Diagram Architects

Rule 5: Mass is Critical

If you’re interested in energy efficient house design, you’ve probably come across the very impressive sounding terms “thermal capacitance” or “thermal mass”.

The principle of thermal capacitance is pretty easy to understand: Heavy (or, more accurately, massive) stuff takes a long time to heat up. Once it’s hot it takes a long time to cool down again.

You’ve probably noticed this if you’ve ever stood against a west-facing brick wall on a cool evening after a hot day and felt the day’s heat still radiating off, or if you’ve felt last night’s coolness of concrete under bare feet on a summer morning.

Thermal mass, coupled with smart glazing and well-designed insulation, is one of the most important efficient-design tools available to an architect or building designer. It is most useful where temperatures swing between hot and cold, or in cold areas where heat sources, like the sun, are occasionally available. (It is less useful in hot, humid climates, where average temperatures are high, and cooling breezes must be maximised.)

Massive materials can store heat for a long time, and if designed properly, release it when it is most useful. Depending on the soil type, depth and structure, some well designed underground houses can access heat or ‘coolth’ from last winter or summer, 6 months or more ago. A concrete slab or thick masonry wall (300-400mm brick, stone or concrete) can release heat or ‘coolth’ from almost 6 hours ago.

Thermal mass can also be useful to draw heat from a space when too much heat is available, and re-radiate it later when temperatures drop. Some house designs can allow you to draw heat from one area (from the outside, for instance) and relocate it to another (to inside) – roof ponds or geothermal heating/cooling work on this principle.

There are situations where thermal capacitance is not useful. In spaces which must heat up or cool down quickly (for instance, when a heater is turned on in an irregularly used room), thermal mass will slow the heat response; a well-insulated lightweight room may be more useful.  And thermal mass can be awful when inappropriately used, as anyone who has baked in an uninsulated hot-box brick apartment on a stifling summer night (well after the outdoors has cooled) will know.

To use thermal capacitance advantageously there are a few questions you should ask:

• Is thermal capacitance going to be useful to me? This leads to some other questions: What are my maximum and minimum temperatures? What is the average between these extremes? How long is the cycle between hot and cold?

• How can I admit the useful and exclude the troublesome? (This might include shielding from extreme western sun, designing windows and eaves to admit more sun in winter than in summer, or insulating the thermal mass from unwanted temperature extremes).

• How much thermal mass do I need? There are rules of thumb for ratios of glazing-to-mass. For underground houses some site testing may be required.

• How do I plan to use the space? Will the usage be regular or irregular? When is heat or cold more useful?

The thermal mass in the earth-coupled polished concrete slab and the internal paddock-stone walls in our Wistow Smart Farmhouse (pictured above) allow the living spaces inside to remain comfortable year-round. The farmhouse needs no mechanical cooling and a minimum of heating in winter, despite baking summers and cold, grey winters.  Note the overhead ‘clerestory’ windows - these can be opened during cool summer evenings to purge the heat absorbed by the stone walls and concrete slab during the day.

Rule 4: Don’t heat with one hand and cool with another

A large part of creating an energy efficient house is knowing how, when and where to keep the outside conditions on the outside.

And that’s not chicken-feed stuff: In Australia we spend about 44.7% of our residential energy usage – about 170 PetaJoules per year (170,000,000,000,000kJ, or about 5.9 million tonnes of coal equivalent )- on heating and cooling our homes. (That figure’s about 5.1 quadrillion Btu in the USA, equivalent to about 5401PJ or 186 million tonnes of coal).

That’s a mighty load of go-juice spent making the inside of our houses more comfy than the outside. And, depressingly, an awful lot of this energy goes to waste.

From my experience, there are three main ways our homes waste this heating-and-cooling energy:

The first path to waste in our homes – and by far the biggest, in the temperate-climate Australian context – comes from building houses that leak heat, inwards or outwards. There are two ways to fix this: To seal the gaps in our houses, where warm air can enter or escape, and to insulate, insulate, insulate.

The second is by failing to use what we get for free. Failing to admit winter sunlight, or failing to store daytime heat for night-time use (through thermal mass or other energy-storage systems). Failing to cool our houses through night-time breezes or convective cooling. Failing to use evaporation, or to plant trees in the right places.

The third – and in some ways the most insidious- is by heating with one hand and cooling with another.

You can see an example of this every time an air conditioner runs with uninterrupted summer sunlight beaming into a room, or trying to cool when inefficient heat-emitting appliances are running. (In our office our laser printer alone puts out enough heat to raise the temperature two or three degrees. Nice in winter, but not so much fun in summer).

I’ve heard it said that a problem well defined begins to solve itself, and that principle can be applied to energy efficient house design. Look at your site, your situation and your energy sources. Know where your heat is going to come from, your sunpath and wind directions, and you’ll begin to know how to shield from it. Likewise, knowing your potential paths of heat loss; your gaps and uninsulated areas, will show you how to block and insulate them.

