Wednesday, 25 March 2020

An Introduction to Designing Facade Walls (or The Secret Life of Moisture)

Facade design can be a bit of a challenge.  Specific project requirements require different responses in design.  The basic parameters requiring consideration include:
  • Different building types,
  • Differences in functional and aesthetic requirements, 
  • Specifics of the local environmental climate,
  • How the building is to perform in relation to thermal mass, thermal resistance, air tightness and resistance to wind-chill etc.
One key performance factor is the behaviour of moisture within the building envelope, which if not designed out thoroughly could lead to deterioration of the building system and rot.  The secret life of moisture in external building envelopes is something I wanted to investigate further, so with the help of Will MacDonald, Head of Facades at AECOM, we have the following answers to unlock how how it all works.

Looking at moisture:
Water vapour in the air condenses on to the face of a cold glass of water, when the temperature on the glass reaches the ‘dew point temperature’; the temperature at which air can no longer hold all the water vapour which is mixed with it, and forces it to condense.  The dew point temperature is always lower or equal to the air temperature.  Water vapour particles hit the surface of the glass and stick instead of bouncing off.  

The amount of condensation created depends on the amount of water vapour in the air, temperature changes and the dew point.  Condensed water has to evaporate again which depends on the temperature of the water in relation to the temperature of the surface it is on.  Therefore, condensation depends on temperature and moisture content, while evaporation requires temperature, as long as the relative humidity of the surrounding air is less than 100%.  If cold enough, the condensation becomes frost.

There are two types of condensation in buildings: 
  • Surface condensation, found normally on an internal or external finished surface, and 
  • Interstitial condensation which occurs between the layers of the building envelope; inside the composition of roof, wall or floor build up.  
Adequate protection and ventilation os required to reduce the risk of moisture causing problems within the building fabric.

Surface condensation depends upon the surface energy balance of the building and the moisture content of the ambient air.  A higher level of insulation increases the risk of condensation on building facades in humid climates, especially in humid climates on clear nights where maximum heat loss occurs.  The risk of micro bacterial vegetation (algae) can accumulate on surfaces.  Variables affecting this include:
  • Internal vapour transmission,
  • Thermal gradient and level of insulation, 
  • Heat transmission and sky emissivity

So what happens to moisture when it migrates in to a building structure?  These are Will’s explanations of how moisture content can be controlled with facade design: 
There are two main sources of moisture that have to be considered when designing a façade. These are: 
  • Water from the external environment, usually in the form of rainwater, but may also include the diffusion of moisture from a wet surface, and,
  • Moisture vapour diffusion. 
In temperate climates such as the UK, the air inside the building usually contains more moisture than the air outside. Therefore, moisture vapour will tend to migrate outwards through the building envelope. 

Water penetration resistance: 
There are two main approaches to water penetration resistance:
  • Face sealing, otherwise knows as a curtain walling system 
  • Secondary defence construction, otherwise known as a rainscreen system 
Face sealed systems rely on the outer skin of the construction alone to prevent water penetration. If there is any moisture ingress past the external skin, there is usually no provision to allow the water to drain back to the outside. 

An alternative approach to water penetration resistance is to provide a secondary defence rainscreen. This usually takes the form of a cavity behind the external face of the wall. The cavity allows moisture that ingresses through the external face to drain back to the outside through openings in the outer surface. The cavity may also have sufficient capacity and openings to allow ventilation, for increased moisture removal. 

A drained and/or externally ventilated cavity may also be beneficial in the removal of any moisture that migrates through the wall from the warm moist internal environment.

Classification of air cavities:
Air cavities may be classified according to a number of different factors. These include the position of the cavity in relation to the main insulation layer, and the size of the openings in the outer surface. 

Cold cavities: 
Cold cavities are located to the cold side of the main insulation layer. Because they are cold the moisture content should be kept low to reduce the risk of condensation forming. A vapour control layer (VCL) on the warm side of the insulation will reduce the moisture ingress in to the cavity.  They should be ventilated to the outside to remove moisture and keep moisture level close to the outside levels. 

