Introduction To Building Physics: Heat, Air & Moisture Movement

In order to design structures that are energy efficient and healthy to live and work in, designers have a clear need to balance the heat, air and moisture movement throughout the building envelope. 

So, understanding these elements and how the interactions between them work, is an increasingly important factor in ensuring today's, highly optimised buildings, are fit for purpose; not just today, but for many years into the future.

Heat

So to begin, let’s review the basic factors relating to heat, air and moisture movement, starting with heat transfer.

In terms of energy performance, managing heat transfer is the most obvious consideration in our design process.

There are three basic modes of heat transfer that are present in all buildings to some extent; conduction, convection and radiation. 

The simplest way to illustrate all three, is heating a pan of water on a hob.

Conduction

Conduction happens where two objects are in contact with each other, like the base of the pan sitting on the hotplate. This direct contact lets the heat flow from the warmer hotplate to the colder pan, until either the temperatures equalise, or the pan is removed. 

Most building insulation boards such as mineral fibre, or polyurethane, work to resist heat transfer by conduction.

Convection

Convection occurs when heat is transferred by a fluid medium, which in buildings is usually air, but in our example here, is the water in the pan. 

As the water at the bottom of the pan heats up, its density reduces, and the warmer water will rise relative to the surrounding cooler water. The cooler water then sinks to take its place, setting up a circulation of water, known as a convection current. 

As the temperature of the water becomes more uniform, the speed of this current will reduce.

In buildings, convective heat transfer can be minimised, by ensuring good air tightness is achieved, restricting this circulation from happening.

Radiation

Radiation is the transfer of heat from one material to another, which occurs by means of electro-magnetic waves. 

Radiation does not require either physical contact of the materials, or a fluid medium. A warm surface such as our hotplate will emit infra-red radiation, which can travel across a space, even in a vacuum. When these Infra-red waves strike another object, they are converted back into heat.

The efficiency of this conversion process is governed by a property called surface emissivity. The lower this value, the less able a material is to absorb, or emit heat via radiation.

In construction, reflective foil membranes and foil faced insulation materials, can help limit heat transfer by radiation. 

Material Properties

So those are the basic mechanisms of heat transfer, but now let’s move on to the specific properties materials have, and how they influence the design choices we make.

The most relevant material properties to our discussion today are thermal conductivity, thermal resistance, thermal mass and thermal transmittance.

Thermal Conductivity

The thermal conductivity, also known as the lambda value, or k-factor, is measured in watts per metre kelvin, and tells us how well a material lets heat flow through it, at a given temperature. 

The lower the conductivity, the better the material will restrict heat losses.

In our example here, you can see that the high thermal conductivity steel beam on the left, quickly transfers heat from the burner; this is because steel has a relatively high conductivity at about

50 Watts per meter-Kelvin.

On the other hand in the concrete beam on the right, it takes a lot longer for the heat to reach the top side of the beam, the k-value of concrete is much lower at around 1 Watts per meter-Kelvin.

Materials commonly used as insulation are lower still, typically in the range of 0.015 - 0.044 Watts per meter-Kelvin.

Thermal Resistance

Thermal conductivity is useful for comparing materials, as it does not account for thickness, making a straight comparison of “how good is this insulation” easier. 

Its less useful however, when we need to consider a specific application, such as a roof or wall, as it doesn't account for how much material we are using.

If we want to include thickness as factor, we need to look the thermal resistance, or R-value. This is measured in metres squared kelvin per watt, and is calculated by dividing the thickness of a material, in metres by its thermal conductivity. 

For a given material, the thicker a block we use, the higher the R-value will be, and therefore the greater its insulating effect. 

Thermal resistance is directly proportional to thickness, so looking at the two blocks of concrete shown here, if we half the thickness of the block, we will double the rate of heat flow through it. 

Thermal Mass

As well as how materials respond to heat flow, their capacity to retain heat is also important. This is governed by their thermal mass, also referred to as fabric heat storage. 

We can see here how the low thermal mass steel beam, cools down quicker than the high thermal mass concrete. In buildings, this property can be used to even out temperature swings, and maintain a consistent temperature, with less heating or cooling energy input. 

Thermal Transmittance (U-Value)

The final property we need to consider here is the thermal transmittance or U-value. The U-value of an element is the inverse of the sum of all the R-values in a construction, and gives an indication of the heat flow through a given element, in this case a wall. 

In practice though, the R-values for each layer need to be adjusted to reflect things like air gaps and structural elements like studwork. 

Locations where structural elements intersect insulation, are known as thermal bridges, and in these areas, heat typically flows faster than though the insulation. 

U-values are therefore adjusted to reflect this, and the methods for doing this are given in the ISO 6946 standard, and the BRE guidance document BR443. 

