Residential Design for Low Energy Homes
Why do we need to build eco homes or have low energy house design?
We need to think of our homes in a different way. Too many of us think of our home as an asset that needs to be built or renovated for the cheapest possible price so that the ‘rising’ real estate market will bring the most profit upon re-sale, so they can move onto a bigger newer home. This thinking has created a new home market that is driven by price and not energy efficient design. To make matters worse, too many designers and builders are ‘afraid’ to suggest improvements because they either don’t know about them or they build to the New Zealand Building Code (NZBC) and no more. The NZBC is the ‘worst’ or minimum you can build to. Some designers and builders see it as a target to achieve, where as it should be a base to start from.
Too often instead of designing and building a smaller low energy home, with higher levels of insulation, or more thermally efficient windows or better air tightness, the owner will opt for 250 or 300 square metres of design, a highly specified kitchen, bathrooms, and internal fitment because, ‘I see that every day, and it will mean more to me and will make the house easier to sell’. They saw it in the show home, they want it.
But for the life of the building (50-100 years) that home will require higher levels of energy to keep warm or cool, and light, at ever increasing cost. The quality of living in that home will be reduced because as humidity builds up in the internal environment, the good health of the occupants will be harder to maintain. More colds and flu, more doctor’s visits.
The NZ Government has admitted that the cost of purchasing electricity rises ahead of inflation. The MBIE website shows that from 2010 to 2015 that New Zealander’s spent on average around $2,000.00 per year to purchase electricity but over the same period we consumed nearly 10% less. Meaning we are using less but paying more for it.When you look at buying a new home or a pre-owned home, how often do you, ask; ‘what does it cost per year to run?’ ‘How easy is it to heat and keep cool?’ You may be surprised.
Yes, it does cost more to build an eco home to add insulation values, increase the quality of the windows, increase airtightness, etc, over doing a standard or minimum NZBC home.
How quickly will I get my investment back? It depends on your level of investment over the cost of a standard home, how much energy costs and inflation increase over that period of time, never mind the savings in doctor’s and medication costs.
Life of the home?
New Zealand doesn’t track the re-sale value of energy efficient homes, but Australia has started and Europe and North America does. The data shows that as energy efficiency knowledge increases within the buying market, the value of those eco homes and quick sale time is increasing. In Europe, every house must be sold with an energy consumption rating, same as when you purchase a car, it shows you how much the yearly running costs for that vehicle is.
Building in a subdivision restricts the size of the home you can build but ask yourself the question – How big a home do you really need? The larger the volume of space, the more energy it will take to heat and cool that space. But then it takes less energy to retain heat than to produce it. So even if you are required to build a ‘larger’ home, insulation and energy efficient windows alone can save you money in the long term.
Sun – Passive Heating
Using the sun to heat the home is free energy, so talk to your designer about your architectural design that you consider placing your main living spaces to collect the sun’s energy. Using thermal mass (exposed concrete floor or tiles on a concrete floor) to store heat is easy, but do consider foundation edge insulation. If you capture that free heat you want to retain it, not lose it the moment the sun sets.
Wind – Passive Venting
Designing the home so that wind movement doesn’t restrict you being able to use outdoor living spaces ensures comfort and enjoyment with the home. Another thing to consider is air movement within the home. New homes are more air tight than previous designs, so air movement between indoor and outdoor is restricted, without opening a window. A home closed up all day keeps building up condensation within the home. Too slow or no air movement and cold leads to mildew and fungal build up, leading to an unhealthy home. You need to consider either passive air vents in window joinery or mechanical venting during the day.
A home that is shaded by trees or neighbouring buildings doesn’t receive the sun’s warming energy, so either ensure high levels of insulation, too keep the cold out or plant deciduous trees so you have shading in the summer and sun in the winter.
Insulation is one of the most cost effective installs you can do to a home. It doesn’t matter where in New Zealand you live, insulation will keep you warmer in the winter and cooler in the summer. To take your insulation values in your walls and ceiling from NZBC standard to two or even three levels above code, is a very cheap investment, you will spend more on your kitchen appliances. It’s a no brainer!
And don’t forget your Foundation. Under slab insulation give little value to thermal transfer of cold. The ground temperature doesn’t change that much from summer to winter, but perimeter foundation is in contact with the air so thermal transfer of cold is very different at that point.
Thermal Bridging (reduced framing)
Timber frame construction of New Zealand homes hasn’t changed for nearly 100 years, but the technology has advanced over that time. Reduced framing design is a method of construction that reduces the amount of timber used without reducing the structural integrity of the frame. The reduced timber means reduced thermal bridging within the timber frame wall.
Thermally Broken Aluminium is being used more and more in energy efficient homes. They provide a thermal brake internally in the aluminium extrusion. You have a standard looking window and door but with added efficiency.
uPVC has been used in New Zealand for a number of years and is gaining popularity. They are very good at air tightness with the three way locking mechanism they use.
Glass Fibre is relatively new to the New Zealand residential market but has many advantages.
Wood is still available in New Zealand, although sometimes they can be imported for a very competitive price. Wood is a very good thermal barrier with great ascetics but when considering wood make sure you ensure they are airtight.
Air movement through the building envelope is something sometimes forgotten by designers with considering energy efficiency. If you have to constantly heat the air changes in a room then it will be costing you money. But then you can’t trap moist air inside the framing either because that will cause degrading of the insulation and wood framing.
Interior/Exterior framing wrap is the cheapest form but check the specifications of the wrap, as some are design to allow movement while others to control movement in one direction.
Ridged Air Barrier (RAB) is often used with volume builders to speed up the process of building but while a RAB slows air movement, it doesn’t stop it. If your builder uses an RAB ensure you have mechanical air ventilation install as well because restricted air flow, and low thermal temperatures help with mould and fungi production.
Structurally Insulated Panels are being used more in New Zealand residential construction. They are designed to replace traditional timber framing and insulation in wall/ceiling/floor construction. They provide high levels of insulation to the building envelope.
Passive venting, as mentioned before needs to be considered as venting the home while unoccupied is necessary to make a healthy home.
If you are going to be using a RAB or want high levels of air tightness and require mechanical venting, removing the warm, moist air and bring in cold clean air is in-efficient, because there will be times when you want to do this and heat the home at the same time. A Heat recovery ventilation system will recover this out going heat and transfer it to the cold, clean in coming air. The incoming air is also filtered at the same time, ensuring healthy, dust free air into the home.
LED lighting is being used/retrofitted in more and more New Zealand homes and you should consider, not only the energy used for the level of luminance but also the life of the bulb. They are a higher level of investment but over the lifetime of the bulb, they more than pay for themselves.
When considering the type of ceiling lights you want to use, remember that every time you cut a hole in your ceiling you are degrading the air tightness of your room/home. Down lights are some of the worst uses of light. Not only are you opening up your ceiling but some down lights can’t have insulation over the top of them. Even LED lights have a certain amount of air leakage.