The following classifications are taken from BS 5250: 2002 - Code of practice for control of condensation in buildings: 
  • Vented air space cavity or void is that which has openings to the outside air, placed to allow some limited but not necessarily through movement of air. 
  • An air layer having no insulation layer between it and the external environment but with small openings to the external environment shall also be considered as an unventilated air layer, if these openings are not arranged to permit air flow through the layer.
  • Drain openings (weep holes) in the form of open vertical joints in the outer leaf of a masonry cavity wall are not regarded as ventilation openings. 
  • A slightly ventilated air layer is one in which there is provision for limited air flow through it from the external environment by openings meeting a specific area. 
  • A well ventilated air layer is one in which the openings between the air layer and the external environment allows air circulation.
Warm cavities: 
Warm cavities are located to the warm side of the major insulation layer. The cavity surfaces will be warmer than those of a cold cavity and the risk of condensation very much less. A vapour control layer may still be required to the internal face of the cavity in order to keep it dry. 

Thermal resistance of air cavities:
A still air layer in a cavity construction will add to the overall thermal resistance of the element and improve its U-value. The contribution that the cavity makes will depend on the level of ventilation between the cavity and the external environment and the direction of the heat flow. 

The U-value of a cavity construction will vary depending on the type of cavity present, and its location with respect to the main insulation layer.   The values given by manufactures are generally 1-dimensional U values and therefore do not include the additional heat loss due to brackets, fixings and cavity ties that may be present in the construction. The constructions are considered to be vertical with horizontal heat flow. 

Notes on cavity construction: 
A cavity that is neither vented or ventilated presents a serious risk of moisture collection. For example, an insulating glass unit requires a hermetic seal and the provision of desiccant in order to remain condensation free.  The higher the level of possible ventilation in a cavity, the more moisture may be removed from it. Liquid water will be removed by drainage and evaporation.  Moisture vapour will be removed by diffusion and mass transfer. 

A fully-ventilated cavity will provide the most reliable means of removing moisture in the cavity. Air movements will remove moisture vapour that has migrated from the internal environment, encourage evaporation of any liquid water and promote drying of the cavity.   

A vented cavity will have less capacity to remove excess moisture. It relies on the diffusion of water vapour in the cavity. Its performance will therefore be dependent on the vapour resistance of all materials to the cold side of the cavity and the size of the openings between the cavity and the external environment. The lower the vapour resistance and the larger the openings, the higher the rate of transportation will be. 

The removal of moisture in a vented cavity may be increased by using a wider cavity. This will improve the air circulation by convection as there will be relatively lower frictional forces to resist the air movements. 

Internally ventilated cavities:
Warm cavities are not externally ventilated and might have openings to the internal environment.  This will allow warm moist air to pass into cooler parts of the wall.  Although this is a warm cavity, the surfaces will be cooler than those of the room, and therefore there will be an increased risk of condensation. 

The risk of condensation will be minimised if there are enough openings so that the cavity is fully ventilated to the inside. This will ensure that the surfaces of the cavity are as close as possible to the internal surface temperatures. 

Building Envelope Energy Transfer:
Energy is gained or lost from a building by: 
  • Radiation or convection from the outer surface of the building, and
  • Air leakage (mass transfer) into or out of the building. 

Energy efficient facades have to be insulated to keep the external surface as close as possible to the external temperature, be sealed to prevent gross air leakage (reduce mass transfer losses/gains), and shield internal surfaces (reduces radiation losses & gains from or to internal surfaces).  They also should allow sufficient daylight in to the building to reduce energy required for artificial lighting. 

Energy transfer mechanisms:
Conduction is the mechanism by which heat energy travels through solids and stationary fluids and gases.  Materials such as metals are good conductors while materials such as mineral wool are poor conductors  but are good insulators. 

