The messy realities of mass
Mass in buildings can help moderate internal temperatures, but it can also be tricky to control its effects. Alan Pears examines when and where thermal mass works well – and when it doesn’t.
This article was first published in Renew 132 (July-September 2015).
The way buildings work is very complicated. That’s why designers increasingly use computer models that simulate hourly performance over a year to try to deliver good performance. Even that has its challenges! Adding mass to a building is no exception; it can bring significant benefits – and some problems.
This article is an attempt to explore the role of mass in buildings and suggest some paths forward for building owners and designers.
First, mass is not actually what we want. The beneficial feature of mass is that it increases the heat storage capacity of a building so that, for a given amount of heat input or loss, the change in temperature inside the building is reduced. This outcome can be achieved by using a lot of material (mass), materials with a high heat capacity per unit of mass (e.g. water can store about twice as much heat per cubic metre as concrete for the same temperature rise in the material), or by storing energy as latent heat in what are known as phase change materials (PCMs, see more on these later).
High mass buildings tend to sit close to the 24-hour average temperature for the time of year, because it takes a lot of energy to shift the temperature of a heavy building. In much of Australia, especially when 24-hour average temperatures are 18 to 24C, this means the building tends to be closer to a comfortable temperature more of the time.
Thick, heavy walls slow down the rate of heat transfer into or out of a building, as the wave of heat has to work its way through the thick material. This can delay the heat flow until it cools down (or heats up) outside, reducing heating or cooling energy.
But it can have a downside. I once lived in a house with a west-facing uninsulated cavity brick bedroom wall. It would delay the heat flow from the afternoon sun until after bedtime, so I would cook at night unless the outdoor temperature had cooled enough for me to flush out the heat.
Note that mass does not provide better insulation – but under varying temperature conditions it can have a similar impact on energy use to a small amount of insulation. Confused? Let’s look at what this all means in practice.
High mass vs lightweight in operation
Consider two houses of identical design and insulation except that one has high mass (actually, high heat capacity), while the other is lightweight. Let’s also assume no heating or cooling equipment is operating.
During daytime, the heat from the sun (and indoor activities) will tend to warm up a house. For the same amount of heat input, the lightweight house will warm up much more. At night when it is cooler, heat leaks from both houses, but the temperature of the lightweight house will drop much faster than that of the high mass house.
The impact of the mass on heating and cooling energy consumption can be beneficial or problematic. For example, on winter days, a heavy building with limited solar input may not warm up much above the 24-hour average temperature without additional heating, while the lightweight building may warm up to a pleasant temperature with a small amount of solar input. But if a high mass building warms up using solar gain, it will tend to maintain a higher temperature into the evening and overnight, while the lightweight building will still cool down, possibly needing more heating.
In summer, the high mass building will tend to stay cooler over the day. But once the mass warms up, it will tend to remain hotter overnight unless it is cooled by ventilation (if the outdoor temperature is cool enough) or by air conditioning. In contrast, the lightweight building may get quite hot during the day, but is more likely to cool down overnight as the outdoor temperature drops. Over a series of hot days without cool nights, the temperature of a high mass building will keep increasing, and will tend to remain hot without active cooling.
Insulation and mass
The better insulated a building is, the slower its temperature will change, regardless of its mass, as less heat flows through the building fabric for a given temperature difference. So better insulation enhances the effect of a given level of mass in stabilising temperature by reducing the amount of heat that has to be stored or released to maintain comfort when outdoor temperatures vary.
It also makes sense to place mass on the inside of the layer of insulation, where its impact on indoor comfort is maximised. Mass outside the insulation (e.g. in a brick veneer wall) does still offer beneficial effects by moderating and delaying the temperature difference across the wall, but internal mass generally works better.
Hot and changing climate considerations
Shading windows from sun (or reducing glazed area) and advanced glazing are critically important in hot climates. Enormous amounts of energy can enter a house through a window (almost equivalent to a single bar radiator per square metre exposed to sun). Certainly, a high mass building is less sensitive to this heat gain but, when the outdoor temperature is high, this heat must be removed from the building, regardless of its mass.
With climate change, we are seeing more hot nights and longer hot spells. So our ability to rely on natural ventilation and fans overnight, or for a high mass building to outlast the hot spell is being undermined. This should be driving a review of design practices, because a need for active cooling on at least some nights is becoming more likely. Houses that cannot be sealed up tightly and are not well insulated will incur high cooling energy costs under these conditions. This is not to suggest that design for effective natural ventilation should be ignored, but it does mean designers need to make provision both for good natural ventilation and, when conditions are very extreme, efficient active cooling.
