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HAMM Principles in Commercial Buildings

Welcome to our Commercial Webinar Page. Here you will find video replay, downloadable video,and transcript of our Hamm Principles in Commercial Building Webinar, held at our head offices on 25th April 2017 , with (Managing Director) Keira Proctor , (Technical Director) Iain Fairnighton and Guest speaker Will Jones. The webinar presentation lasts for 22 minutes, followed by a 10 minute audience Q & A session.

The presentation covers:

  • An introduction to Heat Air and Moisture Management in the building
  • Basics of Heat Transfer
  • Material Properties
  • Basic factors affecting air movement
  • Air pressure phenomena

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Webinar Transcript

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.

Understanding these elements and the interactions between them 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.

There are three basic modes of heat transfer that are present in all buildings to some extent, conduction, convection and radiation. The simplest illustration of all three is heating a pan of water on a hob.

Conduction occurs where there is direct physical contact between two objects, in this case between the hotplate and the base of the pan. Heat then flows from the warmer hotplate to the colder pan, until either the temperatures equalise, or the pan is removed. The rate of heat flow is governed by the specific thermal properties of the materials.

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 or water, known as a convection current. As the temperature of the water becomes more uniform, the speed of this current will reduce.

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.

So those are the basics of heat transfer, but what specific properties do material have, and how do 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.

The thermal conductivity, also known a 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.

BIM is actually relatively well defined and it's about a more efficient way of collaborating on construction projects. Through that efficient collaboration we can improve our design which removes a lot of the surprises and can help get those difficult decisions around details resolved earlier in the process.

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 50W/mK. 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 1W/mK. Insulation materials are lower still, typically in the range of 0.015-0.044.

Thermal conductivity is useful for comparing material as it does not account for thickness, making a straight comparison easier, 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 it’s thermal conductivity.

For a given material, the thicker a block we use, the high the r-value will be and the bigger it’s insulating effect will be. 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.

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.

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 need to be adjusted to reflect things like air gaps and structural elements like studwork. Locations where structural elements intersect insulation are know as thermal bridges, and in these areas heat typically flows faster than though the insulation. U-values re therefore adjusted to reflect this, and the methods for doing this are given in the ISO6946 standard and the BRE guidance document BR443. Non-repeating thermal bridges, known as psi-values, also occur at junctions and corners.

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 product a temperature gradient graph, then a combination of the internal and external environmental conditions and the vapour and thermal resistances of the layers within the construction will produce a corresponding dew point graph.

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

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 me 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.

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 cause air leakage problems.

Stack pressure is caused by convection, 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.

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.

The final air pressure driver is mechanical pressure associated with ventilation and air conditioning systems. In 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 system can be 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 performance as designed both initially and throughout the buildings life.

Traditionally air barrier membranes has 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 subsequently (for example by following trades or later refurbishments) then both air leakage and moisture vapour ingress problems can arise.

If care is not take to seal 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 VCL/Air Barrier.

By moving the air barrier to the cold side of the insulation, external systems such as the Wraptite-SA from A. Proctor Group allows 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, 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 permeablility 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 it’s 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.

Taking a typical office block example, assuming an air leakage rate of 7 m3/m2/hr, the walls require a u-value of 0.125 to achieve the required TER in an SBEM calc, in a typical warm frame construction this will require 165mm of rigid insulation to achieve.

Using a system such as Wraptite-SA, it’s not difficult to reduce this air leakage to 1 with no other changes to the building specification.

If we run this through SBEM with the reduced air leakage value, meeting the TER can be achieved with a u-value of 0.22, a reduction in insulation thickness of 80mm.

To put this in context, while the external Wraptite air barrier membrane will cost a few pounds a square metre more, this is more than offset by the corresponding reduction in insulation cost, not to mention reduced time and complexity in the installation process.

We can further expand this to look at the cost associated with that extra space. For a 10000 sq.ft office, that 80mm translates to an extra £3100 of office rental income, assuming the UK average office rent, depending on location it could be significantly higher. So suddenly that expensive self adhered air barrier membrane looks far more cost competitive.

The final element affecting our building design is moisture management.

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

There area number of potential moisture affecting all building envelopes, the most obvious being form the rain.

Most rainscreen systems commonly used in 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 nothing is designed such that water may 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 or 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. This ventilation ensures moisture vapour form 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 it’s durability, and a carefully designed rainscreen that cannot resist damage will be limited in it’s 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 the building as a whole (and areas with 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 accommodated their design moisture loading as the vapour pressures within the envelope can get significantly higher than normal. Condensation arising form 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 comprises vapour molecules which pass through porous materials because of vapour pressure differences. These vapour pressure differences are created because of temperature and water vapor content differences in the air.

Most building materials are unable to stop vapour diffusion completely, and 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. The example of 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 excess 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.

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.

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 discussed already are properly accounted for to ensure that the building envelope performs as intended throughout it’s life.

Calculating the intricacies of heat flows and energy performance can be accounted for using a variety of modelling tools, from simple u-value and SBEM calculations through to complex BIM models and advanced simulations. The degree of modelling 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 modelling 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 by and equal and opposite drying.

As with energy modelling, 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. 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 modelling 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/WUFI 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 enable by the latest software and improved computing power is not only desirable, but more achievable than ever.

Between WUFI moisture assessment and the more complex energy modelling enable 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.

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