A line of trees on the north boundary prevent solar exposure of the house on winter mornings. To remedy this the Engine Room was added with solar collectors at high level; it acts as a heat exchanger for the entire house.
Airflow on a cold sunny morning
1 The high-level solar collector is replenished with cool air from the thermal storage area below. Warmed air returns to the living area via a high level slot in the internal curatin wall
2 A house fan transfers warm air from the living area throughout the house, returning as cooler air via the corridor
3 Drawn by the solar collector (& back-pressure from the house), cooler air moves through (and is stored by) the thermal storage area
The thermal storage area below includes a 600mm deep concrete manifold, perforated with air ducts, and around 1,000 litres of water in drums, also useful as emergency water in the event of a quake.
Heated air flows into the room via a slot at the top of the inner wall of the solar collector
In summer a flap is opened in the solar collector, which then acts as a heat chimney, drawing hot air from the house.
At night there is no longer a draw from the solar collector and the convection flow stops. Cold air in the collector is trapped, and can’t sink back down to the thermal storage zone.
The system works well, and maintains the house at a consistent temperature for up to 24 hours. If consecutive cold cloudy days occur, we use the original 5.5kW heat pump to top up the mornings and evenings.
Insulation
Walls
90mm boards (R4.5, U0.02) face the solid timber outer walls, with all joints taped. In addition to keeping the structure dry, they also protect from rapid temperature fluctuations (no more ‘rifle-shot’ expansion sounds in summer).
Being inside the thermal envelope, the mass of the walls also help contribute to our thermal storage capacity.
Roof
The roof structure is limited – little space between the purlins was available for filling with insulation. Not wanting to disturb the ‘ceiling ribs’ internally, we opted for placing extra insulation externally. We settled for discontinuous 70mm boards between new purlins, not wanting to lift the roof sheets any higher.
Floor
Underfloor insulation was the most arduous part of the entire project; the sub-floor area enjoys clearances of between 200 and 400mm, so I resorted to polyester fibre and ‘snappable’ polystyrene panels.
Glazing & Cladding
The existing single-glazed aluminium frames were in good condition. Replacing them all with thermally broken double glazing was beyond our budget, so we settled with double-glazed panels (argon-filled, e-coating) throughout, but with new thermally broken frames in the living areas, and retroglazing the old frames for the remainder of the house.
The new frames were bulky, but I managed to hide much of their periphery through the reveal detailing.
Cladding support is via vertical battens placed on the 900mm grid, fixed through the insulation to the wall behind. The cladding is fixed to horizontal battens, with the assembly forming a rain screen which diverts most of the rain, and includes large pressure equalisation cavities to lower the speed of wind-borne water.
We opted for sliding doors and windows to prevent slamming in the wind.
20 solar panels capable of 5.5kW generation are mounted on a rack in the garden, with a grid-connected Fronius Primo Converter and Power Reducer (which prioritises the hot water cylinder with the power generated). We decided to continue using the original HWC, even though it is an energy hog; for most days of the year, its needs are met through solar only.
Inside the house, a 10.5kWh Sonnen battery and power back-up system smooths out our consumption, and helps cope with our frequent grid power failures.
Our healthy energy exports to the grid in summer don’t mean much; electricity supply is privatised in New Zealand — we pay 33c per kilowatt-hour from the grid, but receive 7c for exported energy.
The battery can be seen behind the office door at the end of the Corridor
Total energy consumption vs. imported energy compared over the years
The solar panels and battery aren’t enough during the winter months, so we need to maintain a grid connection
Costs
From the outset, we didn’t want to skimp on the energy-saving components; kitchen equipment, glazing, insulation, solar generation and a battery.
It’s not surprising they make up nearly half of the NZ$190,000 total spend.