Cool climate considerations
In a climate where the outdoor temperature is always colder than the desired indoor temperature, more mass does not replace the need for insulation and other energy efficient fabric features. That’s one reason why homes in many very cold climates are often lightweight: the large, consistent temperature difference means insulation is the big factor in reducing heating energy requirements.
How to design and manage high mass (really high heat capacity) and lightweight (low heat capacity) buildings
It is possible to make both high mass and lightweight houses work well in most climates as long as they are well designed and well managed, and user expectations are realistic. But different design approaches and management techniques are needed.
Lightweight house design
A lightweight house, all other things being equal, will need to be designed with less glazing to achieve the same energy efficiency as a high mass house. And, to achieve very high Star ratings e.g. 9 or 10 Stars, some mass (or PCMs) will almost always be needed to store heat overnight to eliminate start-up loads on winter mornings. This is where interesting discussions can occur about the relative costs of using alternatives such as stored hot water or small amounts of stored renewable electricity to provide morning heating in comparison to the extra cost of upgrading the building from 8 to 9 or 10 Stars!
Fundamentally, the temperature in a lightweight building is more sensitive to the level of insulation, draught proofing, solar gains (mainly through windows) and internal heat generated by lights, appliances and people. This is simply because a given amount of heat energy entering (or leaving) the house has a much bigger impact on the indoor temperature than for a house with high internal mass.
So trapping heat in a lightweight house in winter and blocking heat out in summer (while minimising internal heat generation) are very important. At the same time, large north windows can easily overheat a lightweight house on sunny days in winter – a problem many people would like to have! But without really effective shading, those same windows can cook you in summer.
Lightweight houses are more likely to need some active heating and cooling, although the actual amounts of energy needed to provide comfort can be very small if they are well insulated and shaded. A lightweight beach house I helped to design worked very well. It was very well insulated and sealed, with fairly large areas of low-e double glazing to the north and east, was well protected from west sun. It had a (very small capacity) hydronic heating system. In winter, it was wonderfully warm during daytime, but did cool down overnight, so the hydronic central heating ran for about half an hour on cold mornings.
If the extensive shading was not in place in summer, it quickly became a solar oven: on one hot morning, the living area hit 35C by mid morning! But with effective shading and the occasional benefit of a sea breeze, it was very comfortable, and very rarely exceeded 30C: ceiling fans provided adequate cooling.
Even if it had used active cooling, it would not have used much cooling energy – as long as the shading was in place! Of course, access to sea breezes helped moderate this.
High mass house design
A well-designed high mass house will tend to sit within a few degrees of the 24-hour average temperature for the time of year. In many parts of Australia, this is 18 to 25C, so the house can be quite comfortable with little or no intervention. But in cooler locations (and in south-facing rooms), the 24-hour winter temperature can be 12C or lower, while in the tropics summer daily average temperature can be over 30C. Under these conditions, large amounts of heating or cooling energy may be needed unless the building is well insulated and sealed, and has winter solar gain or summer ventilation (and cool enough overnight breezes).
As mentioned earlier, an uninsulated high mass wall exposed to sun can create an oven, as can high summer heat input from the sun, hot air leakage or unshaded windows. Long periods of hot or cold weather will slowly shift its temperature until it is uncomfortably hot or cold – unless solar gain or active heating is used in winter, and overnight ventilation (if the outdoor temperature is cool enough) or active cooling is used in summer. Then it may need a lot of heating or cooling energy (or ventilation), because a lot of energy must be added or removed to make it comfortable.
Reducing the heat input in summer (e.g. by shading, insulating and reducing air leakage) will allow the high mass house to remain comfortable through a longer heat wave. But if it doesn’t get much solar gain in winter, it may be unpleasantly cool in colder climates without active heating. This is where effective insulation, ventilation systems and adjustable shading can optimise comfort while minimising energy use.
Mass and keeping cool
Weather records and climate science tell us that overnight temperatures are increasing and hot spells are becoming longer and more extreme. These trends mean we are more likely to need some active cooling at times in most climates in the future, regardless of the amount of mass in our buildings.
Further, many people do not ventilate their homes overnight: this may be because of concerns about security, noise or simply because they don’t understand what a difference it can make to their comfort.
Regardless of the reason, it means they are not fully utilising natural cooling to improve comfort and enhance the effectiveness of the mass in their homes. Home ventilation systems that can extract heat overnight are becoming more widely available. And even an exhaust fan can help. Small windows that are high off the ground can also provide secure and effective overnight ventilation.
Mass in slabs
Mass in a slab-on-ground floor is different to mass in a wall. An uninsulated slab is linked to the stable ground temperature (at 3 m depth), so it helps to keep a building closer to that temperature, regardless of the amount of mass in walls. Depending upon your location, this may or may not be desirable.
