Wooden buildings: when and why do they rot? (Part 3)

In our previous article we approached the static aspects of the ground attack detail, and the strong implications of their design on the durability of the building itself.

In this article we are going to talk about another fundamental aspect of the building project: the building physics.

According to the answers to our survey (that keep coming even after 2 months – and for that we are profoundly thankful), we stated that, even though the majority of the attendees got the question n.7 right (which one of the ground connection details shown below is correct? – Solution: the one in which the lowest part of the Xlam panel was placed on a higher level than the external floor), many expressed doubts about the thermal bridge issue.

As stated in the survey-solutions-related article, in the C-detail, which was pointed as correct, there is a thermal bridge. In the drawing, however, the internal counter-wall was not shown, as a matter of simplicity.

Il dettaglio c) indicato come corretto era stato disegnato senza contro-parete per semplicità e conteneva quindi un ponte termico.

Detail of the analysis of the thermal flow c) posted in the survey (Boundary Conditions: internal = 20°; external = 1,2°; ground = 12,6°).

The diagram shows a thermal flow higher than 10 W/m² passing through the concrete curb and heading towards the foundation, dissipating thermal energy in the lower ground.

This phenomenon is due to the fact that concrete, being a material with a strong thermal conductivity, acts as a ‘bridge’ between the heated indoor environment and the colder ground.

Another perplexity concerned the interface layer between the wooden wall and the concrete curb, that many pointed as very likely to induce rot. Many of you suggested to insert an insulating layer (such as cellular glass) to avoid condensation and preserve the wood.

We performed a finite-elements thermal analysis of the discussed detail, with and without the cellular glass layer, using the DarTwin Mold Pro software .

In the following detail, the counter-wall is placed before the internal floor tiling layer. An insulating layer in XPS is eventually considered, to be attached to the curb before the casting of the screed, in order to reduce the thermal bridge.

*A higher value of conductibility was considered in parallel with the wooden fiber (vertical layers), taking into account the conductivity anisotropy of the material.

The boundary conditions imposed were: ambient temperature of 20°C indoors (as required by the Italian law) and 1,2°C outdoors (according to the coldest month in the town of Bolzano); at the interface with the ground a surface temperature of 12,62°C, corresponding to the annual average temperature in Bolzano, was imposed.

In the FEM analysis the 12,5°C and 16°C isotherms were highlighted with, respectively, a green and a blue dashed line. Temperatures were reported of the most relevant points, that are:

1. Internal corner between finished floor and counter-wall: that is the coldest point reached by internal surfaces.
2. Internal corner of the interface between Xlam panel and concrete curb: this point represents a criticality as it concerns the possibility of condensation, which could damage the wooden structure.
3. Internal corner of the interface between Xlam panel and cellular glass: just as explained in point 2, in case of condensation the wooden structure could be damaged.

Isotherms of the detail with cellular glass

Reporting the results in a table we can verify the actual temperature shifts occurred thanks to the presence of cellular glass at the base of the Xlam wall.

An additional study performed on the detail analyses the thermal flow at the boundary conditions stated above:

Flow/flow direction on the detail with cellular glass

Flow/flow direction on the detail without cellular glass

These details show that, thanks to the presence of the internal counter-wall, the concrete structure is correctly insulated and does not evidence high thermal flow.

According to the results it is clear that, from a thermal point of view, placing a cellular glass layer leads to negligible benefits. There is no visible increase in temperature on the internal corner, not considering the rise of static problems due to the brittle nature of the material and the non-linearity of the distribution of forces at the base of the wall, as already mentioned in the second part of this article. From a hygrometric point of view, we can observe that the 12,5 °C isotherm, indicating the minimum surface temperature before condensation, is in both cases more external than the Xlam/counter-wall interface, preventing interstitial condensation.

To sum up, we can maintain that, though it is not the only possible solution, the detail shown in these articles can satisfy all the requirements from the static and the (not less important) building-physics point of view, granting the expected durability of a 50-years-long nominal life (obviously, taking the correct design/execution of all other details for granted!)

If you missed the first two parts of the article about the rottenness of timber buildings click here::

• Wooden buildings: when and why do they rot? (part 1)

• Wooden buildings: when and why do they rot? (part 2)