Recovering knowledge of local, traditional building materials

Why is an old country farmhouse more pleasant in summer than a modern city building? Traditional building materials and natural stone provide old buildings with better thermal and hygrometric properties. Unfortunately, they often lack the technical characterizations they need to find their place in the construction industry. The European regional development project OEHM has set out to resolve this problem. It brings together IMT Mines Alès, the University of Montpellier and the National School of Architecture of Montpellier. Aymeric Girard, a materials researcher at IMT Mines Alès, gives us an overview of the project and the challenges involved.

You’re studying natural building materials through the OEHM project. Why is this?

Aymeric Girard: All building materials require technical characterization. It’s important, since proposals for buildings are always simulated by computer nowadays as a first step. But traditional building materials, which are not produced by industry, lack technical characteristics. By studying local, traditional materials through the project, we are striving to fill this gap.

If the construction industry doesn’t use these materials, is it interested in this knowledge?

AG: Yes, since one of the major observations about current buildings is that they rely too heavily on internal insulation. The main reason for this is a lack of thermal mass in modern buildings, meaning a mass of materials that serves as a heat regulator. In a new building made with conventional building materials, you’re hot in the summer and cold in the winter. So you need heat and air conditioning. But this is far less of a problem in old buildings built with traditional building materials. In Seville, which is one of the hottest cities in Europe, old churches and cathedrals remain cool in the summer.   The construction industry is now seeking to model new buildings after these traditional structures.

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There’s also a second benefit. The construction industry is a sector that contributes heavily to greenhouse emissions. This is partially due to the environmental footprint of transporting materials. Using local stones encourages short supply chains, thereby reducing the environmental impact.

What materials are we talking about?

AG: For the OEHM project, we’re working with a clay brick factory and four natural stone quarries: one for granite and three for limestone. Some of these stones are truly local, since they come from the Occitanie region where IMT Mines Alès is located. Others are local in the sense that they come from France at least.

What aspects of these stones and bricks do you study?

AG : We conduct two main analyses of these stones: a thermal analysis and a hygrometric analysis. Hygrometry allows us to study a material’s ability to absorb humidity. That’s important because in winter, for example, the windows in a house are usually closed and you cook, take showers, sweat etc. All of these things increase the humidity level in rooms, which affects quality of life. Certain stones with very low porosity will not absorb this humidity at all, while others with high porosity will have a buffering effect and provide greater comfort.

How do you obtain the technical characteristics you’re seeking?

AG: The quarries send us small five-centimeter cubes to be analyzed. We use the hot-wire method to study heat transfer. This involves taking two cubes of the same stone, and putting a sensor the size of a post-it note between them. We heat one side and observe the speed at which the stone on the other side heats up. We also study the stones’ heat capacity, by putting even smaller samples measuring 5 mm per side in a mini-oven. This provides us with information about how long it takes to raise the stone’s temperature and about how it behaves.

In terms of humidity, we have a sort of refrigerator where we apply a constant amount of moisture, then we compare the weight of the dry stone with the saturated stone above, and deduce its capacity to absorb moisture. It’s a very long process that can take up to four months.

With whom are you working on this project?

AG: On the industrial side, we’re only working with the quarries for now. They’re interested in the technical characteristics we’re producing in order to provide their partners and customers with data about the materials. It’s important knowledge, just as when you buy glass wool to renovate your home, or when you compare offers to decide what to buy. On the research side, the project is part of a long collaboration between IMT Mines Alès, the University of Montpellier, and the National School of Architecture of Montpellier.

What will the project produce besides these technical characteristics?

AG: We plan to use the data we recover to develop our own material simulation software. And we’re also going to carry out real-site testing in collaboration with the National School of Architecture of Montpellier. They have a replica of a house that can be adapted to test materials. This will give us the opportunity to test our results and share insights with architects about the opportunities offered by natural materials suited to the Mediterranean climate.