We are utilising and developing a new building system for the iDEAL House, called batten and cradle. In our quest to find the best way of insulating the slab, we have decided to insulate above, not below the slab. There are pros and cons with this method.
One major con is that we lose thermal mass inside the home, but unless you have advanced shading systems installed on the home, it can also lead to overheating if you can’t keep the sun off the slab. A second con will be the additional cost of the labour and materials to construct this – but we’ll have a much better understanding when we’re completed.
The single largest benefit that I see, is after being in involved in the first Passive House in Australasia is the highly difficult task of adequately insulating with polystyrene or polyurethane right under the slab, around all the footings, and the side of the slab while leaving no gaps – and avoiding damage while placing the reinforcing steel and mesh. Then there’s the issue of plastering the polystyrene on the side of the slab – and the ongoing issue of protecting that edge from lawnmowers, cricket balls, and just normal wear and tear. I even heard of rats burrowing through the polystyrene and making nests under homes – would be a warm place to live! Another benefit is the fact we approached a number of concrete placers to quote on laying the slab, and none of them were interested in getting involved in a fully insulated slab – I’m sure there’s companies out there that do it, but it seemed it was all too difficult for the companies we approached – some of them quite large businesses.
Instead, we will be placing a typical floating pod-floor type slab on the ground (which against marketing spin doesn’t really help with insulation due to the massive amount of thermal breaks and lack of edge protection) using recycled rubber cradles to hold a light, non structural batten system, and then place a false floor above this. In our case we’ve decided to use Secura from James Hardie for its strength and water resistance, but you can also use a strandboard or particleboard type product. Underneath we’ll be placing around R1.2 of Mammoth high density polyester insulation. R values for the walls should be finalised late April.
The benefits of the system:
1. Easier to find a concrete placer
2. Should be less messy to get an unbroken layer of insulation
3. Removes the thermal mass of the concrete to assist in evening out the temperatures, and risk of overheating (we are deterred from mechanical cooling in the home).
4. Apparently significant gains in ergonomics for the hard bamboo flooring (they use this system under gymnasiums)
5. The system was also developed for noise reduction, and has been used recently in recording studios. As we have no carpet, this could be a large gain for us.
More information and pictures, pricing etc. to come in due course.