Passive House with Timber Frame Structure

For many of our clients, building a passive house with timber frame structure sounds like construction systems from Nordic countries where low temperatures require an airtight and efficient construction system that reduces heat loss from the home.

However, the reality in our country—unless we live in the Canary Islands—is that winters are cold and summers warm. We have 4, or rather 2, very distinct seasons that require very different strategies when addressing energy efficiency. In this regard, today we analyze how a passive house with timber frame structure performs in our territory.

The construction of a passive house with timber frame structure is gaining more and more traction across all latitudes wherever raw materials are available, as it is a renewable, sustainable material that allows for highly efficient construction details. Contrary to what some believe, we still have timber on the planet with controlled logging certifications that guarantee constant resource renewal.

To begin, it would be helpful to define the concept of a passive house, as this will ultimately be the objective of any design. When we speak of a passive house, we refer to a house that achieves a comfortable indoor environment while minimizing active energy consumption—that is, through the use of natural resources such as sunlight or natural ventilation.

These are houses that employ bioclimatic architecture strategies (courtyards, porches, south-facing openings, walls that capture and store heat, cross ventilation, appropriate insulation, and much more) to regulate interior temperature and achieve maximum comfort.

By using bioclimatic architecture (or passive architecture) to our advantage, we seek construction systems and finishes that help us achieve the most comfortable interior temperature and humidity for the majority of the year.

Additionally, during extreme months we can supplement with renewable sources such as solar energy, wind power, biomass, or similar.

At Slow Studio, we are committed to bioclimatic and passive architecture and always work by putting these concepts and strategies into practice. Regarding passive houses, we recommend this article on how a passive house works, which explores the topic in depth and will help you better understand its operation.

Now we’re ready! With the concept of a passive house clear, let’s introduce another element: timber as a construction material.

We will begin by describing different types of structures that exist in the world of timber frame construction. We will continue by explaining where we can source the raw material, how a passive house with timber frame structure works, how much it costs, and whether it is safe in case of fire.

These systems can be combined to better adapt to the characteristics of our passive house with timber frame structure.
Let us first describe each system to better understand how they work:

Light-Frame System

This structural system comes to us from traditional popular culture.

Widely used around the world, it is built using light timber elements interlocked with one another. Since the structure is divided into a framework of small elements, it is not necessary to have large forests to complete a project using this system. In fact, in the United States, it is considered the most common construction system for timber houses.

For it to function correctly, the system establishes a structure in the form of “boxes” every 1–1.5 meters with a maximum height of 9 meters. These elements are enclosed with timber or fiber panels, and the cavity between them is used to incorporate insulation and installations. If we work with ecological awareness, it is advisable for the insulation to be natural, sustainable, and breathable—timber fiber is a good example.

We have the option to install insulation and a large part of the installations in the workshop. This allows us to save considerable time on-site, reducing the budget since on-site, one of the most expensive costs is always the time spent by construction personnel.

In summary: the walls are completed in the workshop while the foundations are prepared on-site, typically made of concrete. Once these are ready, the semi-prefabricated house is transported to the assembly location. The construction process for a passive house with timber frame structure using the light-frame system is reduced to an average of 6 months of work.

CLT Panels

In this case, the structural system operates using cross-laminated timber walls that, once assembled on-site, function as load-bearing walls that can be installed without the need for any additional structure. However, the insulation and surface finish of the façade will be applied afterward on the exterior side of the wall.

These cross-laminated timber walls are formed by a series of several timber boards glued and subjected to pressure, always placed in alternating directions consecutively—that is, horizontal-vertical-horizontal.

If necessary, the resistance of the panels can be improved by increasing to 5 or 7 layers that make up the panel.

The most commonly used timbers for this system are pine, fir, or larch, among others, and these can be combined with one another.

It is important to consider the design and organization of the installation pathways, as these can be included within the panels (if they have adequate thickness) or, alternatively, we can plan voids and grooves to facilitate their installation and access inside the dwelling.

Combining the two systems is also an option we can consider. In these cases, the walls are constructed using light timber framing, allowing us to prefabricate the relevant conduits and openings for installations from the workshop. On the other hand, the floors and ceilings are formed using cross-laminated timber structure, allowing for larger spaces and rapid assembly on-site.

Origin of Timber for a Passive House in Our Country

Currently, the timber industry in our country is still a minority sector. This does not mean it is deficient—quite the opposite.

The timber from our forests—mainly the Pyrenees—is of high quality, but since the trees are small and slender, companies limit themselves to the commercialization of small-format systems, such as light frames or small-format cross-laminated panels. Unfortunately, large-scale structural systems using local timber have not yet been developed.

In comparison, the timber industries of Northern Europe produce highly standardized products at prices that are difficult to compete with, using timber from forests managed by owner cooperatives that have organized to manage and exploit timber, such as the company Metsä (formerly Finnforest).

Thanks to the experience of these companies and the quality of the raw material—they have forests with large, slender trees—they offer a very diverse range in the market, from small-format products to large-section elements such as glulam beams that can span great distances.

It is understandable, given this situation, that the local timber industry sector is fragmented. The companies working in it are small and technologically less advanced. Only 2 or 3 large companies stand out working with timber for construction, although some of them do not work with local timber, which, in addition to being a disadvantage, does not contribute to activating the timber sector in our country.

Is a Passive House with Timber Frame Structure Safe in Case of Fire?

Having doubts about the behavior of a timber house in the event of fire is completely normal. In fact, this is often precisely the point that holds us back when choosing this material for construction.

It is true that timber, being a material composed of cellulose and lignin fibers—both carbon-, hydrogen-, and oxygen-based—is a combustible and flammable material.

Despite this, we must understand that its combustion occurs primarily on the surface, so structural elements—which are thicker—will take longer to be affected, should a fire occur.

A timber structure will take longer to burn the larger its cross-section. When timber burns, it chemically decomposes, forming a surface layer of char that acts as thermal insulation, thereby delaying its combustion time.

Another advantage of this material is its low thermal expansion. Therefore, in case of fire, timber structural elements will not be affected by thrust or deformation.

There is a specific regulation in Spain—CTE DB-SE-M: Timber—that determines the requirements for structural timber elements.

In it, timber is classified according to its reaction to fire since, despite being a material with a moderate reaction to fire, it still contributes to surface spread and the formation of harmful smoke.