Radiation heat transfer occurs because all bodies at temperatures above 0ºK emit heat energy. Two surfaces at different temperatures will emit energy at different rates and energy transfer will occur. 

In a layered construction all layers resist heat transfer but a layer that has a significantly greater resistance to heat flow is usually be specified as the insulation layer, to reduce heat flow through the wall. 

Heat transfer through a sandwich panel:
Assuming that a sandwich panel comprises a 120mm thick core of mineral wool with thermal conductivity of 0.035 W/mK and is faced on both sides with a 1.5 mm aluminium sheet with thermal conductivity of 160 W/mK, the heat transfer can only occur if heat is gained at one surface and lost at the other by convection and / or radiation (otherwise known as surface resistance).

In practice when a building element is exposed to the environments, the surface temperature on the cool side will be warmer than the air on that side.  The surface temperature on the warm side will be cooler than the air on that side.  These temperature differences exist and cause convection and radiation heat transfer at the surfaces.  It is possible to calculate the surface temperatures but it is more convenient to calculate heat transfer knowing just the air temperature on each side of the construction.  The thermal resistance for convection and radiation at each surface can be combined into a surface resistance measuring the total resistance to heat flow.  Temperatures through out a layered construction can be calculated from the resistance of each layer. 

Thermal bridging:
A thermal bridge occurs where a material or component of high conductivity pierces an insulating layer of lower conductivity. This allows heat to bypass the insulation with two effects:

1. The rate of heat transfer through the combined materials is greater than it would have been through the insulation alone, and
2. The warm surface is cooler and the cool surface is warmer. 

The second effect gives rise to the term ‘cold bridging’.  In cool climates thermal bridges cause localised cold patches on the inner surface of the building envelope and are associated with problems of condensation, dampness and mould growth. 

Isotherms are lines of equal temperature and these may be plotted to show how temperature is distributed through a construction system. 


Design principles:

Moisture is introduced into buildings through life processes such as heating, cooking, bathing etc.  Water vapour may be removed by natural ventilation or air conditioning, however, there must be water vapour in the air to make a room comfortable for habitation. 

Water vapour disperses through the air and migrates through porous solids in an attempt to give a uniform vapour pressure.  In the UK external air moisture contents are lower than internal levels and water vapour, which then  migrates outward through the wall.  For air-conditioned buildings in some warm and humid climates water vapour moves inward through the wall. 

Water vapour should not migrate past the first vapour control layer only to be captured by a second vapour control layer or a breather membrane which incorrectly acts as a vapour control layer.  

Cavities formed on the warm and humid side of a vapour control barrier may be affected by severe condensation. Cavities may be formed as part of the internal building fit out when window boards and internal panels are added. These will often provide insulation so that the cavity is cooler than the room but provide very little resistance to vapour movement. Solutions include ventilating these cavities to the room so that they are warm, and incorporating a vapour control layer to the inner wall panel such as a foil-backed dry lining. 

Rainscreen cavities should be ventilated so that any water ingress stopped by a breather membrane will dry out after drainage has removed most of the water. 
Two layers having a similar vapour resistance should be avoided.  A metal sheet might be perforated to avoid the creation of an unwanted vapour barrier and it might be necessary to provide a breather membrane to prevent external water ingress by-passing the perforated sheet. 

Mould growth on the internal surface is prevented by limiting moisture levels at the internal surface.  This can be achieved by ventilating the room or improving the wall insulation to raise the 
temperature of the internal surface.  BS EN ISO 13788 recommends that the relative humidity at the internal surface should be less than 80% to avoid mould growth. 

Condensation assessment:
Condensation assessment requires knowledge of the moisture contents and temperatures within a wall.  Building envelopes containing a single well-defined layer offering high vapour resistivity at the warm side of the construction can often be acceptable. For walls with no well-defined vapour control layer, or one that is in a cooler part of the construction, an analysis will be required. 

