Know Your Cavity Part 4: Benefits of Sealed Cavities

This is the final of four major articles in our ‘Know Your Cavity’ series which analyses key hygrothermal issues for the performance of wall construction systems. By Jesse Clarke

As discussed in the earlier articles, drained and ventilated cavities provide two functions: for reducing the wetting of a wall system and to increase the drying rate through airflow. Ventilation rates and drying potential were discussed in Part 3 of the Know Your Cavityarticles (Building Knowledge #7, Mar 2015) where it was shown that increased airflow increases moisture removal and drying potential.

Other functions of cavities however can be enhanced by restricting airflow and creating a static body of air. The thermal performance of wall systems is enhanced through the use of still air cavities in cooler climates.

While closing off the cavity may create a still air condition, the cavity provides a capillary break, as with drained and ventilated systems.

Porous cladding materials will allow rain to migrate through the cladding material at cracks and joints between cladding panels, window junctions or door junctions through capillary action – once the water reaches the air cavity it can no longer continue to travel via this mechanism. In this case the liquid water will appear on the back side of the cladding or drain down towards the bottom of the cavity if the volume is sufficient.

 

Figure 1 Capillary break prevents liquid water migrating between material layers

As discussed in the Part 1 of Know Your Cavity (Building Knowledge #5, Sept 2014), all construction systems should follow the principles of Deflection, Drainage, Drying and Durability (the 4 Ds).  While ventilated and drained cavities separate the deflection and drainage planes, non-ventilated cavities are not drained, therefore they are not designed for large volumes of water within the cavity.

Waterproofing and air sealing for these facades is managed at the external cladding surface, forming both the deflection and drainage plane. Coating systems and sealants generally provide the water proofing and air barrier on the external cladding surface and around window and door junctions for non-ventilated systems.

In hot humid climates as discussed in the Part 2 of Know Your Cavity (Building Knowledge#6, Dec 2014), heavy rainfall can be managed in two ways: the wall system can be drained to reduce wetting and ventilated to increase drying; or suitably sized eaves can reduce the amount of driving rain reaching the façade. In either case water vapour which may reside on the exterior cladding surface, in cavities or in the external air will generally migrate to the cooler drier internal plasterboard surface. In order to protect moisture sensitive timber structures and internal plasterboard from high humidity conditions and mould growth, the use of a waterair and vapour barrier sarking is essential on the outside of the insulation. The sarking must be tape sealed to work effectively as an air and vapour barrier.

In cold climates the general rule is to use only vapour permeable materials on the inside of a ventilated cavity. This includes the wall wrap, insulation and plasterboard. With a ventilated cavity the material properties of the cladding are irrelevant and vapour permeable or vapour barrier claddings such as metal sheet can be used safely.

Figure 2 Vapour permeability is important in ventilated cavities in both tropical and cool climates

In Europe wall constructions rarely contain ventilated air gaps as this is a construction practice which can introduce uncontrolled air flows and energy losses – reducing the performance of the thermal envelope. Ventilated air cavities can compromise the air tightness of constructions as well as the ‘real vs theoretical’ performance of construction systems (the importance of air barrier sarking was discussed in the Part 1 of Know Your Cavity).

When no cavities or unventilated cavities are present in a construction system, the thermal performance is improved and more stable. The primary mechanism for drying now becomes vapour permeance of building materials and the ability of moisture to migrate to either the external or internal surfaces without being trapped by vapour barrier materials (metals, glass, foils, foams or plastics). There is always the need to carefully consider the vapour permeance of all material layers in the construction.

Better Thermal Performance

In cold climates introducing cool air flow within a cavity will reduce the thermal benefits of the air layer. According to NZS 4214:2006 ventilated air gaps behind cladding materials reduce both the thermal performance of the air gap and any insulating materials located outside of the air gap. In 2004 the Building Research Association of New Zealand (BRANZ) published an article, R-Values and Cavities by Eddie Bruce which stated:

“If the insulating layer is on the outer side of the timber framing and a ventilated cavity is introduced, then air moving through this space will remove heat. The flow of air through a 20mm cavity is not great, but any airflow reduces the insulating effect of the outer-most layer. This reduction occurs regardless of the outer layer material. For most cladding types the cladding R-value itself is small, so the effect on the overall wall R-value will be minimal, but where the insulation relies on the external cladding, as in Externally Insulated Finish System (EIFS), the effect will be significant.”

Some systems such as Autoclaved Aerated Concrete (AAC) cladding have an improved R-Value of the external cladding. With insulation properties more than double common cladding and brick materials, AAC itself can give a boost to the thermal performance of walls when used in non-ventilated wall systems. In ventilated wall systems this benefit is mostly negated by airflow.

