Designing a bioclimatic timber house

In our studio we mainly work with two materials; our homes are generally designed in timber or in masonry. Today we will discuss the design process for a bioclimatic timber house: a home that, as its name suggests, has a load-bearing timber structure and performs in harmony with its surroundings to reduce energy consumption.

What is a bioclimatic house

Bioclimatic architecture is a type of architecture that adapts to the environment where it is built. It is the architecture our grandparents knew: architecture that is consistent with the climate and the conditions of the place where it is built.

If we are in a cold climate with access to large forests, we will build in timber, insulate the house heavily, and open large windows to capture the sun.

Conversely, if we are in a warm climate, we will want plenty of ventilation and solar protection. In fact, there are homes in South America or Asia that, instead of glass windows, have openings protected by louvred shutters that block direct sunlight and allow constant ventilation throughout the house.

In this sense, bioclimatic architecture is based on traditional knowledge learned over several generations, so that our home works in our favour by harnessing the sun’s heat or protecting us from it when necessary.

When we talk about designing a bioclimatic timber house, we mean a home that can maintain a suitable, comfortable indoor temperature using only natural resources, minimising the use of artificial heating and cooling and therefore reducing energy consumption.

In this case, we will look at the specific case of designing a bioclimatic timber house: a home whose main structure is timber, but not all of its materials are. In other words, the façade cladding does not have to be timber, and we can combine lower-maintenance materials as well as materials with thermal properties that complement timber. Let us look at this in detail.

Designing a bioclimatic timber house

Integration with the surroundings

Integration with the surroundings is one of the main design characteristics of a bioclimatic timber house that aims to reduce consumption or provide natural thermal comfort. Building in accordance with the site’s existing conditions allows the home to perform better and adapt to the climate and characteristics of the place.

Orientation

The best orientation, of course, is always south-facing, as it provides the greatest solar exposure throughout the day.

It is important to have at least one south-facing façade to heat a house. On it, around 60% to 75% of the surface area should be windows or glazed surfaces in order to capture solar radiation.

On the other orientations, of course, this percentage will be reduced, since glazed surfaces are also a weak point in terms of heat loss. Depending on the site’s possibilities, we can open windows to the east and west to capture the first and last rays of the day; however, this must be assessed on a case-by-case basis, taking into account possible obstacles such as neighbouring buildings.

On the north façade, the most important thing is to reduce the amount of glazing or, failing that, ensure it is very well insulated with double glazing, and to insulate the walls thoroughly. Let us remember that at our latitude the sun does not strike north façades, which is why they are considered cold façades.

It is important to remember that protecting ourselves from the sun is just as important as capturing it, so all glazed surfaces must be able to be shaded in summer, especially those on the south and west orientations.

Ventilation

If in winter heat is achieved through solar radiation, how do we design a bioclimatic timber house to achieve a good temperature in summer?
Undoubtedly, through ventilation. An air current at 3 metres per second—10.8 km/h—reduces the perceived temperature by 1°C. Even if the air is warm, having a breeze will help dissipate heat and improve thermal comfort.

For it to work in a home, it is important to have cross ventilation. To achieve this, you must ensure there are windows or openings on opposite façades.

This can be difficult if the home is very large or if it has many internal partitions. In these cases, we can design internal courtyards to naturally ventilate all rooms.

This is what we did at Slow Studio, when designing the home in Colomers, to ensure that all rooms had cross ventilation.

Insulation

Capturing heat or cooling a home is of little use if we then allow it to dissipate. To prevent this, we must install an appropriate thickness of thermal insulation.

When designing a bioclimatic timber house, depending on the area, we typically use insulation with a minimum thickness of between 15 and 20 cm.

This is one of the main design differences between a bioclimatic timber house and a conventional home.

Traditionally, homes often had no insulation at all, or thicknesses on the order of 5 to 10 cm—something completely insufficient by today’s standards and technical knowledge.

