Nailing micro-environments

By - , Build 154

The structure of a building creates micro-environments that impact on building performance. Ongoing studies at BRANZ are finding out more.

Figure 1: Environmental monitoring instruments on BRANZ test house.
Figure 1: Environmental monitoring instruments on BRANZ test house.
Figure 2: Wall surface temperatures of BRANZ test house.
Figure 3: UVA irradiation on different walls of BRANZ test house.

BUILDING MATERIALS gradually deteriorate, losing qualities such as their aesthetic appearance and structural integrity as they interact with the immediate environment.

These environments differ according to the actual service conditions. The atmospheric environment, for example, is different for a material that is directly exposed to the atmosphere compared with a material that is less exposed.

Affected by building shape and structure

Typically, construction materials are made into components and assembled onto buildings.

A building’s geometry and structural or functional features can affect the exact load of climatic factors. A large wide overhanging roof or eave, for example, can change wind patterns, rain wash and pollutant accumulation on façades. This creates micro-environments close to the building envelope so materials no longer interact directly with the atmosphere.

Three micro-environments defined

Three types of micro-environments have been defined and used by the New Zealand Building Code and relevant building standards, such as NZS 3604:2011 Timber-framed buildings. These include open, closed (or hidden) and sheltered.

Corrosion rates not that simple

Most New Zealand land areas are marine influenced. Sheltered locations are generally believed to be more aggressive than open locations due to the lack of rain washing of accumulated salts.

Some studies show, however, that this might only be true in severe marine environments where large amounts of airborne sea salt particles can be produced.

In mild marine environments, zinc corrosion rates are very similar in open and sheltered locations. Some shelters can decrease salt accumulation, reducing material deterioration risk.

Research in industrial environments also indicates that materials in sheltered locations may deteriorate more slowly than those directly exposed to the atmosphere.

Location on building also important

The position on a building envelope clearly affects the degradation of materials, a fact supported by the BRANZ House Condition Survey.

However, the characteristics of micro-environments and their exact influences on material performance are complicated by the:

  • severity of the atmospheric environment
  • type and concentration of pollutants
  • geometry and orientation of the building
  • in-service configuration of the materials.

Materials specification approach

An atmospheric environment and building micro-environment approach is being used to specify materials that will meet or exceed the minimum durability expectations set by the New Zealand Building Code.

Earlier work characterised New Zealand’s atmospheric environment. From this, several exposure zones were defined by the atmospheric corrosivity map in NZS 3604:2011.

However, our understanding of building micro-environments is limited. This probably explains why some inconsistent or confusing specifications can be found within durability clauses and regulation documents.

Over or under-specification during design, construction or maintenance can lead to either higher material costs or higher premature failure risks.

Study using real buildings

Most studies use purpose-designed structures to investigate how building micro-environments are created and affect building material performance. While these are convenient for experiments, they could change or even exaggerate the influences of some environmental factors.

For example, some enclosures for rain-wash exclusion may reduce dew cycle, increase temperature and cut out ultraviolet (UV) radiation. These changes will affect material deterioration compared to the equivalent exposure on a real building envelope.

It is challenging to simulate micro-environments with simplified exposure racks or structures. Monitoring on real buildings gives more meaningful results.

Some studies monitoring real buildings have focused on wind-driven rain. Although moisture is the most important contributor to material deterioration, other factors also have a role. For example, UV can act with rainwater synergistically to produce erosion on polymers. Thus, it is as important to monitor other climatic factors in micro-environments on building envelopes.

Instruments and material on BRANZ building

Instruments have been installed on a test house at BRANZ’s Judgeford campus (see Figure 1) to monitor the climate close to the building envelope, including surface temperature, humidity, wind-driven rain, UVA irradiation and surface dirt deposition.

Steel samples, including steel plates and fasteners in treated timbers, have been installed at various positions to investigate their direct interactions with the local environments. Polymeric samples will be added.

Figure 1: Environmental monitoring instruments on BRANZ test house.

Already some solid results

BRANZ’s building micro-environment project is providing solid data to show how climatic factors can vary on a real building envelope. Monitoring from 16 December 2015 to 15 January 2016 showed that the maximum surface temperature on the east wall can be 2–3 times higher than that on the south wall (see Figure 2).

Figure 2: Wall surface temperatures of BRANZ test house.

Meanwhile, UVA irradiation on east and west walls can be 5 to 10 times stronger than that on north and south walls (see Figure 3). The wind-driven rain difference on walls is even larger (see Table 1).

Figure 3: UVA irradiation on different walls of BRANZ test house.

Table 1
Wind-driven rain on BRANZ test house

WALL WIND-DRIVEN RAIN ON: REFERENCE
ORIENTATIONBOUNDARY SHELTERED AND EXPOSED EXPOSED  
North 2.6 mm  3.8 mm  
South 0  0 51.4 mm
East 0.2 mm  0.2 mm  
West 0  0  
      Data recorded 16/12/15 to 15/1/16 

Filling in the gaps

Full-year data on the house will be available in due course. Monitoring will also be performed on buildings in other typical atmospheric environments.

The complete dataset will help us better understand micro-environments in terms of atmospheric environment, building orientation, construction features, position on envelope and seasonal variation.

More importantly, it will contribute to quantifying and correlating material performance with material position and building detailing.

Note

This research is supported by the Building Research Levy.

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Figure 1: Environmental monitoring instruments on BRANZ test house.
Figure 1: Environmental monitoring instruments on BRANZ test house.
Figure 2: Wall surface temperatures of BRANZ test house.
Figure 3: UVA irradiation on different walls of BRANZ test house.

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