This depends upon whether reliance is being placed only on solar / environmental gains, or alternatively on inputs from network energy sources. In my earlier EBOSS Detailed blogs regarding the insulation of concrete floors, (Thermal Mass: Is it Useful? and Is Slab Edge Insulation as Effective as it Seems?), I have stressed that my thoughts relate only to floors which are not heated by a 'Network Energy' source, (as defined in NZBC-H1), which in relation to a building means "...the energy from a network utility operator or a depletable resource." If network energy is used then there are ongoing payments to be made for an 'active floor', whereas with only solar / environmental sources for a 'passive floor', the energy is free after the setup costs are paid, therefore the quality of the insulation design becomes more important in the former than the latter.

From time to time I am asked, in casual settings, about the detailing of insulation for 'active floors' heated with network energy. In these cases I am not able to give specific information because there are many inter-related factors to be considered — my usual answer is, "It depends". Yes, unhelpful I know, but it is very true. Most enquirers begin their slab edge detailing with the standard 10mm thick polystyrene vertical strip below the inner face of the wall framing, or insulation panels fixed to the outer face on the foundation wall / slab edge, but do not see obvious solutions beyond these. In my various blogs discussing thermal mass (Slab Edge Insulation: An Interesting Experiment, Thermal Mass: Is it Useful? and Is Slab Edge Insulation as Effective as it Seems?) I have raised questions about the effectiveness of surface applied external panel insulation. From the comments I received I am still not convinced of the long-term thermal and economic effectiveness of this detail. I also cannot see how a 10mm thick strip of insulation within the slab (regularly bridged by reinforcing bars), is okay when the timber wall above is required to have a construction R-value of R1.9/R2.0. If a floor is to be actively heated then by the NZBC, the construction R-value is to be a minimum of R1.9.

Throughout New Zealand most houses with a concrete floor have some form of covering, be it carpet, vinyl, cork, etc., or a water-proof membrane under tiles. All these provide some level of insulation which reduces the heat transfer ability, be it into or out of the concrete thermal mass. In the primary living spaces carpets provide very effective resistance to heat transfer from and to the slab.

In my opinion, if the brief asks for a 'passive' concrete floor, then after fitting horizontal insulation panels under the slab, the most important features to influence the comfort performance within rooms is first the selection of the site for the house, followed by a carefully and considered plan layout and fenestration arrangement (see Site: It is Never To Early to Begin). If the plan layout does not make the best use of the available solar gains, and the windows are not appropriately sized and located, then fiddling with applied insulation to control heat loss is just playing with the Titanic's deck chairs. Passive solar design is often associated with heating, but the control of the cooling aspects are equally important.

In my opinion, for a house briefed to have an 'actively' heated floor, the first approach to the design of the thermal insulation should not be to look at the insulation, but instead be to maximise the passive heating and cooling so as to reduce the demand for network energy, and then to use zoning to direct the heating / cooling to best satisfy the comfort requirements of the various activities within the building. With this design approach there is less network energy being used and the floor insulation can be targeted to and designed for the areas of greatest need.

The second step is to consider the 'fuel' to be used for the heating / cooling system. Electric resistance cables use only network energy, so the control of unwanted heat losses is critically important. A concrete floor with a circulating hot water hydronic heating system using electricity for a heat-pump or storage cylinder, or a gas or diesel fuelled boiler also requires a high level of insulation. On the other hand, a hydronic system could be solar driven where a large proportion of the input energy is free, and so heat losses are less expensive, and consequently the quality of the insulation is less important.

It is my understanding that to achieve a reasonable level of temperature within a floor slab heated room, the circulating water needs to be in the mid-30°C range for an exposed concrete surface, and the mid-40°C range for a carpeted floor. Given the thermal conductivity of concrete and steel reinforcing (and the need for structural continuity), this makes the design and construction of the slab edge insulation very critical and complex if the slab edge heat losses are to be kept at sensible levels. I don't see how a 10mm thickness of polystyrene is sufficient to contain the adjacent 35°C — 45°C temperature of the concrete. As I have said in previous Detailed blogs, the design of effective and economical thermal insulation for actively heated concrete floor slabs is difficult and should be tailored to each of the differing situations and locations around the building's perimeter.

In my view, installing an actively heated floor slab as the primary heating source for a house is not warranted in the geographic locations where the majority of new houses are being built, especially given the difficulties of controlling the significant heat losses when standard construction methods and materials are used. If active heating is still required then the house design should make the fullest use of the freely available solar and environment energy so as to minimise the proportion of network energy required.

I'm sure there are many contrary views and so, given the financial and social importance of energy savings, I welcome comment and discussion on my thoughts as expressed above.