Today, some homes still use thicknesses between 8 and 12 cm which, while improving on the insulation of decades ago, force reliance on heating or air conditioning consumption to achieve a good indoor temperature.

It is also necessary to check that there are no thermal bridges. A thermal bridge is a specific point in the building envelope where, due to design or construction shortcomings—i.e., omissions by the architect or builder, or lack of knowledge—the continuity of the insulation is broken. This is usually due to poor resolution of construction details and results in heat loss that must be taken into account.

Timber as a building material

As we have mentioned, a bioclimatic house can be designed with many materials; however, in today’s article we focus only on timber design, and this has prior and construction implications to consider.

First of all, timber, as a natural and living material, can move due to changes in temperature or humidity, be attacked by insects or fungi, or even rot under extreme conditions.

For this reason, timber is a material that requires prior treatment, from deciding the ideal time to fell the tree to the drying and pre-construction treatment process.

In general, at Slow Studio, we recommend avoiding chemical treatment in residential construction, so that we do not release toxic emissions into the indoor environment, which can be very harmful to health. We also recommend cutting timber in winter, when it is dormant.

Structure

Timber is a good structural material, as it performs well in both columns and beams, and it is a natural material because it is obtained directly from the forest without undergoing many transformation processes.

Different types of structures can be built thanks to timber’s versatility. Among them, there are three basic types that should be known.

Balloon frame structure

The balloon frame—or stud framing system—is the most widely used worldwide. It is a framework of modular timber studs in which the walls, both internal and external, become the structure that supports the house. The advantages of this type of structure for designing a bioclimatic timber house are its speed of construction, its affordability, and the fact that it does not require specialised labour.

CLT structure

CLT structures refer to cross-laminated timber panels that work together as a system. In other words, they are timber panels bonded with an adhesive that acts like a load-bearing wall. The layers are alternated so that the grain direction of one panel is horizontal and the next is vertical. This increases the structure’s capacity and reduces timber expansion and contraction to a negligible minimum.

Glued laminated timber beam structure

Finally, the glued laminated timber beam structure is the most similar to conventional systems with concrete and metal. These are longitudinal timber sections bonded together to create a beam, typically intended to span large distances—that is, large spans between timber columns. Compared to a standard structure, it offers higher strength and minimal deformation thanks to the different bonded sections.

Fire protection

Concern about fire is one of the reasons timber has not achieved wider use. Even so, timber has been one of the main and most versatile building materials for many years.

Fires usually start in kitchens or in an electrical appliance. A fire occurs when a spark reaches combustible material such as furniture, curtains, or carpets.

Timber is a material with very low thermal conductivity, and in the event of a fire, temperature will not easily be transmitted to other rooms. In addition, since its expansion due to heat is almost nil, it will not affect the rest of the house’s structures and materials.

In Spanish regulations it is classified as an unfavourable material in relation to fire. However, timber’s easy combustion occurs only on the surface. For this reason, the thicker the timber structure, the longer it will take to burn through.

Façades

At Slow Studio, we do not usually recommend timber façade finishes unless the client expressly requests it, or unless timbers such as larch are used, which darken over time and require no maintenance.

If you opt for a timber façade, the possibilities are very varied, but the different systems will always have in common an outer layer protected against water, a wind barrier membrane, and a protection system to prevent water and moisture from rising by capillary action where it meets the ground.

Interior walls

Timber partitions are built very similarly to plasterboard partitions. The idea is to build a structure of studs—timber in this case—and install plywood boards, which for timber partitions are preferably Fermacell, a cellulose-based board.

In this way, services can be run between the boards on both sides and/or acoustic or thermal insulation can be installed to separate rooms.

Interior walls can also be made of solid timber, although in these cases they may also be used as part of the house’s structure.

If you wish to have radiant walls, a very healthy heating system, we will need to combine these partitions with a panel with greater thermal mass to retain the heat generated by the heating pipes.