So what’s the Plan?

C and I have always had a pretty clear vision of how the house would look in terms of it’s form – clean, minimalist look with Scandinavian-style touches: whites, concrete, timber, lots of natural light etc.

What perhaps took a bit longer to come together was the function, but that’s to be expected – you don’t know what you don’t know, and one of the reasons I wanted to do this blog was offer the insights I didn’t find when I started researching construction methods.

So in term of function, what did we want?  Because we’d already settled on having polished concrete in the living area/kitchen/bathroom etc, we’d also decided in pretty short order that we needed underfloor heating.  As a result of that, it became pretty obvious we needed to spend our money on a solid level of insulation to protect the ‘thermal envelope’:

  • 70mm Expol ThermaSlab H (under-slab insulation)
  • Expol Expol-X (slab-edge insulation)
  • All exterior walls are 140mm framing (R4.0)
  • Ceiling insulation is R6.0
  • Downlights are all LED, and IC-rated (as are the drivers)
  • Joinery is all double-glazed, thermally-broken, argon filled, with Planitherm XN (Low-E) glass in the living area and bedrooms

Based on my research, the most crucial factor to effectively mitigating thermal loss when running an underfloor heating system is actually the one I see done the least: insulating the edge of the slab.  The so-called Second Law of Thermodynamics states that heat will always flow from an object of higher temperature to that with a lower temperature.  People seem to go way overboard with underslab insulation, and leave the edge of their slabs exposed – underslab insulation is important to be sure, but with a damp-proof membrane down first, and an assumption that there’s no water source running under the slab, the ground temperature directly under the concrete changes very little.

Consider what can happen then if your slab is being heated to 18 degrees, but the outside ambient temperature drops to -5 (as it does where I live).  This is why it’s often stated that ~75% of heat loss occurs through the slab edge.

The problem however is that there seems to be very few effective, reasonably-priced and compliant systems.  Maxraft have a nice system that friends have used to good effect, but I opted for Expol’s solution, as it’s simple to apply, and sufficiently waterproof.  An important point to note, Extruded Polystyrene, (XPS), is much more preferable for this application than Expanded Polystyrene (EPS) due to its closed-cell construction, which means it absorbs much less water, and conducts heat much less readily.

Beyond that, we wanted a robust, clean cladding which fits in with the aesthetic we were after, so we’re using Sto’s Render system over AAC (lightweight concrete).  This gives a significant thermal advantage over standard 70mm brick, the AAC can be installed quickly, and gives much greater impact resistance than polystyrene-based plaster substrates.

One other structural product we’ve opted for is RAB Board, a James Hardie product which is applied directly onto the framing prior to installing the AAC panels  This has a few nice advantages over conventional building wrap, in that it provides necessary bracing elements, it’s easy to fit, and when fitted to spec, it’s weathertight, which allows the builder to crack on with ‘first fix’ tasks without being dependent on a block layer.

The north corner of the living room will be clad in Firth stack bond 10-series veneer block, which is purely for aesthetics.

The house also features a ‘sail wall’ façade of vertical shiplap cedar, that effectively comes inside into the foyer, creating a nice transition from outside to inside.


One thought on “So what’s the Plan?

  1. Thanks for the informative post Chris. Really looking forward to following your progress.
    We have just engaged a designer for our house build so im well down the rabbit hole when it comes to research and reading a blog like this really helps.
    What brand of insulation are you going for?
    As for under floor insulation, what flooring are you going to be using?
    I would love to know more about the other products you have mentioned, like the downlights, windows and underfloor heating system.
    Good luck and looking forward to more posts.


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