New Zealand Sustainable Building: Thermally efficient building, Passive Solar Design, Solar Water Heating & Solar Underfloor Heating.

This page is a summary of what we have learned and how to achieve an energy efficient building. This is based on our experiences to date, two year's after opening. Feel free to challenge this advice or put us straight on any specific issue - we appreciate feedback.

SUMMARY

• Consider energy efficiency inextricably linked and equally important to architectural form. 
• Allow 5-10% more to the cost of your project to ensure super-good insulation. 
• Use solar water heating to mitigate energy costs.
• Choose your preferred room heating method. More important is reducing the need to heat in the first place, and the building's ability to retain it.

If done properly, energy savings will outweigh increased building costs from the outset. 

THERMAL EFFECIENCY

Wanaka Homestead is insulated at every level: with R3 polystyrene under the floor slabs, R3 poly-blocks around (but not under) perimeter footings, R3 to R3.2 in the walls, R3.6 to R4 in the ceilings and double-glazed timber joinery. These are good standards, but we could do much better: 

Walls & Ceilings Expert sustainable architect & lecturer Robert Vale recommended minimum values of R4 in walls and R6 in ceilings, during a recent Wanaka seminar. We believe this is the single most cost effective strategy - insulation is cheap. Insulation materials? no strong opinions.

Windows we all love them and the more-the-better is certainly "in" right now. However, windows are not thermally efficient and good ones are expensive. Even top of the range argon-filled double glazing has poor insulation qualities compared to well insulated walls. Try to reach a sensible balance, avoiding unnecessarily large window areas, especially avoid too many high windows where heat loss is greatest. Double glazing is a minimum requirement for thermally efficient houses in temperate climates. Double glazing does not significantly reduce the ability to absorb solar radiation. It is required on windows on all faces of the building.

Money is wasted if you have a weakness in your overall insulation stratgey. For example a straw-bale home with single glazed windows and R2.6 ceiling would be be criminal misuse of the excellent insulation qualities of straw.

PASSIVE DESIGN

Why heat or cool your building, when the sun, or lack of it, can do it for you?

In this respect overhanging eves are essential. Design eves to "just" allow full winter sun to enter each window. Conversely, overhang the eves (or verandahs) enough to shade the windows at summertime midday sun. Minimise number and size of windows on non-sunny faces. Try to make your building face north (in the southern hemisphere) and place living areas on the sunny side. Overhanging eves also protect your walls from rain & weathering and help protect against"leaky building syndrome."  Avoid flat roofs or parapet walls for the same reason. 

We have two cottages, identical in construction quality - one has great winter passive design, the other is shaded. The latter costs about 30% more to heat during winter.... passive design is that important.

Thermal mass is important - but can work against you. Try to consider the indended use of the building and also whether you have compromised passive design principles. If you have properly designed in accordance with the previous paragraph, then a sensible design might benefit from a well placed thermal mass (eg, an exposed concrete floor) located exactly where the winter sun hits it. Conversely thermal mass exposed to too much direct sunlight on a hot summer's day might cause discomfort or the need for cooling. Thermal mass is only useful if it is inside your insulated building shell, but of little use otherwise. The more mass, the slower the response time to either heating or cooling.

SOLAR SYSTEMS

Solar water heating is a very cost effective way of producing hot water. Over the course of the year it will typically save 50% of your domestic hot water heating bill. Manufacturer's recommendations vary, but all work best facing north, and the ideal angle of pitch for most is about the same as your latitude. Eg. Wanaka is 45 degrees south, so 45 degrees is an ideal pitch. They work well, but slightly less efficiently at other angles too, and also produce more heat than you might expect during cloudy days. We find that solar hot water, combined with instantaneous gas water heating, is an extremely good combination, but there are plenty of other solutions too.

Solar Underfloor Heating  In the darkest depths of winter when heating is most demanded, the sun's ability to heat water is very limited. In our case solar heating has negligible effect mid winter with almost all of the energy required coming from "bought in" sources - gas in our case. In spring and autumn, the sun has more energy and heat is in less demand - solar heating begins to have benefit. It works for us, but it is complicated and relatively expensive to install. Passion for solar energy can blur objectiveness with regard to solar heating and whilst it is a worthy goal, our general advice is to keep it simple and first spend the money on vastly improved isulation and solar heating for your hot water system. 

Solar heating for radiators and wood burners. Same advice. Keep it simple and spend the money on improved insulation and solar heating for your hot water system

Underfloor heating with water pipes. Our experience with this form of heating is extremely positive. Its a wondeful, healthy and unobtrusive form of heating. Our system runs from a gas boiler, is timed, and is linked to a room thermostat.

A thermally efficient home looks great, feels great, costs less each month (on a mortage repayment + energy bill basis) and lasts longer.

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