Figure 3 Ventilated and non-ventilated cavities in cold climates. Ventilated Wall Cavities allow for air flow to promote drying which reduce thermal efficiency. Non-ventilated cavities provide still air for enhanced thermal performance and reduced air leakage. Drying is achieved in cold climates by allowing water vapour to permeate through building products.

NZS 4214: 2006 states that the ventilated air gap shall be taken as 0.45 times that of a corresponding enclosed air gap and the thermal resistance (R value) of each layer between the ventilated air gap and outside air shall similarly be de-rated by a factor of 0.45.

The following figure outlines the thermal performance of a standard brick veneer wall system, an AAC system and a Polystyrene system and the thermal degradation effects of ventilating the wall systems. When non-ventilated cavities are used, the thermal performance of the walls increased by 7%, 11% and 32% respectively compared to de-rated ventilated cladding and air gap as per NZS 4214:2006.

Figure 4 Comparable R-Value for ventilated and non-ventilated Brick, AAC and Polystyrene systems

The most common sources of moisture in construction systems includes driving rain (commonly around windows), exposed structural timbers or cladding materials during construction, high moisture content of in-situ concrete slabs and humid air inside or outside the building. For these reasons it is essential that both ventilated and non-ventilated systems be able to dry sufficient amounts of moisture according to the local climate.

In non-ventilated systems water vapour must permeate to either the inside or outside of the construction. The amount of water vapour exiting the system must outweigh the amount of liquid and water vapour ingress into the system. The vapour permeance of the external cladding and coatings therefore becomes critical to drying in cold climates.

Figure 5 In systems with non-ventilated cavities, permeable cladding and waterproof coatings are required to allow vapour to escape through the cladding assembly.

In a non-ventilated system in cold climates we need to consider how the internal moisture (humidity) within the building will escape from the construction. To do this we consider a fully ‘vapour open’ construction in which water vapour is free to diffuse through the internal plasterboard lining to the external side of the cladding material with all vapour permeable layers being used.

Constructions seeking higher efficiency utilise still (non-ventilated) air cavities with cladding materials of lower thermal conductivity. However in the quest for energy efficiency, controlling heat and air flow is essential – but must be balanced with moisture flows. Permeability of materials is key to doing this: additionally, moisture storage capacity of materials allow buffering (storage and release) of moisture like a desiccant sachet added to your packet of noodles to keep them dry. Porous materials like Hebel act like a drying sachet to keep the timber frame (the noodles) dry.

To demonstrate the effect of permeability working together, CSR carried out computer simulations to compare a non-ventilated non-permeable cladding system with a non-ventilated permeable cladding system in a cold climate region of Hobart. The critical moisture sensitive material in typical walling systems is the timber frame, this can provide a substrate for mould growth (>18% moisture content) or begin to rot (>20% moisture content). AS 1720.1 ‘Timber structures – Design methods’ states the typical moisture content of seasoned timber to be 10-15% and is the ideal range for long term operating moisture content.

Figure 6 below shows results from a simulation using the Software package WUFI 2D 3.4, this indicates the moisture content of timber studwork under two different cladding assembly types. Under ideal operating conditions with no water leaks, the moisture content of the timber frame is expected to operate within the ideal 10-15% range in the Hobart climate region when a vapour permeable cladding assembly is used. However, with a non-vapour permeable cladding assembly (vapour barrier) the moisture content of the timber is likely to operate just outside of the safe zone – however still below the danger zone.

Figure 6 Stud moisture content for a perfectly sealed wall in Hobart with a vapour permeable cladding assembly and a wall with a non-vapour permeable cladding assembly.

The risk of damage increases when the system fails to perform from a weather tightness perspective. This is the fundamental reason why the NCC 2015 contains new verification methods for facades which tests the waterproofing quality of the cladding and assembly. From a durability perspective it is the ability of a construction system to cope with undesirable real world water leaks that has led CSR to use predictive software tools.

With minor water leaks modelled the moisture content of the studs increase – vapour permeance is now imperative to achieve suitable drying and reduce the risk of structural degradation. Figure 7 shows the benefit of vapour permeable cladding to non-ventilated cavity systems. The timber moisture content is roughly 2% lower with permeable cladding assemblies under water leak condition, albeit operating at elevated moisture content, but below the threshold for mould growth. With a non-permeable cladding the non-ventilated wall crosses the mould growth limit and the risk of mould and structural issue increases in the mid to long term.

Figure 7 Stud moisture content when a water leak occurs in Hobart for a wall with a vapour permeable cladding assembly and a wall with a non-vapour permeable cladding assembly.


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