The trick is to address your energy gains or losses at the source, rather than developing active systems to counter them.

It’s simple stuff, but even professionals get it wrong sometimes.  It involves a change of mindset – a new thrift, rather than just throwing more energy at a problem.

I once worked on a multi-purpose hall for a primary school where a mechanical engineer proposed an air conditioning system that over-cools the air, then precisely adjusts the temperature by re-heating it again. As he explained the drawings, I sat, dumbfounded. Worse, he was astonished at my astonishment.

This type of inefficient thinking is everywhere, and it’s madness.

Sources:

National Greenhouse Gas Inventory, 1990, 1995 & 1999 Australian Government Department of Climate Change

2005 Residential Energy Consumption Survey, US Energy Information Administration

Rule 3: Insulate, Insulate, Insulate

Houses are, on the whole, fantastic things. They give us all sorts of benefits: Privacy, space to store our stuff and a whole life-support system of services and systems, from hot water to waste disposal..

But one of the main reason we decide to live in houses, rather than bedding down in a swag under a coolibah tree, is the controlled microclimate our houses allow us to achieve.

Face it: It’s hot out there. Or cold. Or windy, snowy, rainy or whatever.

So it makes sense to try to keep the weather we want on the inside, and the weather we don’t want on the outside. But there’s one big problem: our homes leak heat, both inwards and outwards, meaning the inside climate we’ve worked so hard to create is always escaping, and the less-than-ideal outside climate is eternally working its way in. What’s the biggest way to reduce that leaking? Three words: Insulate, insulate, insulate.

Most building codes worldwide now have minimum standards for insulation, and more and more people are retrofitting their existing homes with one sort of insulation or other. We all know that it’s good, and that we should get ourselves some. We have a rough idea that it works something like a blanket, slowing the transfer of heat from a hotter medium to cooler. We know that it helps to keep the outside out, and the inside in.

It should be simple – but it’s not.

A quick trawl of your search engine of choice for information about insulation will reveal a bewildering cornucopia of different types, materials, performances, measurement systems and purposes. The range of products can be bewildering, while differences in measuring insulation can be downright misleading.

Some insulation companies companies measure the thermal resistance of their products by ‘U’ value, most by ‘R’ value.  Each measurement means the opposite (A low U-value means a high R-value, and vice versa). To make things even more confusing, R-values are measured differently in metric (kelvin square metres per watt) and imperial (Degrees Farenheit square feet per Btu). (Note: 1K-m²/w = 5.67446 °F-h/Btu). Check carefully when you compare claims for R-values from overseas.

Some insulation companies measure the performance of their product by itself, and some publicise the ‘whole of wall’ or ‘whole of roof’ performance. Check the small print on any manufacturer’s literature for this. And it’s difficult to compare the performance of reflective insulation with bulk, as they work in entirely different ways.

So what do you need to know when insulating your low-energy house?

The first question you need to ask is “How much do I need?”. Statutory R-levels of insulation are usually set by your local Building Code (The BCA in Australia), but you do have to remember that these are minimum levels only. While a higher R-value is normally better, there are exceptions and there are points of diminishing returns. Getting an energy rating performed on your house design by an accredited house energy rater is often a good place to start. Some insulation companies also publish insulation guides for different locations; these can provide worthwhile advice.

The second question you need to ask is “What type of insulation is best for me?”. There are three main types of insulation available. Bulk insulation uses tiny pockets of air, which resist heat flow, and include Glasswool, Polyester, polystyrene boards and the various types of fluffy batts, blankets and blow-ins. Reflective insulation is great for reducing radiant heat transfer, and includes the foils and other shiny things. Composite insulation uses a combination of reflective and bulk insulation, and includes foil-faced boards, blankets and bubblewraps.

To choose the right type of insulation, you need to know what kind of heat it will be resisting; conductive, convective or radiant. Will most heat be moving outwards or inwards? You also need to know where it will be located and what the limitations of that location will be (how will it be installed? What conditions will it be under? Does it need to be fire resistant? Will it get wet? Will it need an air-space? Will it be crushed or moved by other building elements?).

The third question you need to ask is “How will it be installed?”. This is a biggie. As in every aspect of house design, detail is everything. A good insulation system will be worthless -or even dangerous- if badly or inappropriately installed. Ask your insulation supplier for their standard details or installation instructions, and don’t be afraid to change systems if the installation system doesn’t suit your house. Push your Architect/Designer to detail the installation well, and push your installers to do the job properly.

Insulation is, in many ways, the starting point for most low-energy house design. Get this wrong and everything else you achieve will be leaking out your roof and walls from day one.