The behaviour of moisture:
Condensation occurs in the ventilation cavity and not on the outside face because the heat flow direction is from inside to the outside. The warm air in the cavity has a higher lever of water vapour and as it cools it forms condensation. The temperature difference is very small between the cavity and the outer face, hence no condensation.  On some well-constructed unitised façade system (front sealed) you can often see condensation on the metal spandrel panel.  

The higher the value of insulation, the greater the risk of condensation.  The value refers to heat flow of the material. The higher the value the more heat will be lost. As more heat is lost you will get more vapour changing to liquid, promoting a risk of condensation.  The condensation isotherm lines change with the different value of insulation.  Good practice with rainscreen design recommends these lines are kept within the cavity.

Looking at different cladding systems and why those used in Antarctica are different to each other, and different to solutions of other climates:
Interstitial condensation a risk of material breakdown with the build-up of a wall in an environment which experiences freeze-thaw cycles is caused when warmer air is trapped in an unvented cavity and is cooled so that moisture is formed. Warmer air has higher relative humidity levels and more moisture than cooler air, and as it cools the problem occurs. This is often a problem on rainscreen systems when EPDM is used to seal up a façade before the cavity.  It was a principle carried out in Canada which has caused major problems.  The extent of material breakdown depends on the type of material used. Past projects suggest that precast concrete has no problems, however dry lining showed signs of mould after a relatively short time. Any seals and gaskets at the interfaces of the façade should be robust because they could suffer damage in freeze-thaw cycles and allow moisture to ingress the building fabric from the inside. 

The cladding system at the American South Pole base does not have a ventilated cavity or use membranes in the same way as cladding systems do in temperate climates.  Finished panels are applied directly to timber sips, but the outside temperature never rises above freezing.  The only natural heating affect might be from solar gain. The moisture risk here is mitigated because the South Pole cladding system  is a front sealed, air tight system which stops air-driven ice and snow at its outer surface.  The temperature on the inside of the building would be 20ºC and so the heat should flow would from the inside to outside. As the outside air temperature is always below 0ºC the relative humidity is either very low or zero, hence no condensation.  As the aluminium covering the SIP are impervious the only area moisture that can be a problem is the interfaces, or at cold bridging points, although these were carefully designed out. 

Similarly, Halley VI for the British Antarctic Survey was built with closed panels encapsulating their insulation core.  They intend to eliminate moisture ingress from both inside and out, that could degrade the insulation under freeze-thraw action. The solution resolves a lot of the problems of the natural environment which includes resisting the accumulation of wind driven particles of microscopic ice, known as spin-drift.  GRP was used as the outer skins to the panels.  It is a workable material to shape and make bespoke complex panels and eliminate cold-bridging.  Closed cell insulation was also used for the core to resist any accumulation of moisture.  The fixing details were carefully detailed to prevent cold bridging and create a robust structural skin. 

Could the cladding principles of the Halley system and its ability to lock out moisture be replicated in cladding systems in temperate environments, either with GRP panels or with a timber SIP system, and eliminate the ventilation cavity?  Yes, however you relying on workmanship and you do not have a second line of defence.  The application of the single ply membrane is depending on workmanship, weather conditions, application methods, location and material used.  The vented cavity system acts to equalise pressure.  When there is a difference in pressure on a façade, some areas will suck in air and other area allow air to be exhausted, which happens in the cavity.  This system reduces the pressure on the façade system.  Due to the temperature difference between the inside and outside of the building, there always will be a pressure difference. 

Different environmental conditions pose different design problems to be resolved with cladding systems.  Even across a single country (albeit a continent) varying and localised environmental conditions might require specific and bespoke design responses.  Importing a fabricated system used in a different country for a project in the UK might not necessarily be suitable if it performs differently because of differences in local climatic conditions.  The conclusion is that we need to thoroughly and carefully study how cladding systems work for each project.  It is not enough to work on a basic understanding of system types, and where relevant, involve a Cladding specialist to help ensure the designed proposals are robust.

Thanks to Will MacDonald, Head of Facades and AECOM for their input in to this post.

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