Author: Janne Steinmetz
The article is original from bachelor thesis (German) : Anforderungen an Methoden zur Einbindung von Holzbalken in die luftdichte Ebene bei der Sanierung von Gebäudehüllen

The report presented in this article builds on the results of the research report Integration of wooden beams into the airtight layer (German) [Peper et al., 2014], in which the sealing of wedge-shaped cracks in historic wooden beams was tested using various methods.

In order to define specific requirements for sealing methods, the typology of cracks in old wooden beams found in building envelope refurbishment projects was analysed in more detail. On the basis of these investigations, four specific test specimens were developed which represent the relevant crack shapes and sizes. In order to represent the most realistic conditions possible, in addition to the expected small (2/50 mm) to large (10/120 mm) wedge-shaped cracks in the test specimens, as shown in Figure 1, a kinking crack was also realised in the average of the selected dimensions.

Four different solutions for sealing systems were applied to these test specimens, tested with regard to their airtightness and compared with each other by measuring the leakage volume flows. The most successful sealing method from previous investigations at the Passive House Institute [Peper et al., 2014], the injection channel, was selected as method 1 in order to test it on the selected crack types and compare it to other solutions. Crack sealing is the main focus of the investigations carried out. The sealing collar with adhesive tape to the airtight layer shown in Figure 2 was realised to carry out the airtightness measurements, but was not considered further. Due to the good accessibility and simple geometry, the adhesive collar does not generally pose a particular challenge.

All methods use non-trailing sealant, as this is suitable for use in all crack orientations. The four different approaches to sealing the crack cross-sections are presented below. With the exception of method 1 (injection channel), the methods were newly developed.

The measurements are based on DIN EN 12114, an established test method for determining the air permeability of building elements [DIN EN 12114, 2000]. Each sealing method was applied three times to all test specimen types and tested for airtightness in order to record the general functionality. Figure 7 on the right shows the four series of measurements for the different test specimen types. In addition to the total leakage volume flow, noticeable leaks were identified and documented during the measurements.

The measurement results of the airtightness measurements at a test pressure of 50 Pa were classified into the PHI airtightness classes A+, A, B and C for penetrations in order to be able to estimate the influence on the overall airtightness of the building. For this purpose, the mean values of the leakage volume flows from overpressure and underpressure measurements were related to the circumferences of the test specimens.

As the measurement results in Figure 4 show, Method 1 (injection channel) achieves the highest sealing effect with a length-related leakage volume flow of 0.12 m³ /(hm) and thus PHI airtightness class A. Method 4 (injection dowel) achieves a similarly good value of 0.24 m³ /(hm) and also achieves PHI class A. Method 2 (injection with limitation) and 3 (injection with blade), on the other hand, achieve comparatively higher leakage volume flows of 0.78 m³ /(hm) and 0.86 m³ /(hm). With these results, method 2 can still be assigned to PHI airtightness class C after taking the measurement error into account, while method 3 just fails to achieve any of the classes.

In order to obtain additional information on the effectiveness of the sealing methods in relation to the different types of cracks in the test specimens, identifiable leaks were recorded during the measurements and the individual test specimen types were measured separately.

In order to be able to assess the possibility of integrating the knowledge gained into construction practice, the handling of the methods in practice is evaluated. The criteria of difficulty of application, duration of application and material efficiency are taken into account.

In summary, it can be seen that none of the methods is rated as optimal in all criteria yet. Method 2 fails in the comparison due to the longer application time due to the duration of the curing of the foam (approx. 30 minutes) and the high degree of difficulty of application. Methods 3 and 4 both turned in variable results and differ in terms of material efficiency and application time. Method 1 only falls short of the optimum in terms of the difficulty of application and therefore proves to be the best for the time being.

The results provide important information for the optimization and further development of sealing systems for the construction industry. The analysis of the investigations reveals clear requirements for sealing systems for cracked wooden beams. A high-performance method should fulfill PHI airtightness class A, be flexible and quick to apply and efficient to use. The injection channel and injection dowel methods show convincing results and should be developed further. The injection dowel proves to be the most effective sealing method for kinked cracks, but offers potential for improvement in terms of handling and material. The other solutions, injection with blade and injection with limitation, do not yet achieve the desired performance in the form tested.

[Steinmetz 2024] Steinmetz, Janne : Anforderungen an Methoden zur Einbindung von Holzbalken in die luftdichte Ebene bei der Sanierung von Gebäudehüllen, Darmstadt 2024 (Bachelor Thesis(German))