Non-repeating thermal bridges, such as floor and roof junctions and corners, are also adjusted for. The heat losses associated with these features are called psi-values.

Where these bridges occur the temperature in the element will be lower, and if they are not properly designed, cold spots can occur internally, which can lead to condensation problems, both on the surface and internally. 

If and where condensation will occur in an element, can be predicted by looking at the dew point temperature. 

We can use the R-values for the layers in the element, to produce a temperature gradient graph. From this graph we can then use a combination of the internal and external environmental conditions, and the vapour and thermal resistances of the layers within the construction, to produce a corresponding dew point graph. 

Provided the actual temperature gradient through the structure remains above this dew point line, no condensation will form, however if the dew point and temperature gradient graph lines intersect at any point, then condensation will occur. 

We’ll come back to condensation and other moisture sources later on, but for now let’s consider air movement in structures.

Air

It’s important here to note that what we’re talking about here is unplanned air movement. Planned air movement, provided by ventilation systems, is an important part of maintaining good indoor air quality, so when we talk about air tightness, we really mean unintentional air openings, and the uncontrolled exchange of air from outside to inside, and vice versa. 

This can have a significant impact on the overall energy efficiency of a building, and as fabric insulation levels increase, this becomes an increasingly influential factor.

The two basic factors affecting air movement are infiltration and exfiltration, air moving in and air moving out of the envelope, and there are various mechanisms which drive this.

The three main air pressure phenomena affecting air movement are; stack pressure, wind pressure and mechanical pressure. 

Stack Pressure

The first, stack pressure, arises from the stack effect in the building. This is an important driving force behind passive ventilation strategies, however if not properly designed for, it can lead to air leakage problems. 

Stack pressure is caused by the convection current we discussed earlier, where warmer air rising through the building draws cold air in at the lower levels. 

This creates pressure gradients as shown, from outside to inside at the lower levels, and from inside to outside higher up - this effect becomes more pronounced the taller the building gets. 

Wind Pressure 

The second, wind pressure, results from a difference in pressure between the windward and leeward sides of the building. 

The positive pressure on the upwind side pushes from outside to in, and the negative pressure downwind draws internal air outwards. 

Being driven by weather, the directionality, and extent of this pressure can vary considerably. 

We also have to consider the effects of shelter provided by surrounding structures, and any variations that may occur between the upper and lower storeys of a building. 

Mechanical Pressure

The final air pressure driver is mechanical pressure, associated with ventilation and air conditioning systems. 

In high rise residential and commercial structures, large scale air handling equipment is capable of creating substantial pressure drops within the building. Natural sources produce pressures of up to 10 pascals, while mechanical systems can go as high as 60 pascals. 

If this is not managed correctly it can contribute by drawing air into the building envelope from unintended sources. 

Additionally, if the system is not installed well, leaking ductwork may draw air into the system that has not been accounted for, with a corresponding detrimental effect on the efficiency of both the system, and the building as a whole.

So the common factor across all these mechanisms is the presence of holes in the building envelope, through which uncontrolled air movement can occur. 

This air movement can not only significantly reduce the energy performance of the building, but introduces a discrepancy between the “as designed” and “as built” performance, that can lead to complex and expensive remedial action. 

It’s therefore important to ensure that the air barrier system used is robust and flexible enough to perform as designed, both initially and throughout the buildings life. 

Air Barriers

Traditionally air barrier membranes have been installed internally, on the warm side of insulation. Typically these also function as vapour control layers.

This positioning requires the air barrier to be sealed at all service, and structural penetrations, and remain sealed. If the membrane is damaged, either during installation, or by following trades or later refurbishments for example, then both air leakage and moisture vapour ingress problems can arise.

If care is not taken to seal all external elements effectively, this can also allow cold air movement around insulation, known as “thermal bypass” or “wind washing”, which can lead to reduced energy performance, cold spots and condensation issues, particularly if there are continuity issues with the airtight VCL line.

By moving the air barrier to the cold side of the insulation, external systems such as our Wraptite, allow for an almost penetration-free airtight layer, which can be installed faster and more robustly.

Far simpler than internal options, the Wraptite external air barrier system maintains the envelope’s integrity, with less building services and structural penetrations to be sealed, and less room for error. 

Being fully self adhered to a substrate, it’s also better able to resist external forces such as wind loads, ensuring good performance is maintained by minimising wrinkles and poor sealing. 

External systems require good vapour permeability to ensure moisture is not trapped within the construction, and this opens up a third possibility……..This is to position the air barrier layer within the construction, over the structural sheathing board, but behind the outer layers of thermal insulation. 

This increases the protection to the air barrier layers, and can help simplify detailing of the membrane. Care must be take with this method to ensure wind washing through the outer insulation does not occur, for example with the use of a vapour permeable jointing tape on the insulation boards.

