iHaving insulated the loft space and the underfloor it was just the walls left to do and there only really seemed to be three options -
1) Remove the gib, insert batts behind, then reinstall gib and replaster
2) Install polystyrene sheets on the inside of the external walls and plaster over the top (not sure what happens to plug sockets though so that wasn't really an option
3) Airfoam - a cavity wall insulation inserted into the walls
In the end it really was a no-brainer - the re-gibbing, though not difficult, would become very expensive after a while - each sheet of gib costs $25 and covers 1.2m *2.4m - with 3.6m high walls it doesn't take much to work out that a single wall would probably cost about $500 to regib in gib board alone, but then you add the labour and the batts and the plastering... Also, our house has what's called sarking, or horizontal boards behind the gib, which add insulation, and are very useful if you want to fix anything to the wall - and to fit batts they'd all have to go.
So we went for Airfoam which seems to be the only provider of cavity wall insulation in NZ. Because our house (with the exception of the bathroom which we insulated anyway) hasn't been decorated we elected to have the foam squirted in from the inside - this reduced the cost a bit (presumably because it wasn't necessary to hire scaffolding).
The process only took a day, and cost $4000. As you can see they drill holes between each joist and squirt in the foam which starts at the consistency of shaving foam, before firming up over a couple of days. The company then patch up the holes left behind ready for repainting.
After it was done we were supposed to leave all the windows open for a month to ventilate and allow the foam to cure! Which was a problem as we had it installed in the middle of winter so we didn't ventilate much, but lived to tell the tale. Anyway a month or two since it's difficult to quantify the effect, but basically unless we're in the middle of a "polar blast" the house is warm enough just with the electric heaters, so I think that's a notable improvement. We definitely won't save on heating bills, but just making the house a nice temperature was always the main goal so we're pretty happy.