Rule 2: Stop fighting the universe.

The universe wants you to design an energy-efficient house. No, seriously; it does. Why else would so many things, from the tilting of the earth to the laws of thermodynamics, give you the tools to heat or cool your house for free?

More energy lands on the roof of your house than you’ll ever need to heat it. Last summer’s heat lies stored only a few metres under your feet, and last winter’s ‘coolth‘ only a short distance below that. Hot air rises, where it can be recirculated as required or vented to the sky at night. The evaporation of water draws latent heat, cooling the air. Glass transmits light (short wave radiation) but traps heat (long wave radiation). Light-coloured surfaces reflect. Mass provides thermal capacitance, carrying the day’s heat into a colder evening. Heat travels from warm to cold, but at different speeds through different media.

These basic physical laws give us passive solar heating, earth-coupled heat storage, night-time heat purging, subsidence (cool) towers, sunrooms, reflective foil and bulk insulation and the many other tools we can use for energy efficient house design.

The same tilting of the Earth on its axis which gives us winter also sets the sun lower in the sky, allowing heat and light to pass under deep eaves and further into a north-facing room (south-facing in the northern hemisphere). The increased azimuth of the sun in summer allows it to be screened, provided you calculate your sun angles and eave dimensions carefully. (Pritzker-winner Glenn Murcutt calls this “Letting the planet do all the moving”.)

These are not new discoveries. Xenophon of Thebes wrote about passive solar design in his Memorabilia (3.8.8-10) in 371AD, but the 20^th Century experience of cheap ,easily accessible fossil fuel has allowed us to heat and cool our buildings through the application of brute force.

Now, brute force is a fantastic thing if you’re planning to wrestle a gorilla or launch a payload into orbit, but using it to warm or cool your house is just pure bloody-mindedness if there are cheaper and easier options available.

So before you turn the first sod or draw the first line for your new, energy efficient house, we recommend you ask yourself (or your architect/designer) the following questions:

• Are we best using the house’s environment, climate and conditions?

• Is there a path of lesser resistance to achieve the same levels of comfort and delight?

And my personal favourite:

• What is the universe trying to give me for free?

By all means, create an energy efficient house because you’re concerned about energy independence, peak oil or climate change. But personally, I like energy efficient design because any other choice is just beating your head against the laws of the universe. 

Rule 1: Pick the Low Hanging Fruit

Its easy to get excited about the bleeding-edge gadget-end of eco-house design. Solar photovoltaics, thermal phase change materials, home automation, in-house fuel cells and other enviro-toys help us to shout our green credentials from the rooftops and still play with the coolest technology.

And that’s OK if you’ve got the dollars to spend on it.

But it’s not where the real action is in energy efficient design. In fact, concentrating on this stuff while ignoring the fundamentals – the flow of heat, light and electricity through the everyday materials of your home – is throwing money away for minimal ecological benefit.

The trick is to start with the easy, effective changes; the things that will give you the maximum impact for the minimum cost, risk and commitment. In fact, some of the most effective changes involve the smallest investment, and start paying you back almost immediately. This is what we mean by “Picking the low-hanging fruit”.

The benefits of this approach are obvious and immediate. Here’s an example:

Some time ago, we were approached by a homeowner who was looking at installing a roof-mounted solar panel system to fully offset his home electricity usage. His electricity bills were substantial, and a solar system with the capacity to offset them entirely was well out of his budget

A quick review of his home revealed (amongst other issues) 110 50w halogen downlights cluttering his ceiling. Assuming a conservative average of 3 hours on every night, that’s about $940 on his annual electricity bill for halogen downlights alone.

Just replacing his 50w globes with 35w IRC halogens (same light output and quality) at a total cost of about $900 would save about 1806 kilowatt-hours per year – that’s an annual saving of about $280. (He’d save even more replacing them with compact fluorescents.)

Compare that with that a 1.5kw grid-connected solar system costing about $12,000 (including the federal government rebate) which only generates about $233 worth of power over the course of a year (that’s a payback period of 60 years).*

That’s right – an investment of $900 on everyday-tech light fittings saved more electricity and greenhouse pollution that $12,000 of solar technology.

If this homeowner had spent his $12,000 on solar power, without upgrading the efficiency of these and other basic items, every single watt of power generated from the sun would have been used to power a ceiling packed with unnecessarily (and expensively) greedy downlights.  Instead, we recommended that he spend less than 10% of the cost to make up the difference through simple, off-the-shelf efficiency measures.

I don’t want to knock solar photovoltaics. We’ve designed several buildings fitted with them, like the Wistow Smart Farmhouse in the picture above. But if you don’t sort the simple things first, you’re paying serious money to waste the energy you generate with them.

So what are the simple things? Watch this space.

*Costing information www.citipower.com.au