Tests with hybrid sill are promising

The demand for sustainable and environmentally friendly construction methods is growing worldwide. High-rise timber-frame construction is a notable trend in this regard. This construction technique offers numerous advantages: it is lightweight, renewable and has a low ecological footprint. However, new technical challenges are also emerging in the process, particularly in the area of waterproofing facades. Low-relief window sills play an essential role here. The tests currently being conducted by Venster Techniek BV to increase the water tightness and drainage of their low-relief sill solutions are promising in this regard.

Low-relief lower sills are the first line of defense against water infiltration. Insufficient drainage through these components can lead to serious moisture damage and mold, among other things. In particular, drainage in wood frame construction - HSB for short - poses specific challenges due to the sensitivity of wood to moisture and the structural properties of this construction method. These difficulties arise primarily because wood, unlike more traditional building materials such as concrete or stone, is susceptible to damage from water infiltration and moisture build-up.

One of the critical points in achieving watertightness in wood-frame construction is the hollow low-relief lower sills. In fact, traditional (full) lower sills protrude through the watertightness surface of a wood frame element. Furthermore, with hollow lower sills, it is common practice to implement drainage through the sill. That water cannot escape at that location in the wood frame element. The design and installation of under sills must therefore be done with particular care, paying special attention to sealing and water-proofing, to efficiently drain and direct water away from the wood frame.

In the search for a sustainable and reliable application of timber frame construction in high-rise projects, a working group has been established in recent years that has taken an important step forward. This working group, in which several customers of Venster Techniek BV and the Dutch Trade Association for the Carpentry Industry (NBVT) are represented, aims to realize the certification of timber frame elements for use in high-rise buildings.

In a study - in collaboration with its customers - on improving drainage solutions for wood-frame elements, Window Technology found that a hybrid under-sill produces much better results than traditional full sills. The hybrid sill has the feature that it does not protrude through the water retaining surface, allowing the water retaining surface to continue behind the sill. This innovation offers a promising solution to the problems often encountered with waterproofing in wood structures, especially in high-rise buildings.

Further improvement is achieved by sealing openings (created by operations, such as the ends of lower sills) and better directing water to where it can leave. To effectively seal these openings and drain water properly, Venster Techniek has explored new options. 

Recently, the product developers at Window Technology, in collaboration with established sealant suppliers, tested a new material that is proving promising for achieving watertightness, particularly with wood-frame elements. This material has proven to be highly effective in all tests, with excellent performance in terms of moisture resistance, temperature variation and strong adhesion. However, this does mean that traditional header treatments can no longer be used and that cavity battens must be caulked.

All in all, the introduction of the hybrid sill and the new sealing material promise significant improvements in the drainage and water tightness of (low-relief) sills. With the progress of testing and implementation of these innovations, Window Technology is contributing to the achievement of certification of wood frame elements for high-rise buildings.