The New Zealand Building Code clause H1 (Energy Efficiency) [NZBC-H1] states that floors are required to be insulated to a minimum of R-value R1.3. Regarding concrete floors, it further notes that “The floor R-value is met by concrete slab-on-ground” i.e. no insulation is necessary under an unheated concrete floor that is laid directly on to a prepared ground surface. Nor is there any requirement for slab edge insulation.

Of course the situation is different if the concrete slab is suspended above the ground surface so that there is free-flowing air beneath, as with a timber floor, in which case the construction R-value is to be a minimum of R1.3. The normal “slab-on-ground” has an R-value very substantially below R1.3 depending upon concrete thickness, type of hardcore and earth below, height of water table, etc.

(Note that with a heated floor the R-value rises to a minimum of R1.9, with the thermal resistance of the construction from the heating plane to the inside air to be less than the resistance to the outside air. A heated floor slab requires careful and specific insulation design so as to maximise the benefit of such a system.)

To achieve compliance with NZBC-H1, carpet or floor coverings are excluded from the makeup of the concrete floor’s "construction R-value".

With the Building Code not requiring any insulation under a slab-on-ground, nor edge insulation, why is it that there is an emphasis on slab insulation when the industry and real estate market are happy to work with Code minimums for the thermal performance of walls, roof, and glazing? Of course concrete floor slabs, and all aspects of a building, should ideally be constructed to greater than the minimum requirements of the Building Code, NZ Standards, etc. but so often they are not.

When I began my career, if a slab was to be insulated it was only a 4’0” strip of polystyrene around the perimeter – a recommendation still supported today. By leaving the centre portion of the floor in contact with the earth, there is additional thermal mass available to further moderate the temperature fluctuations in a room with an exposed concrete floor. While there is this emphasis on the heat loss from slab-on-ground floors, most are actually carpeted – especially the living spaces and bedrooms – resulting in the thermal mass being substantially removed from the interior air, making the under-slab insulation much less effective.

I am intrigued by the current promotion for thermal insulation to be applied to the outer face of concrete floor slabs. Yes, I know we are told that research shows that around 80% of the heat in a slab escapes through the slab edge, which of course it does. There is little difference in temperature between the centre of the floor and the ground below it – compare this to the rapidly fluctuating temperature of the constantly moving exterior air at the perimeter. When speaking with architect and designer colleagues I find that there is nowhere near the enthusiasm for the detail as is suggested by the various promotions, especially if carpet is to be fitted over the slab.

Once cured, the outer face of a concrete slab is quite waterproof and, being vertical, it sheds water very quickly so there is little time for it to soak into the surface, leaving the exposed concrete dry with the overhanging cladding above giving additional protection. For those faces that are exposed to the sun there is additional drying and a thermal gain that transfers into the floor behind to partly counter any nighttime loss.

When a layer of insulation is applied to the slab edge, yes there is a slower heat transfer, but the drying and heating effect of direct sun exposure and the moving air is also lost. Most insulation systems have a robust plaster-like outer coating as protection to the insulation but if this is breached – by gardening, lawn-mowing, accidental impacts, shrinkage, etc. – then water can enter through the damage or the hairline cracks and then not be able to escape so it is pushed further in as more moisture enters the insulation. Also, due to the thickness, there is the potential for water to enter from the top surface and by wicking up through the bottom edge that is usually below ground level. 

It is common knowledge that when insulation becomes wet its thermal performance rapidly diminishes, which is why there is such an emphasis on keeping it dry in walls and roofs. While polystyrene looks as if it doesn’t absorb water, the ‘weathertight homes’ problems demonstrate otherwise. Once wet, the value of the insulation is greatly reduced. Also the water within will be attracted to the adjacent dry concrete making it damper than if exposed to the air, and therefore of greater thermal conductivity allowing the faster heat transfer from the room above. There will also be small gaps and crevices between the insulation and the slab edge that will hold free water, allowing faster absorption into the concrete.

Last millennium the industry told us that the outer coatings of monolithic insulated claddings would keep the timber framing dry; are we repeating the situation? It is my understanding that the gains shown by thermal modelling research are based on the assumption that the insulation remains dry. Is this a realistic simulation of actual concrete floor slabs over their lifetime? If not then how much gain is there and is this worth the trouble and additional expense of applying insulation to the edge of a concrete floor, especially if it is to be carpeted?