Passive Heating: Inside The Box

WindowCovering
Window coverings and building materials make a big thermal difference. Photo By Mara Ripani

By understanding the principles that underpin passive heating design, the building you create or retrofit will produce enduring comfort, be far more sustainable and deliver significant financial savings.

The most important design principle when retrofitting or building a new home is to ‘build tight’. This means eliminating air gaps in walls (due to poorly installed insulation), drafts around doors and leaky windows caused by single-paned glass or aluminium window frames. Things like chimneys above open fireplaces, air vents in older houses and even recessed downlights all release heat from your home leaving it less comfortable in cold weather, so identifying these areas are your top priority when looking to reduce the need for auxiliary heating.

The Envelope

The building envelope constitutes your home’s windows, walls, floor and roof. Directing most of your finances to these four areas makes the most sense because your building envelope is the key ingredient for creating an extremely comfortable home. A home that genuinely supports you by keeping you comfortable all year round at minimum cost. According to Sustainability Victoria, the cost of heating a medium-sized house is on average $1263 per annum, that’s $12,630 over ten years.

The building envelope drives and informs so many other decisions. If it’s airtight and insulated to the highest standards, it will negate the need for expensive heating units like in-slab floor heating, hydronic heating, split reverse-cycle systems, all of which have high startup costs and high long-term costs regardless of their efficiencies. Instead it allows you to purchase standalone space heaters for the most important rooms and to use those heaters the least number of times.

Windows

Make windows your design highlight. High-quality double-or-triple-glazed units reduce heat loss and dramatically reduce outdoor noise pollution.

The Australian Window Association (AWA) estimates up to 49 percent of a home’s heat can be lost through windows. When purchasing windows, look for products with very low U-values which signify how well a product prevents heat from escaping. Window performance is measured using an Energy Rating Scheme (WERS) based on whole-window U values determining how readily a window conducts heat: window frame, edge of glass and central glass area. Another important measure of window performance is the solar heat gain coefficient (SHGC) which measures the percentage of solar heat gain that passes through a window in the form of reflection, absorption and transmittance. Where you live impacts the best SHGC for your home, use a high SHGC coefficient for very cool climates and a low SHGC for warm climates.

But regardless of its U value or SHGC coefficient, glazing is still a poor performer compared to a well-insulated wall. The recommended minimum insulation value for a wall in Australia is R 2.8 (with the exception of Thredbo, NSW, where it’s R 3.8). For context, a high-performing double- or triple-glazed window with thermal breaks will have an R rating of 0.5 or 1!

While it is important to orientate your biggest windows to the north, don’t overdo it. The supersized windows so fashionable in contemporary architecture bring far too much heat into your home in the summer months in temperate and warm climates.

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As good as they look, a lot of heat will escape from large windows. Photo By Mara Ripani

Shutters And Blinds

Keep winter warmth in your home with curtains and internal or external shutters. Curtains have gone out of favour in contemporary design, however their positive benefits far outweigh fashion fads especially when combined with pelmets which conceal the curtain fittings, as they can reduce heat loss by as much as ten percent. All of these approaches seal your home and allow you to take control of your thermal comfort.

Walls

INSULATION INSTALLATION

Even the best insulation installed badly is almost like no insulation at all. For example, if the insulation is heavily compressed during installation, or if gaps are created by poor workmanship, it’s a huge waste of your time and money and happens surprisingly regularly. If you can, oversee the installation process in order to maximise the efficiency of your expensive insulation.

Insulation is either sold as batts or as reflective foil and a combination of the two is recommended to achieve the best result. Bulk insulation relies on pockets of trapped air within its structure and reacts the same regardless of the direction of the heat flow through it. Reflective foil insulation, however, relies on the presence of a 25 mm air layer adjacent to the shiny surface, so the direction of the heat is important.

NATURAL MATERIALS

One of the best ways to build a passively heated home is by using natural materials when constructing walls, such as strawbale, lightearth, rammed earth or cobb. Strawbale, lightearth and cobb all use straw to varying degrees, which is a natural insulator and creates buildings with excellent thermal performance.

Strawbale homes perform best with a typical wall R value of 10, lightearth walls have varying R values depending on straw-slip density and wall thickness, while cobb, too, will have varying values depending on the straw-to-clay ratio. Rammed-earth walls, if built with an insulation layer, also have excellent thermal performance.

Not only are natural materials biodegradable at their end of life, but the building and insulation materials are the same entity, which eliminates the risk of installation error and ensures a tightly built envelope.

