Going back to first principles of a passive house the aim is to achieve internal comfort and energy efficiency through five different requirements:
- high levels of insulation – walls, foundations, windows, doors
- a design and construction method that avoids thermal bridges
- an airtight building envelope
- ventilation that makes use of highly efficient heat or energy recovery
- shading and architectural design to prevent over heating
Item 4 is delivered via a heat exchanger, which I will provide more detail on when we install our unit and I can share photographs. Item 5 is delivered through the design of the house; locations and sizes of windows, amount of overhang provided by the eaves, and shading provided by external structures such as pergolas.
Items 1 to 3 relate to the construction materials and methods. When I think of these three requirements I think of the outside edge of the house; floor slab, walls, windows, doors and roof. Each and every one of these external surfaces must tick the box for each of the three requirements so as to keep heat (or cool) inside and control the effects of the outside environment.
People of my age who lived in the UK in the 70s and 80s will remember TV ads for Ready Brek, a breakfast cereal with the stripling “central heating for kids”. The ad showed children going about their day in cold winter weather and they had a a glowing outline to show they were toasty warm (see the screen grab below). When I think of PassivHaus I think of it in this way – holding warmth inside, or keeping excess heat outside!
In a previous post we covered the foundations and how the concrete slab is insulated and avoids thermal bridges. Needless to say, a concrete slab naturally achieves airtightness! I will address doors and windows in a future post.
Our build makes use of THECApanels for both the external walls and roof. The panels are made here in Christchurch by Theca using as many local products as possible. The house design is converted to 3D drawings and put into a CAD software system to enable sizes to be determined that are accurate to the nearest mm. Such accuracy is a significant contributor to the speed of construction and the airtightness requirement.
The wall panels are 185mm thick, and the roof panels are 300mm thick. They comprise an engineered timber frame, a layer of Pro Clima wrap for waterproofing (the blue layer), and a layer of Intello wrap for airtightness (the white layer) and in between a recycled fibreglass insulation “fluff” is pumped to fill the cavity. All holes, such as the one made to insert the insulation fluff, are sealed using a tape that is compatible with the Intello wrap and is also airtight.
It is important that the wraps are larger than the actual panel as they are overlapped and taped down once put into place, so as to ensure the airtight envelope is constant.
It’s like a big reverse vacuum cleaner that pumps the fluff into the cavity:
The fluff is in big bricks that have been compressed and the pumping machine separates and aerates it to make it fluff up:
The panel below has the window inserted already (it wasn’t for our house) but it was decided to install the windows onsite in our build so as to allow us to stack them one on top of each other on the flat-bed trailer for transportation from the factory to our house. Similarly, the panel below already has the battens for the service cavity installed, whereas we installed ours onsite.
It’s imperative that no penetration is made of the airtight membrane as this would be a compromise and could lead to failure of the blower door test (and thus no PassivHaus certification). It’s such a critical factor that Glenn, our Passive House Consultant provided Pete, our foreman with signs like this to place prominently all over the site!
There will be a service cavity between the Gib internal lining and the Intello membrane, into which will be placed all cabling, plumbing, plug sockets etc. Of course the cabling has to go from the ceiling to the walls, and that’s where a bit of cunning came into play – circles were cut in the factory at the end of the ceiling units. These circular holes will allow the cables in the ceiling to pass through and then down the back between the Intello wrap on the wall and the solid wooden end of the ceiling unit. The insulation layer provides good cushioning and is able to be flexible enough to allow the cables to pass easily. Cables going right the way down the wall will pass between the battens and the Intello all the way down the wall until they arrive where they need to be.
The above photo shows where the tape has been placed over the holes made to insert insulation and vertically where the Intello wraps of the two adjacent panels were overlapped.
At the junction with the slab the Intello is glued to the slab to finish the airtight seal.
Guy and Pete from Ethos Homes spent a lot of time with Glenn at Theca to plan the sequencing of placement of panels so as to ensure that at all junctions the Intello wrap could be overlapped and taped. I understand it was quite a long drawn-out process in relation to a few tricky junctions!
So what will we achieve by having these panels made the way they are? Firstly the Intello wrap membrane ensures requirement 3 (airtightness) is met. Secondly, the level of insulation complies with requirement 1 (insulation). And finally, the use of wood and recycled fibreglass fluff mean that thermal bridges are minimised, and so requirement 2 is achieved.
But there is another benefit to a panellised construction, and that is speed to build onsite. While our concrete slab was being poured and then curing, Theca were busy constructing these panels, which meant that the moment the concrete was ready the internal walls could be put in place and the panels brought to site. The entire two storeys and roof took a total of three days to assemble onsite, which is substantially shorter than the many weeks that a traditional build takes.
I took some time to research the NZ building code requirements for wall and roof insulation and learned that the requirement for minimum R values are:
- walls – 2.0
- roof – 3.3
- windows – 0.26
Our house, built with these panels will achieve:
- walls – 4.7
- roof – 8.9
- windows – 1.4 (average)
Clearly a significantly higher level of insulation than NZ building code, and especially in the roof which is the area of an uninsulated house where the vast majority of heat is lost.