Hygrothermal performance of ventilated wooden cladding

Abstract

This PhD project contributes to more accurate design guidelines for high-performance building envelopes by analysis of hygrothermal performance of ventilated wooden cladding. Hygrothermal performance is defined by cladding temperature and moisture conditions, and subsequently by risk of degradation. Wood cladding is the most common façade material used in rural and residential areas in Norway. Historically, wooden cladding design varied in different regions in Norway. This was due to both climatic variations and the logistical distance to materials and craftspeople. The rebuilding of Norwegian houses in the 1950s followed central guidelines where local climate adaptation was often not evaluated. Nowadays we find some technical solutions that do not withstand all climate exposures. The demand for thermal comfort and also energy savings has changed hygrothermal condition of the building envelopes. In well-insulated wall assemblies, the cladding temperature is lower compared to traditional walls. Thus the drying out potential is smaller, and the risk of decay may be higher. The climate change scenario indicates a warmer and wetter future in Norway. Future buildings should be designed to endure harsher climate exposure than at present. Is there a need for refined climate differentiated design guidelines for building enclosures?

As part of the Norwegian research programme “Climate 2000”, varieties of wooden claddings have been investigated on a test house in Trondheim. The aim of this investigation was to increase our understanding of the relation between microclimatic conditions and the responding hygrothermal performance of wooden cladding, according to orientation, design of ventilation gap, wood material quality and surface treatment. The two test façades, facing east and west have different climate exposure. Hourly measurements of in total 250 sensors provide meteorological data; temperature, radiation, wind properties, relative humidity, and test house data; temperature, wooden moisture content, time of surface wetness, relative humidity in cavities and wind-driven rain (WDR). Four years have been analysed and recalculated by numerical simulation. The moderate climate of Trondheim provides thorough boundary conditions for hygrothermal analyses of building envelopes.

The WDR was measured in the cardinal directions in a free field and on each façade of the test house. Eight WDR gauges were mounted on the west facing wall with the highest amount of WDR. The WDR measurements are provided in a database on the web that is available for the validation of WDR simulations.

A statistical analysis investigated which climate parameters contributed most to the fluctuations of the moisture content in the wood. It was found that air temperature, global radiation and wind velocity were the three main parameters. WDR was the fourth most important parameter. WDR only defines moistening and not drying, which might be the reason for not being a determinate parameter for the fluctuations in the moisture content in the wood. The time of wetness was further investigated and compared to WDR. The surface wetness sensor measures describes periods with liquid water moistening more accurately and includes the period with free water on the surface after rain and by condensation.

The importance of the wind velocity led to a separate CFD study of the air flow in the cavities when including the bulk wind flow around the test house. The cavity flow is not measured at the test house. The CFD study resulted in a function describing the air change rate of the ventilated cavities dependent on wind velocity, wind direction and cavity opening. The function was tested in WUFI 1D calculations. The calculations showed good correlation with measured data when including air change rate in calculations of cavity temperature and RH.

It was intended to measure the moisture profile in the wood cladding with moisture pins, by measuring the electrical resistance in different depths, but these measurement results were not possible to interpret. However, the methodology might work, even on thin wood boards, if the set up is thoroughly calibrated prior to mounting

The test house study shows the advantage of two-stage tightening, with a ventilated cavity behind the cladding. The cavity reduces the risk of moisture problems in wall assemblies; it serves as a safety valve, discharging excess moisture by drainage and ventilation. At the test house, with open fields around, it is shown that by having only a few millimetre cavity opening the cavity operates sufficiently. In a dry climate, where the wall will be mostly dry, the results indicate that a design with the cavity openings closed will give the driest wood cladding. No significant conclusion can be drawn regarding the surface treatment and material quality. Although the four year study shows some results, the service life of a wooden cladding might exceed a hundred years with correct design and maintenance.

The Voll measurements were used for validation in HAM simulations of wooden claddings. Models were defined for the separate wooden cladding with controlled outdoor and cavity climates, and with the complete wall assembly including the cavity ventilation. The model had good correlation to measurements and is a useful step towards calculating climate adaptation of wooden cladding. The climate influence and wall design can easily be altered and the hygrothermal performance defined. This enables simulations to find the best suited cladding to different climates. However, there is still a need to define the validity and range of action of the simulations. In any case HAM simulation tools provide a breakthrough in terms of geographically dependent façade designs.

The findings within this PhD project reveal that the current design recommendations on wood cladding provide sufficient performance. Wood cladding can still be promoted as a high-performance rain screen, also in harsh climates.