A key part of the design for our house extension was increasing thermal mass, which helps to keep the house cool in summer, and warmer in winter. Two major elements contributed to this: the rammed earth wall and a reverse brick veneer wall.
Of these, we were expecting that the rammed earth wall was going to be the wacky-difficult-to-build element, but it proved easy. In comparison, I’d been reading about reverse brick veneer walls for ages in the green community, and was aware that they are widely used in places such as New Zealand.
And in general, they are simple: you put the single brick wall on the inside, where it provides valuable thermal mass, and you put the highly-insulated wood frame on on the outside. In otherwords, the reverse of a traditional brick veneer construction.
Surprisingly, however, when it came down to finding out exactly how to construct the wall, I came up short. I could find no clear diagrams on the web, nor was this type of construction covered in the books I had on hand. No builder I talked to had ever done one.
So we worked it out ourselves, and in the interests of sharing our experiences, this is what we did:
Layering of our reverse brick veneer wall
From the inside to the outside:
- Gyprock. Traditional plasterboard inside wall surface, nothing unusual there.
- Single brick wall. This is constructed in the usual way, and isn’t load-bearing. It’s anchored to the wood frame via brick ties (lots of them!).
- Air gap. This is one of the key elements of the insulation, just like a normal cavity brick wall.
- Wood frame (4a) and bulk insulation (4b). The wood frame is completely run-of-the-mill, and is a load-bearing structure. Inside the frame, we put Grenstuf R2.5 wall batts, which provides the bulk of the insulation.
- Reflective insulation. Air-cell Permishield provides a layer of reflective insulation, helping to keep radiated heat off the inside layers.
- 20mm batons. The reflective insulation only works when there’s an air gap, which we created via batons nailed through to the wood frame.
- Gyprock Fyrchek MR. Unfortunately our side wall was closer than 600mm to the property boundary, making it necessary to meet fire rating standards. Thus the layer of fire-protecting Gyprock.
- Wallwrap. Strictly speaking, the Gyprock Fyrchek isn’t weatherproof, despite the “MR” rating. So we added a cheap wallwrap, as per Gyprock’s installation instructions.
- Weathertex weatherboards. Finally, the outside cladding!
- This is a lot of layers! There were a few different combinations possible, particularly relating to the Air-cell Permishield (inside the stud wall, air gap inside or outside). I think we made the right decision, but your circumstances may vary.
- The requirement to have the wall fire-rated added a bit of complexity.
- This type of wall is thicker than a traditional double-brick wall (with or without a cavity). So be warned, and factor this into your designs from the outset.
- Make sure you seal the top of the wall (against the weather) as well as the bottom (against bugs). All the air gaps could otherwise cause problems down the track…
The good news is that this type of construction really isn’t any more effort than a traditional brick veneer, and it should pay us positive dividends for the lifetime of the house. As opposed to “normal” brick veneer, which is perhaps the most foolish possible building technique for the Australian climate…