Insulating the perimeter of the foundation slab is one way to increase the thermal performance of your home. Depending on the circumstances, combining underslab insulation with slab edge insulation can result in thermal performance of the slab improving by 100% or more. Perimeter insulation can bring significant gains in energy efficiency.
Changes to the insulation requirements within the NZBC has meant a push to reducing the thermal loss from the floor slab. There are a few ways of reducing this loss, one is install insulation under the slab the other is to insulate the perimeter foundation of the slab.
Perimeter insulation generally consists of a rigid foam insulation such as Expanded or Extruded Polystyrene applied to the vertical face of the exterior edge of a floor slab. The foam insulation extends from just below the bottom edge of the exterior wall cladding to the bottom edge of the wall footing.
The exterior of the perimeter insulation should also be protected from impact damage and moisture accumulation with additional layers of plaster (including Hydroplast)
By insulating the perimeter this will give a relative improvement to the thermal performance of the floor slab.
The main reason we are seeing the uptake in perimeter edge insulation is when there is a underfloor heating system being utilised.
Typically a 100mm floor slab with no insulation and 600mm foundation wall would have an R-Value of around 1.01
By adding on 10mm XPS Insulation with an R-Value of 0.35 you will increase the overall R-Value of the floor slab to approximately 1.43
If you keep a gap of no more than 20mm between the bottom of the cladding and the insulated foundation you will get some thermal loss, this would only reduce the R-Value to around 1.31 still an overall improvement of 0.3 for the addition of 10mm of XPS to the foundation.
Combining this with Underslab insulation (such as 50mm EPS) will yield even larger increases.
It has been shown that using polystyrene any thicker than approximately 50–60 mm EPS or 30–35 mm XPS achieves only slight improvements in R-value. It is generally accepted that 50mm EPS insulation is adequate unless the designer is trying to achieve a zero-energy house using construction methods with better than average R-values for other building envelope components.