Your Home provides guidance on this on p. 255:
Ground coupling in mild climate zones such as Perth, Brisbane or coastal NSW allows the floor slab of a well-insulated house to achieve the stable temperature of the earth: cooler in summer, warmer in winter. In winter, added solar gain boosts the surface temperature of the slab to a very comfortable level.
In climates with colder winters, such as Melbourne or the southern highlands of NSW, the deep ground temperature is too low to allow passive solar heating to be effective enough. In these locations, slabs should be insulated underneath, which reduces the amount of heat required to achieve comfortable temperatures. In northern Australia, ground coupling still works well, unless the building is to be air-conditioned, in which case insulating the slab – especially the edges – is essential.
Insulating the edges of floor slabs is beneficial in all but the mildest climates, but protection against termites needs careful attention.
Other design considerations
When considering high mass design, it is also important to look at overall building cost. Some high mass solutions can be expensive relative to other options.
It is possible to reduce dependence on mass by using effective shading, advanced glazing, and other energy efficiency measures including insulation and sealing that reduce the amounts of heat flowing into and out of a building. And high efficiency reverse-cycle air conditioners with rooftop solar can help to maintain comfort in an energy-efficient building at relatively low capital and operating cost – with beyond zero emissions averaged over the year.
But a well-designed high mass building provides a very stable and comfortable environment with minimal need for active management – which can be very nice.
Where should the mass be?
If mass is hidden by a layer of other material, such as a carpet or plasterboard, the rate of heat flow into and out of the mass is reduced, as these materials act as insulation. So a given area of mass may be less effective in the short term at stabilising temperatures, but the extra insulation may also be beneficial at times.
In principle, to maximise the benefit of mass, it should be exposed to the interior air. And if winter sun shines on it, it can absorb heat more quickly. But it can still be useful even if it is covered. And increasing air movement within the house over the surface of the material, e.g. by using a fan, can increase heat transfer, improving the effectiveness of the mass.
Ideally mass should be located on the room side of the layer of insulation in walls (and ceilings), so that its effectiveness at stabilising temperatures is maximised. However, computer modelling and field measurements show that even an outer layer of mass (e.g. brick veneer or stone) does offer some benefit in many climates, by moderating the effective temperature difference driving heat flow into or out of the building. But the benefit is not sufficient to avoid the need for insulation unless the climate is very moderate.
Phase change materials: can we get higher heat capacity without mass?
Phase change materials (PCMs) provide a way of increasing the heat capacity of buildings without adding mass. As a material changes phase from solid to liquid, or liquid to gas, it absorbs a large amount of heat energy without its temperature changing. This is called latent heat: the energy is used to free molecules from the attraction of other molecules.
For example, ice melting in a drink absorbs a lot of heat and keeps the drink cold: as ice melts, it absorbs 80 times more energy than it would if liquid water was heated by 1 degree. When liquid water evaporates, it absorbs over 500 times as much energy as a one degree temperature rise! Of course, when steam condenses or water freezes, it releases the enormous amounts of heat energy it took to melt or evaporate.
Advanced PCMs can be designed to change phase (i.e. shift between solid and liquid) at selected temperatures, so that they act like high mass materials around those temperatures. PCMs are being incorporated into plasterboard, installed in lightweight walls and even incorporated in concrete; however, they can be costly. PCMs can’t currently be modelled in NatHERS, although it is being investigated.
Some people incorporate water tanks or containers into the interior of homes. Water stores two to three times as much energy per cubic metre as common high mass materials. But it can leak!
Life cycle energy and emission issues
Common high mass materials such as brick and concrete have high embodied energy and emissions – large amounts of energy are used to produce these materials. From a life cycle perspective, this investment of energy has to be offset by lower operating energy use and emissions if there is to be a net benefit.
In moderate climates and as buildings and appliances become more energy efficient, the break-even period for operating savings to offset the investment can reach decades. And the importance of ensuring a long life and reusing or recycling the materials increases.
However, there is an increasing range of lower emission high mass products. For example, eco-cements that replace some or all Portland cement with materials such as fly-ash and slag from steel production can have significantly lower embodied emissions. Recycled steel reinforcing or using fibres for reinforcing can cut the embodied energy of the concrete: steel can comprise around half of the total embodied energy of reinforced concrete.
More-energy-efficient gas-fired brick kilns can significantly reduce the embodied energy and emissions from brick production. The best kilns use less than half as much energy as the worst. One manufacturer has even begun to produce a zero carbon brick that uses sawdust mixed with the clay to provide much of the energy needed to fire the bricks. Recycled bricks are a good option.
Some other materials such as mud brick, rammed earth (pise), hempcrete and even strawbales can provide substantial mass and heat capacity without high embodied emissions and energy.
This article was first published in Renew 132 (July-September 2015). Renew 132 delves into the world of innovative, sustainable building materials.
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