An internal air barrier is only as good as its installation. If all the service penetrations are not adequately sealed, performance will be compromised. 

Switches and sockets represent the more obvious paths for air leakage, but there are many others, which may be unseen. A huge variety of ‘airtight’ accessories will be required when using an internal air barrier system. 

Examples of these will include the airtight VCLs, pipe and cable gaskets, junction boxes, extractor fans, switch boxes, light fittings and sealing tapes. These airtight accessories are generally more expensive compared to standard non-airtight versions, and take more time, care and attention to install correctly. 

There can also be supply chain issues acquiring these products at short notice.

In contrast, external systems generally require far fewer components and do not require as complex an installation process, leading to faster completion of the airtight envelope, with fewer potential delays and construction errors.  

The increased speed of achieving airtightness means pressure testing can be carried out earlier, potentially simplifying any remedial action that is required. Combining these benefits in turn, allows stricter design air leakage rates to be used, with greater confidence that the lower air leakage rates can be achieved.

Performance Benefits

So let’s now consider the benefits of this more reliable air leakage performance.

If we can be sure we’ll achieve a low rate of air leakage, then we can reduce the design air leakage rate used in our modeling, which means we can reduce our requirements in other areas. 

In a recent project we were involved in, this improvement in air leakage, was traded off against the fabric insulation.  in this case however, the goal was not to reduce the thickness, but to switch from a rigid foam insulation, to a less combustible mineral fibre product.

Ordinarily, this swap would increase the thickness of the wall assembly significantly, but here the reduction in emission rate delivered by Wraptite, allowed this switch to happen, with almost no additional thickness.

This delivers cost savings across the building envelope, as depths of flashings, fixings and various trims can be reduced across the entire facade. 

So while a high performance product like the Wraptite may appear more expensive at first glance, when we factor in the decisions and trade offs it enables, it’s not as straightforward as it might appear. 

Moisture

The final element affecting our building design is moisture management. 

Controlling and managing the moisture flow in a building is fundamental to the core principals of HAMM and also to maintaining the durability of the building envelope, maximising energy efficiency and protecting the health and safety of the occupants.

There are a number of potential moisture sources affecting all building envelopes, the most obvious being the external environment, weather and rain. 

Most rainscreen systems commonly used in high rise residential and commercial construction, are not designed to provide a complete barrier to water ingress, therefore the cavity behind should be provided with a provision for drainage to the outside, and care should be taken at design stage so that water may not track into the inner layers.

As well as allowing for the drainage of liquid water, rainscreens should be designed to ensure moisture from the interior of the building can escape, and that any wetting associated with water ingress through the outer rainscreen, can dry out.

This is typically achieved by fully ventilating the void between the outer cladding panels and the inner leaves of the wall. By ventilating, we ensure moisture vapour from all sources can escape freely to the outside, however it does also mean that achieving a good airtight seal of the inner leaf is critical to minimise air leakage.

A well designed weather tight layer is only as good as its durability, and a carefully designed rainscreen that cannot resist damage, will be limited in its usefulness. 

Internal moisture sources must also be considered. Achieving an effective and efficient moisture control strategy, depends on the design being carefully matched to the buildings function. 

The same envelope design that works for an office may not work well in a sports centre for example, at least not without significant over engineering and added costs. 

Developing a thorough understanding of how both the building as a whole, and specific areas within the building, will be utilised by its occupants, and the corresponding moisture production is the key to achieving a robust and cost effective design.

Wet trades such as plastering and screeding, and “wet” materials such as concrete, also contribute significant quantities of moisture to the building envelope, which will evaporate to the internal atmosphere in the initial period following the buildings completion. This is known as the drying out period.

During this period, problems can arise even in buildings that have been well designed to accommodate their design moisture loading, as the vapour pressures within the envelope can get significantly higher than normal. Condensation arising from this excess moisture, can still cause damage to the building fabric, even if it only occurs for a short period after completion.

As well as understanding the sources of moisture, it’s important to understand how moisture moves around the building. 

Vapour diffusion is the movement of vapour molecules through porous materials, because of vapour pressure differences. These vapour pressure differences are created because of temperature and humidity differences in the air on either r side of the material.

Most building materials are unable to stop vapour diffusion completely, and therefore, building science uses the term “vapour control layers” suggesting that they will control  (i.e. slow down) the process, but not completely prevent the movement of water vapour. 

Low permeability materials are those which can significantly slow down vapour diffusion.

Transfer by convection occurs when air flow acts as a transport mechanism carrying moisture in or out of buildings. Holes, cracks, penetrations and leaky ductwork may all provide potential pathways for moisture movement. 

As the moisture passes through the building it will condense on surfaces with temperatures are below dew point. Condensation levels will be affected by the difference of temperature between inside and outside, relative humidity, and the speed of air movement.

