U shaped membrane against moisture: problem or solution?

We often discuss with clients/colleagues the role of U shaped membranes, which is a system applied at the bottom of timber based walls to prevent from rising moisture. Those membranes most of the time are bituminous and provide water tightness via a butyl based adhesive and are presented as the “final solution to the durability issues of timber buildings”. Is this right?

The answer is absolutely no and we’re going to explain you why.

As written in the previous articles (Timber buildings: how long do they last? (Part 1) ; Wooden buildings: when and why do they rot? (Part 2 – Structural aspects) ; Wooden buildings: when and why do they rot? (Part 3)); the most critical detail in terms of durability is with no doubts the connections interfacing with the ground. Applying a U shaped membrane under timber walls essentially creates a sort of channel in which water is trapped, not being able to evaporate out. As a result, stagnating water will create the perfect environment for rotting.

How does water get in?

The potential sources are numerous, we’ll report some of them, keeping in mind that we could also have unexpected and unpredictable situations:

• Infiltration from the inside: domestic accidents, floor cleaning, installation losses etc.
• Infiltration from the outside: damages on the membrane during the assembly operations, atmospheric events, capillary ascent from the R.C. foundations, etc.
• Inner water on the wall due to the exposure of the panel during the transportation phases and on the building site
• Interstitial condensation phenomena due to not previewed weather condition, even if temporary.

The last scenario is particularly dangerous because if steam truly isn’t an issue; its condensation creates local accumulations which cannot evaporate, taken down to the bottom of the wall by gravity forces.

It must be said that those disastrous phenomena could happen in the short term but also a few years after completion of the building.

Le foto seguenti mostrano una situazione reale dove una parete a telaio si è abbondantemente bagnata durante il trasporto e durante la posa per mancanza di protezioni adeguate. La guaina ad U ha impedito all’acqua di uscire ed è stata quindi rimossa non senza difficoltà.

The following pictures show rotting platform frame walls provoked by an accumulation of moisture due to the exposition of the panel during the transportation and assembly phases. This situation could have been avoided by protecting the walls with waterproofing membranes during those phases.

The picture shows how the U shape membrane trapped the water in and for this reason, it has been removed.