Tall Timber Facades

Problem description

Wood as a natural material is – in opposite to the competing materials concrete and steel – much more moisture sensitive and based on detrimental moisture subject to decay by following fungi. As long as dry conditions can be secured, timber is one of the most durable materials known. Fire safety and structural design therefore will not be the main restrictions, but adequate moisture safety against water from outside and sometimes inside (sprinklers) has to be observed more consequently in future.  The better knowledge of moisture risks of tall building envelopes is required as well as optimal risk reducing measures. These measures need to be identified for cost-effective and resilient envelopes for durable wooden constructions.

Challenge

With an increasing height of timber buildings the challenge is increasing to provide dry conditions of the envelope for the expected lifetime of the building. Tall buildings are particularly exposed to high wind pressures combined with driving rain.  Additionally, large buildings require longer times of construction in which the structural elements are especially exposed to moisture. Furthermore urban buildings often show expressive facades with complex details like loggias, bay windows, recessed roofs and other, that are more at risk than simple exterior walls. Last but not least inspection, maintenance and repair possibilities are limited in high rise structures.

Today most of the ‘nearly high-rise’ and multi-storey timber buildings are built in ‘close to passive-house’ standard. The heat energy consumption of these buildings is on average 25 kWh/m²a. The very good floor area / building envelope ratio in combination with insulation layers of 250-350 mm thicknesses helps to meet high energy-efficiency requirements. Excellent airtightness values are self-evident, as well is the noise insulation on the highest level. Fortunately up to now only few problems occurred in the realised buildings. But thick insulation layers that meet high energy requirements results in very complex details (especially in the northern climate). In addition it’s in their nature to have a high moisture storage capacity and leakages are often recognized very late, with dramatic consequences. Therefore the aim is usually to decrease the thickness of walls using more efficient insulation, a subject that will be taken into account in the project.

Objectives

The main objective of the project is to facilitate the confident design of durable and therefore cost-effective design solutions for tall timber facades. Moisture-safe design will be enabled by a risk-based design tool taking into account exposure and vulnerability of façade components consistently. The risk based design concept for wooden facades is developed with relation to existing computational fluid dynamics (CFD) models for wind driven rain coupled with heat and moisture (HAM) transport models and mould- and fungi development models. Moreover, a simplified semi-probabilistic design framework (comparable with the Eurocode 1990 load-resistance design format) is developed that enabled easy utilisation of the results of this project by practitioners.

Additional results from earlier research projects e.g. TES EnergyFacade and smartTES (WW-Net 2008-2013), WoodExter (WW-Net 2008-2011) and WoodBuild (Vinnova 2008-2012) will be used for propagating innovative solutions for tall buildings adapted for industrial processes and stakeholder demands.

RiFa Tool

The variety of options and configurations of the building envelope and the diversity of the effects of the external and the internal climate need assistance in the selection of material and design options. For this, based on risk analysis methods, a risk facade tool (RiFa-Tool) will be developed that will enable planner and producer to make substantiated decisions for specific constructions. In addition, the tool is used to create a guideline for Tall Timber Facades – envelope constructions, which are developed together with industry partners for practical application.