PANEL SYSTEMS

Structural Insulated Panel Systems (SIPS) also remove the risk of insulation installer error. The insulation and structural building frame are one product creating a tighter energy envelope, however the big downside is that SIPS use expanded polystyrene foam. Polystyrene is not biodegradable and creates environmental pollution. Some are better than others, however, so thorough manufacturer research is required if this is your preferred option.

Floors

Twenty percent of your home’s heat is lost through the floor. Floors can be one of the hardest places to insulate in an existing home. In a new build, a thermal-mass floor can be insulated with slab-edge insulation, and the inside of the slab can also be packed with insulative material to increase its thermal performance.

Because standard slab floors are made from concrete filled with expanded polystyrene waffles, the challenge is finding ways to make truly sustainable slab floors. Sustainability wise, earth floors are the best from an embodied-energy perspective, but we’re a long way from the mainstream building sector innovating and embracing earth floors.

If a slab is not used, then high R-rating insulation batts and foil can be used to protect the subfloor, and a similar system is a relatively simple and effective way to retrospectively insulate raised timber floors.

Ceilings

Because heat rises, up to 35 percent of the heat lost in your home is through your ceiling and needs to take priority when it comes to insulation. Many homes built in the last 20 years have recessed downlights in the ceiling, which completely compromises the performance of ceiling insulation. Halogen downlights can reach well above 300 °C, meaning that ceiling insulation must be placed well away from each individual light. While it is possible to safeguard your home by purchasing downlight covers for each light, the very best thing to do is to have them replaced with non-recessed LED lighting. Only then can you maximise the performance of your ceiling insulation.

Insulation batts can be updated to newer, more efficient materials in older homes, and if the roof cavity can’t be accessed from inside, it is possible to remove the roofing material to retrofit insulation.

Shape

While a sphere is the most efficient shape in terms of passive heating, it’s expensive to build and impractical to live in. A rectangular-shaped box is the cheapest to build and will still produce a positive thermal response because it diminishes the envelope’s surface area.

‘A lot of passive houses are a rectangular box, because if you have a small perimeter it’s easy to insulate,’ explains passive-house architect Talina Edwards.

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Clockwise from top. Thick walls made from natural materials have excellent thermal properties; Well-placed plants can provide cooling shade for a home in summer months; Tilt-and-turn casement windows offer flexibility. Photos By Mara Ripani

Orientation

No matter how beautiful the views, no matter where the street is, do not let either view or streetscape persuade you to undermine the passive-solar orientation of your home. In reality, most of us are so active cooking, caring for others and working that what we experience the most isn’t the view, but how comfortable we feel. A house built with passive-house principles will maintain a temperature between 20-25 °C with no more than 10 percent of the year exceeding 25 °C. When you wake up in the morning, it’s not the view that you notice first, but the temperature of the room. Orientate solar north: 15° W–20° E, and if your best views are to the south, create small picture-sized window openings there.

West-facing walls will heat up the most in direct afternoon sun, so making efficient use of deciduous plants that mirror and provide our seasonal shade needs is a clever way to resolve this.

Body Heat

Your body heat is equal to about 100 watts of energy, so in an airtight house with four individuals, this equates to 400 watts per hour heating your home. This heat is not to be underestimated because in an extremely well-designed passive home, turning up the heat can be as simple as inviting a few friends over for dinner.

Smaller Is Smarter

A small home’s footprint combined with good ceiling heights of three metres makes a small home feel spacious. The extra ceiling height, if coupled with ceiling fans, can ensure winter heat is pushed back down into the room by reversing the fan’s orientation. A smaller footprint frees up money to create an excellent building envelope, too.

Passive heating is 100 percent dependent on the home’s building envelope and its airtight design. In a passive home it is the windows and doors that steal the attention and spark conversations. An airtight, draft-free home is a thing of great beauty, and you will fall madly deeply in love, happily ever after.

Outside the box

North-Facing Greenhouses

Building a greenhouse to the north side of your home is an excellent way to passively heat it. In a perfect world, a greenhouse could be built to run the full length of the house or along the areas that need the heat most. In this example, your bathroom, loungeroom, study and bedrooms would sit along the north face of the house and the greenhouse would be built parallel to these rooms.

Leave the more active spaces such as the kitchen, where heat is produced from cooking, without a greenhouse and allow natural light to fill the areas you spend the most time in during the day.

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