Passive Solar Design

Passive solar design is based on the principle that there is a non-polluting, always on, nuclear reactor in the sky flooding planet earth with free energy year-round, and that ignoring this fact during house design leads to unhealthy, expensive to operate buildings.  We should simply build for the sun that sustains all life on earth.

A passive solar house basically lets the sun do the work (that’s the passive bit), by orientating the building towards the sun and storing energy captured from the sun for winter heating, and then avoiding unwanted energy capture from the sun in summer using unheated thermal mass and ventilation for cooling.

Passive solar design is a big subject and, like the other sections in this site on strawbale building and eco-system design, we will just touch on it lightly here highlighting some key elements that we are employing for Nga Kereru.  If you are interested in learning more about passive solar design an internet search is a great place to start, as there is a huge body of knowledge on-line.

Siting the house

In the southern hemisphere the Sun tracks across the northern sky, so for maximum passive solar efficiency the house should be orientated to face true north (map North, not magnetic North) and the winter sun exposure should be as uninterrupted as possible. Our building site at Nga Kereru ticks all these boxes. 

Managing heat gain

The sun naturally changes its inclination as it tracks across the sky through the seasons, tracking low in the sky in winter and tracking more overhead in mid-summer.  Nature allows us to design the roof, ceiling and eaves of the house to maximise heat gain from the sun in winter and minimise heat gain from the sun in summer.

Our house design has broad eaves that help protect the plastered walls from weather and also to control the amount of sunlight entering on the northern face of the house.  The ceiling pitch is set to 25 degrees on the north side of the house to allow penetration of the sun to the back of the house in mid-winter, directly heating the entire open plan living area and two of the bedrooms, as shown in the diagram below.

Thermal mass

Thermal mass refers to materials that can retain heat and discharge it gradually. Stone, concrete and water are all good examples of materials that can be used for heat capture and storage.

Our house has a super insulated black concrete slab floor.  The floor is polished to a salt and pepper finish and left exposed so it can absorb or radiate heat most efficiently.  The walls are clay plastered inside and clay/lime plastered outside, with the internal plaster making a significant contribution to the thermal mass of the house.

This heat storage and transfer from the thermal mass within the house is used for warming or cooling the surrounding air depending on the season, regulating the air temperature within our house year-round within a couple of degrees range (21 - 23 degrees celsius) without any other energy inputs.

Thermal efficiency

The siting of our house for the sun and building in thermal mass are a great start, but without insulation and airtightness the thermal gains are offset by losses that require additional energy inputs. We have therefore given special attention to maintaining the integrity of the thermal envelope of our living space.

The concrete slab is the primary thermal mass storage so it has been fully insulated, using Cuploex Thermal Wrap, Cupolex Pods and Magrock Insulfound.

The Cupolex Thermal Wrap and Pods are insulated from the ground and provide for heat transfer within the slab, and the Magrock Insulfound insulates the perimeter of the slab from ground and air heat gains/losses.

80% of the heat loss in a slab is to the air from the exposed sides of the concrete slab if uninsulated.

In our frame walls 1 layer and in the ceiling 2 layers of TeraLana R.4 wool insulation are being installed over a Pro Clima Intello airtightness membrane for the living spaces.

The weak point in passive solar homes, for thermal efficiency, is generally the extensive north facing (in the southern hemisphere) glazing that is required. To minimise unwanted heat loss in winter, we have specified Natureline European profile all timber hardwood exterior joinery from ThermaDura of Mosgiel.

Thermal bridging through the frames is eliminated by the all timber construction, and the glass is 52mm double glazed, low-E and argon gas filled throughout.

Nga Kereru ongoing energy requirements

Most importantly our house is designed to passive solar principles and super insulated, and so has minimal energy requirements for climate control as it does not require additional energy inputs for seasonal air conditioning to heat or cool.  As designed our house should maintain a 21-23 degrees celsius temperature range year-round - we will update with our actual experience here after a couple of years living in it :-)

Our house has also been designed for low total energy consumption, with induction cooking hobs, a heat-pump hot water system, a high efficiency wood stove for backup heating (if required) and LED lighting throughout.

The house has been connected to the Grid for its energy supply but has been built ready for future solar power generation.  Why?

The grid in New Zealand is currently 85% renewable power and with a small population to fund renewable power generation and power distribution connecting to and using power from the grid is generally the greenest, most socially responsible option in New Zealand.

As the grid is predominantly renewable power in New Zealand the solar power rebates available in other countries e.g. Australia and USA are not offered here, and grid connected solar energy buyback rates in New Zealand do not incentivise householders to use the grid as a virtual battery.

With steadily falling prices from improved design and economies of scale as demand has increased, Photo Voltaic (PV) panels are now competitively priced for power generation in New Zealand without subsidies, however to be effective they need to be used in conjunction with batteries to supply continuous power when the sun is not shining.  The cost of batteries, like PV cells, has reduced dramatically over the last decade – by an order of magnitude in fact, however… they still have some way to go before they are competitive without incentives in New Zealand vs grid-based power.  Within the next 5 to 10 years we expect it will make economic and ecological sense to invest in PV solar and batteries for on-site power generation and storage at Nga Kereru.

We have connected to the grid at the front gate with a new transformer installed, and we then trenched and cabled the power underground approx. 150m uphill to the building site.  We have three phase power connected to the house, setup for three phase 400v charging of two electric cars and also have cable laid for a future PV solar panel array on the North facing hillside directly behind the house.

Passive House

Passive house design has been developed in Europe, originating in Germany, and building standards have been developed there to certify building practitioners, materials, designs and finished buildings.

Passive house is not the same as passive solar, passive house is a design philosophy principally concerned with the comfort of the occupants which pays particular attention to the integrity of a buildings thermal envelope - air-tightness and ventilation management are the key aspects it addresses.

Thanks to our builders and architects knowledge and input we are applying many Passive Haus principles in the design and build of Nga Kereru, but we are not actively seeking Passive Haus certification.

We are using several specialised products, primarily sourced from Pro-Clima NZ, to wrap and tape for air-tightness of the houses thermal envelope during construction.