Country home build diary (part 3)

Meet the McCrosties

• The McCrostie family – Dominique and Daryl (both 30) and children Shaun (4) and Jessica (nearly 2) – are building their dream home off SH63 in Marlborough.
• Their home is being designed and built by Hybrid Homes and Claymore Construction respectively. Their budget for the build, including solar power system and amenities such as rainwater collection and sewage, is $530,000.
• The McCrosties’ aim is to get back to the simple life by being self-sustaining and having a minimal impact on the wider environment.

Dominique: We have just watched the foundations going in. It is quite nice now – we can actually walk around and see where everything is. It is really starting to take shape.

Daryl: It’s a really wicked progress. You can see the dreams that we started out with over two years ago taking shape.

Dominique: You can stand in the master bedroom and look out and say, ‘This is going to be my view from the bed’. You can see where things are going to go, and it is really nice to be able to see that.

The concrete pad is really important. It is something we talked quite a lot about. Hybrid Homes initially told us that having concrete throughout the house would suck in all the heat from the sun and release it slowly throughout the evening. But we have always preferred carpet because it’s softer to walk on and not as hard on your feet.

So we have compromised and put carpet throughout the house, but left a strip of concrete along the north side in the dining area, family room, three bedrooms; the whole north side of the house. It’s about a metre wide, so it will still trap some of the sun’s heat and help warm the house overnight, but we will still have the nice comfort of carpet as well.

Daryl: We won’t get the same benefits as if we had the whole house in concrete but it’s just a case of usability really. Being right on the north face, the concrete still gets pretty much all-day sun.

It sort of worked hand in hand that we wanted the house to be north facing and we wanted that sun coming in to the rooms we are using the most. We wanted big doors and windows along there anyway to maximise the views. We never planned on the concrete strip when we were first drawing up the house. But as it turned out, we didn’t have to adjust much because we had the right idea to start with.

Dominique: From the start we wanted carpet because we felt it was more comfortable. But it was kind of a compromise that to utilise that heat from the sun we needed a concrete strip and it was just convenient that we had already planned for the big windows and door openings which made use of it.

I think in winter and on sunny days, we will need less heating than we otherwise would. I think we will be putting the fire on a bit later. All going to plan, it will trap quite a lot of sun heat.

Daryl: I would like to think that from mid-autumn to mid-spring we won’t need a fire at all, but we will find out. The weather we were getting in September still had a bit of a chill, but we are hoping that with the concrete and the extra insulation we have got we wouldn’t need to put the fire on at that time of year.

Dominique: The house is also thermally broken. They have laid 100mm of polystyrene under the concrete pad, and there’s also polystyrene around the perimeter of the house. So the foundation around the edges is separate to the concrete pad inside. It means the outside temperature will be more effectively kept to the outside and the inside to the inside. We have been watching a few builds since deciding to do it for ourselves and none of them had polystyrene in the concrete pad.

Hybrid has talked a lot with us about thermally broken homes and that is always what we wanted, but to actually see it put into practice at the build site and to see exactly what they are doing is really nice. It’s becoming a lot more real.

What is thermal breaking?

Thermal breaking is a way of stopping heat moving between the outside and inside surfaces of a home. Many people have heard the term used around windows – thermally broken windows are an excellent way of stopping heat-loss through the frames – but few people realise it’s possible to insulate the concrete pad on which a new home sits.

The concrete pad beneath the McCrostie’s home was isolated from the ground with a layer of polystyrene in a process that is currently being patented by their designers, Hybrid Homes. The layer stops heat seeping into the ground, and allows the concrete pad to function as a kind of battery, storing warmth on cold days. Natalia Harrington of Hybrid says a thermally broken home works a little like a sleeping bag or a chilly bin. “It’s fully insulated right the way around.”

As Natalia explains, a properly thermally broken home doesn’t just isolate one part of the building – it addresses all the surfaces. “When you have two different building materials, say timber and aluminium joinery; when there is a difference in thermal conductivity between the two materials, naturally the energy – either hot or cold – will be attracted to the weakest point.

“You can put all the insulation you like in your home, but the moment you have two materials meeting with different conductivity properties, they create a thermal bridge. Thermal bridges basically just dump the energy out.“

To be truly thermally efficient, you need to eliminate that thermal bridge throughout the thermal envelope of the house.”

Thermally broken homes don’t cost a great deal extra, says Natalia. They just take a little more time and a little more labour.

“The advantage of thermally broken homes is that they give you greater thermal efficiency throughout the year. It basically takes ordinary building materials and makes them work the way they should.”

And of course thermally broken windows are a well-known example for a good reason: according to the Window Association of New Zealand, nine out of 10 New Zealand homes have aluminium window and door frames. And with aluminium being a notoriously poor insulator, the opportunity for energy loss is high.

However this can be prevented with thermally broken aluminium joinery, which is now widely available. A rubber seal within the aluminium framing becomes a thermal break between the inside and outside, and stops condensation from forming.

“It gives you a much more efficient structure,” Natalia says. “And it doesn’t promote condensation, so therefore mould and mildew and all those issues are eliminated.”

Using thermal mass

When the McCrosties opted for exposed concrete flooring in their home, they were actually using an ancient building technique from the Middle East.

Middle Eastern builders have known for centuries that using thermal mass is a smart and energy-efficient way of dealing with extreme spikes in temperature. Heavy stone blocks used in buildings throughout the region absorb the heat of the sun during the day and release that heat during the cool of the night – and the concrete slab in the McCrosties’ home is designed to do the same thing.

It’s an idea that leading New Zealand architect Gordon Moller understands well. The designer of the Sky Tower recently completed the parliament buildings in Oman, where he studied local buildings to discover their thermal mass secrets.

Visiting several ancient structures during the searing heat of the day, he found that their interiors were several degrees cooler – despite the fact they used no powered air-conditioning. And the interiors were kept comfortably warm during the cold desert nights, thanks to the slow leak of heat from the stones.

Thermal mass has really come to the fore in New Zealand in the last 10 years, Moller says, as people think more about the way in which they use energy.

“The sun is still the cheapest form of energy, but obviously you’ve got to control it.”

Moller says that most older New Zealand houses were designed by European settlers who brought designs better suited to the climates of their previous home countries. That has contributed to the damp, cold conditions in many homes here.

But now New Zealanders are starting to switch on to ancient concepts such as thermal mass as a way to create healthier homes that require less energy to either heat or cool.