Moisture can also be transported by capillary action. This describes the ability of water to travel up through a building or material, against the flow of gravity, like for example how water will wick up through a paper towel following the direction of the fibres. 

Capillary action works best with smaller pores rather than larger holes, for example the fine pores found in brickwork or concrete provide an excellent mechanism to be used for this wicking action.

In order to avoid the occurrence of excessive condensation, which can result in mould growth and damage to building fabric and/or contents, designers, should assess the amount of water vapour likely to be generated within the building and determine the resultant increase in internal vapour pressure above that of external air. 

They should then consider the physical properties of the construction separating inside from outside and how this interacts with the various moisture sources present.

Lastly designers should consider the effects of the external climate, which, being site related, is beyond direct control but may be moderated by the building’s form and orientation in relation to topography, prevailing winds, sunlight and over-shadowing.

Moisture can have a significant impact not only on the fabric of the building but also on the health and safety of its occupants. Moisture and condensation can lead to mould and bacteria growth, which if left uncontrolled, can cause asthma and allergies.

Assessing Heat, Air and Moisture

Now we’ve looked at all the foundation elements of heat, air and moisture in building, we can begin to appreciate how to balance these effectively, and how we can ensure this balance is achieved at the design stage. 

The basic principle here is to ensure that all the factors we have already discussed are properly accounted for to ensure that the building envelope performs as intended throughout its life. 

Calculating the intricacies of heat flows and energy performance can be accounted for using a variety of modeling tools, from simple u-value and SBEM calculations through to complex BIM models and advanced simulations. The degree of modeling complexity required will vary from one project to the next.

Depending on the system used these models can account for everything from basic insulation levels to complex life cycle assessments and allow comprehensive optimisation of the building design by accounting for all the factors we’ve discussed. 

As well as our comprehensive technical support service, the A Proctor Group make BIM objects available for a range of our products, enabling easy integration into whatever design workflow you choose.

In addition to fully modeling the energy performance, we must also make sure we properly assess how the building design balances moisture. 

Balance in this context meaning we must ensure that all moisture associated with the building can dry out, and that any moisture accumulation, wetting, is balanced and equaled during the life of the building.

As with energy modeling, it’s important to use the right design tools. Traditional methods of assessment have in the past been based on the Glaser method – a standard static interstitial moisture calculation. 

We talked a bit about this in last weeks webinar on refurbishment, but its worth reiterating as it’s also relevant in a new build context. 

Developed back in 1958 for use in lightweight buildings, the simplified calculation used by the Glaser method is based on average monthly temperatures, vapour pressure and steady state conduction of heat to determine if critical condensation points are reached within one year.

The limitation of this approach is that Glaser assumes vapour moves only one way, from inside to outside. It also completely omits the key feature of driving rain from its calculations, does not measure absorption or porosity, and therefore fails to identify potential risk attributed to the aspect of moisture storage.

While this basic model is acceptable in some cases, and useful for providing a rough guide to avoiding the worst and most obvious problems, a proper assessment of the wetting and drying balance of structures requires a more substantial approach. 

Outlined in the EN15026 standard, this enhanced approach dynamically predicts moisture movement and storage as well as condensation for each location, taking into account climate effects such as driving rain and solar gains. 

The most common software used to assess structure via this method is known as WUFI. Using this modeling means the designer can achieve a minute-by-minute prediction over a given period of years, which proves invaluable when assessing the correct position for high performance vapour control and vapour permeable membranes to ensure a healthy building fabric.

We can see from the example here that while the Glaser method calculation simply shows that no condensation risks are predicted within the construction, a more comprehensive assessment with WUFI shows the rate at which construction moisture is drying out from the wall.

We can also examine the differences between the placement and specification of various components within the wall. Here we can see the difference in drying out periods between internal and external placements of insulation. 

Placing insulation externally leads to a much faster reduction in the moisture contents in the masonry compared to placing it internally.

So although there’s no problems predicted using the Glaser method, WUFI clearly shows a better hygrothermal balance is achieved with external insulation over a longer term.  

If we change the insulation vapour permeability, we can again see a substantial shift, particularly with internal insulation, which will cause the base wall to pick up moisture on a recurring basis. 

The latest code of practice for the control of condensation, BS5250:2011+A1:2016 details when it is more appropriate to use EN15026 or WUFI software and when the simpler Glaser method is acceptable for various types of building element. 

It is however clear that for more complex large scale structures, a more dynamic and extensive approach enabled by using the latest software, and improved computing power is not only desirable, but more widely achievable than ever.

Between WUFI moisture assessment and the more complex energy modeling enabled by modern BIM software, it’s now possible to model the balance of Heat Air and Moisture accurately and hence to design and specify with much finer tolerances. This helps ensure a cost effective and robust building that remains fit for purpose for many years.