The many layers of a concrete foundation
Things are finally starting to happen on site for our family of four, building a home in West Auckland.
Our south-facing site (with a pretty good sized slope) has meant that our architect has gone slightly outside the box in the ground floor design.
Well actually, he's gone inside the box. Inside a concrete box - and designed a self-buttressing structure to form the base of the house.
What is a self-buttressing structure?
The whole bottom floor is a solid concrete box: concrete slab on the bottom, concrete block walls and a suspended concrete floor which forms the ceiling of the ground floor (and the base of the first floor).
And rather than using steel to provide the structural support, all that concrete does the job instead.
Mr BuildingBoxes suggests it's like a cardboard box which can be easily flattened, until the lid is folded in and then become much more rigid. And of course it's concrete, so it's already super strong.
There is steel, just not the amount we might have needed without this design. And once all that concrete is insulated inside and out, it becomes the thermal mass to give us a passive heat source and form the basis of the passive design of the project.
Passive design is quite a green buzz phrase, but the principle has been around for a long time.
Boiled down to its essence, it's about using free, renewable sources of energy such as sun and wind to provide household heating, cooling, ventilation and lighting.
As it's been explained to me, the most important part of the design process is to assess the location and orientation of the site: get the sun in, but not too much, and when it's in, keep the heat.
It's about making living in the house super comfortable, so there is good, fresh, air flow, and just the right about of heat.
There is a German standard in passive design, but we can't afford to go through the whole Passivhaus certification, so we're using the principles as far as we can.
And for us that means a big thermal heat sink, (the concrete) lots of insulation, (Knauf XPS on the outside of the concrete and DriTherm on the inside as well as Earthwool, Glasswool in the walls and ceilings), double glazed windows, and a high spec ventilation system (a Zehnder system from Fantech).
I'll talk more about all of this another time.
Concrete. Lots of concrete.
It's kind of an oxymoron as concrete itself isn't exactly a super green building material, but once it's in, we're going to make it work pretty hard for us.
From providing that structural support, to sucking up as much heat as possible in the suspended concrete top floor, it's got a lot to do.
Lucky it started its life in a pirate ship… Okay, not a ship, but the guy from the concrete pumping team had our build crew folded over like something from a French bakery because he had the best cackle. And an eye patch. I asked him if he had a parrot and he gave me a knowing look, and a wink.
Now that the first round of concrete is down (four whole truck loads), the concrete blocks will go up, (to be filled with more concrete) before they get topped off with… more concrete.
It feels like things are about to speed up, but it's one of those parts of the build that will provide a quick, visual burst of progress, only to be followed by all of the other work you don't see as easily: the retaining, backfilling, and… more concrete.
Catch up on the progress from week one below;