the fire safety of a timber house, it is important to distinguish between lightweight timber-frame construction and a massive CLT structure. Although both are made from wood, their behaviour under extreme heat and in the rare event of a fire is very different.

A CLT panel consists of a large mass-timber cross-section. Under very high temperatures, the outer surface begins to char. The resulting char layer acts as insulation and slows the transfer of heat deeper into the timber. This allows the inner part of the panel to remain load-bearing while the structural cross-section reduces gradually and predictably.

The load-bearing members in lightweight timber-frame construction have much smaller cross-sections, and their fire resistance depends more heavily on the entire layered wall build-up and protective boards. With mass timber, the CLT panel itself contributes significantly to the fire resistance of the structure.

This is a clear advantage of CLT: in the rare event of a fire, mass timber does not suddenly lose its load-bearing capacity. Instead, it chars gradually from the outside in at a predictable rate. Engineers can take this into account when designing the structure and dimension the element so that it retains the required load-bearing capacity for the specified period.

CLT can ignite and burn when exposed to sufficiently high temperatures for a prolonged period, but because of its massive cross-section, its behaviour is gradual, predictable and can be accurately calculated.

CLT is a long-lasting structural system

The lifespan of a timber house is not determined simply by the fact that its structure is made from wood. What matters most is that the house is correctly designed and built, and that the structure is protected from prolonged moisture exposure.

A functioning roof, correctly designed junctions, proper water management, a dry foundation area and properly functioning technical systems are just as important to the longevity of the house as the structural material itself.

CLT is a strong and dimensionally stable mass-timber structure with a very long service life when used under appropriate conditions. Happy Home provides a 50-year warranty on the structural elements and a 2-year warranty on construction works.

The longevity of a good house ultimately comes from the whole: its load-bearing structure, building envelope, details, technical systems and quality of construction.

CLT can offer a lower carbon footprint

One important advantage of CLT is its lower environmental impact compared with many conventional concrete and steel structural systems.

Wood is a renewable material, and as trees grow they absorb CO₂ from the atmosphere. Part of this carbon remains stored in the timber throughout the building’s service life. In the load-bearing structure of a long-lasting home, timber can therefore act as a store of biogenic carbon for decades.

The environmental benefit of CLT does not come solely from the carbon stored in the wood. It also depends on which construction materials it replaces, how much material is required, where the timber comes from, how it is processed and transported, and what happens to it at the end of the building’s life.

Life-cycle studies of mass-timber construction generally show that its carbon footprint can be significantly lower than that of comparable reinforced-concrete or steel structures.

CLT also has an excellent strength-to-weight ratio. A mass-timber structure is considerably lighter than a comparable concrete structure, which can reduce transport loads as well as the amount of material required in other parts of the building.

Environmental impact should not, however, be reduced to the simple statement that wood stores CO₂. The actual impact depends on the entire life cycle of the material — from forest management and manufacturing to the building’s service life and the eventual reuse, recycling or disposal of the timber.

For Happy Home, it is the combination of these qualities that matters: CLT allows us to create a strong, durable and architecturally flexible home while using a renewable material whose carbon footprint can be significantly lower than that of a comparable mineral-based structural system.

What does choosing CLT mean for a Happy Home client?

For us, the value of CLT does not lie in one individual property, but in the way its different advantages work together. It allows us to combine individual architecture, large open spaces and slim structural solutions with factory precision, fast installation, excellent energy performance and a long service life.

Just as importantly, CLT supports the principles according to which we design and build our homes: the structure should be technically strong, architecturally flexible and carefully designed in advance, while the entire house should work as one coherent whole — from the structure and insulation to the technical systems and interior solutions.

CLT is therefore not an end in itself for Happy Home, but a material that supports one of our core principles: we want to design and build houses that we would be happy to live in ourselves.