City home build diary (part 2)

Meet the Mitchinsons

• Bruce and Annabel Mitchinson are in their 40s and have two children, Bailey (18) and Anthony (14).
• They designed their Glen Innes, Auckland home themselves through Bruce’s architecture firm Mitchinson Simiona Limited. TDC Construction handled the build; the budget was $550,000.
• The Mitchinsons’ green goals were about minimising energy and water use and taking a responsible approach to urban intensification and global environmental issues.

Bruce: We wanted to maximise the use of timber framing, and plywood, to be as sustainable as possible. The structure of the house is designed to the New Zealand Light Timber Frame Standard which is commonly used for housing here. It was basically about trying to make use of timber resources that are grown here, in sustainable forests.

I think it was an easy choice. However, there are limitations. If you’re trying to design with timber framing, it doesn’t span that far, and large room sizes are hard to do using standard timber sizes. What normally happens is you either end up using laminated timber or steel beams to make up the difference. This is where forward planning pays off. We’ve only got two steel beams in the whole house. We really tried to minimise the amount of steel because of the high embodied energy.

A giant battery

Because it’s a three-storey building, the standard required a concrete ground floor, which I was going to do anyway, because we planned to install hydronic underfloor heating. What we’ve got on the ground floor is quite a hefty slab. It’s 150mm thick.

This means we have a decent mass of concrete, and that’s parked on top of polystyrene insulation that separates it from the ground. With our underfloor heating, the idea is to use the thermal mass on the slab to store the heat energy so the slab stays warm and at an even temperature.

Standard energy efficient design relies on a building’s orientation to the north, to bring the sun through to directly warm the floor slab. The slab absorbs the heat and when it gets cold outside, the energy that was stored in the slab warms the space. It’s very slow to warm up and very slow to cool down.

However, with this house, we didn’t have the luxury of direct sunlight because we weren’t facing north. Instead, we’ve used technology to transfer energy. Solar panels on the roof grab the sun’s energy by heating water, which we are then able to send down to the slab. The thermal mass of the concrete floor acts like a battery, storing the warmth.

I also wanted the house to sit between two solid walls. The one on the north side is a concrete brick that looks like stone – again, quite a solid mass of concrete. The idea is that the sun warms up this face, and the warm air is brought into the house through the fresh air intake grille, located in the middle of this wall. In summer, when the sun is directly overhead, the wall has a cooling effect.

On and in the walls

Inside the house, I didn’t want to use a lot of solid timber. Instead, we used timber veneers which are thin and attached to a plywood structure. Exotic timbers look great, but by using them in veneer form it’s a lot more sustainable than carving up great hunks of slow-growing wood. The kitchen and bathroom joinery looks like solid timber, but it’s all veneer.

With the wall and roof insulation, there are a lot of different products out there to choose from. In the end we used a polyester product called Greenstuf which is made from recycled PET plastic.

The plastic is ground up into sugar-size granules and then they put it into a machine and whip it up like candy floss. That spins the particles around and heats them and turns them into fibres that are fused together to form the insulation blanket. You can get it quite dense, which I like, and at the time it had one of the highest R-values around. I liked the idea that it was made from recycled product and not from new materials. You’ve got to get rid of all those plastic bottles somewhere.

Our green choices

• Maximise local timber, minimise steel
• Concrete used for heat retention
• Double-glazed windows
• Greenstuf polyester wall insulation made from recycled PET bottles
• Block work designed to minimise on-site cutting and waste

Why not steel?

Making steel takes a lot of energy and releases significant amounts of greenhouse gases. And because steel construction members are all imported to New Zealand, transportation impacts can also be considerable. Steel has its place in construction, but it pays to be mindful when designing for it.

Catch the next city home build diary where the Mitchinsons find the latest green technology.

Share your building story by commenting below.