Overlapped watertightness strategies (142)

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Overlapped watertightness strategies (142)

A few days ago, one of my students asked, regarding water tightness, why do we place a membrane all around the perimeter of the openings in the rain screen façade solutions while in the panel façade a sealing seems to be enough. The question is really appropriate and is directly linked to the taxonomy.

When we build a façade we need to give continuity to the different functions the façade must fulfil even though the constructive solutions or the systems being used may be different along it. The thermal behaviour, water and air tightness, acoustic performance, mechanical continuity, among others, need to be achieved on the façade as a whole and so on each of its parts. Ensuring the continuity of these functions fulfilment among systems is a design challenge the architect must assume.

In the specific case the student was referring to we observe that, regarding water tightness, not only we need to give continuity to the function fulfilment but we need to solve the fact that the window and the rain screen façade solve this function resorting to different strategies.

While the window with the glass, the frames and the gaskets between them, entrusts water tightness to a material barrier, the rain screen façade entrusts it to drainage. In the façade panel that does not happen: all the systems, window and panel, entrust water tightness to a material barrier and so giving continuity to the function fulfilment only requires sealing the join between them. When the strategy between systems changes, this specific joint to be seal does not exist and so we need to seek for a mechanism that permits overlapping them. The membrane is this mechanism: a thin and flexile water tight material that enters into the drained cavity the necessary length to overlap the two different strategies.

From a conceptual point of view, we are trying to give continuity, regarding water tightness, between a system that in our taxonomy would be placed on the upper rows, with one placed in the bottom one.