Heating costs could be cut if we do as Americans and Europeans do and add perimeter edge insulation to heated concrete floor slabs.
A BRANZ BUILDING RESEARCH LEVY -funded project has been investigating the practicalities and effectiveness of adding exterior perimeter edge insulation to concrete floor slabs. This is common practice in North America, Canada and Europe but less so here. Reasons include that it can be difficult to install and make durable and that it can be visually intrusive.
Particularly useful for rebuild in Christchurch
The BRANZ project aimed to counter negative perceptions by demonstrating how it can be done and the performance that can be achieved.
The option to add exterior perimeter edge insulation to floor slabs is particularly pertinent for the rebuild of Christchurch. These floor slabs may contain more concrete and steel and the ground underneath the slabs is compacted to a higher density, which increases potential heat loss from the slab.
Perimeter insulation is most effective with heated floor slabs, including ones that capture solar energy.
Research involved modelling and monitoring floor slabs
As well as using computer models to predict performance, the project monitored heat loss over the winter months from three floor slabs at BRANZ and a residential floor slab in Christchurch.
Exterior perimeter insulation consists of rigid foam insulation applied to the vertical face of the exterior edge of a floor slab. The foam insulation normally extends from just below the bottom edge of the exterior wall cladding to the bottom edge of the wall footing. It is protected from impact damage and moisture accumulation.
Figures 1 and 2 show the modelling results for a square 100 m² floor slab with a 40 m perimeter.
Figure 1 shows the impact of changing the thermal resistance of the perimeter insulation while keeping the vertical height fixed at 600 mm. Including insulation underneath the floor slab provided a significant additional improvement in thermal resistance.
Figure 2 shows the impact of changing the height of the perimeter insulation while keeping the thermal resistance at R1.0. A thermal resistance of R1.0 can be achieved using 30 mm of good-quality extruded polystyrene foam (XPS). A greater thickness (35–45 mm) will be required if expanded polystyrene (EPS) is used.
Key modelling findings
Increasing the thermal resistance of the perimeter insulation above R1.0 is relatively ineffective, and even using only R0.8 (25 mm of XPS) should still provide a reasonable improvement (see Figure 1).
Likewise, increasing the height of the perimeter insulation above 0.6 m becomes increasingly less effective (see Figure 2).
In practice, it is often difficult to extend the insulation much below the bottom edge of the footing. The results of modelling for larger slabs lead to the same conclusions about the optimum perimeter insulation R-value and height.
Field trial in Christchurch
XPS was chosen for the field measurements because it enables a thinner and therefore less visible insulation system. For the same reason, 3 mm grey uPVC sheet was used to protect the insulation.
Another reason for selecting XPS rather than EPS or alternative foams such as polyurethane, polyisocyanurate or phenolic was that it has a history of successful use in this type of application.
The floor slab monitored in Christchurch (see Figures 3 and 4) provided results for a heated slab in a climate with a significant difference in temperature between the interior floor surface and the outside air and ground temperature.
The floor slab system was a waffle-pod style incorporating 220 mm high EPS pods.
Because the insulation needed to be retrofitted, only a short section of the Christchurch floor slab perimeter was insulated and monitored. A nearby section of uninsulated slab perimeter was also monitored.
Heat loses revealed
Measurements were carried out over the 2015 winter months (see Figure 4). Measurements included March and October, but the months with significant heat loss from the perimeter of the slab were from April through to and including September. The data points are averages over successive 24-hour periods.
Typical floor slabs, including this one, often have a total slab perimeter length of 70 m or more. While the heat losses shown in Figure 1 may seem relatively small, the heat loss is occurring 24/7 for 5 months and needs to be integrated over the 70 m perimeter length. The average heat losses over 5 months are 2.2 W/m for the slab perimeter with perimeter insulation included. This compares with 5.7 W/m for the section without the perimeter insulation.
Integrating the difference of 3.5 W/m over the 70 m perimeter length gives an estimated heat loss of 245 W. When integrated over the 3,660 hours of the data, this gives an accumulated difference in heat loss of approximately 900 kWh.
Savings recover cost relatively quickly
The estimated cost for the perimeter insulation, including fitting to a new floor slab and the uPVC sheet for protection, is around $20/m.
For a slab with a 70 m perimeter, that would be an additional $1,400 for the cost of the floor slab. Heat-loss reductions in the order of 900 kWh per annum would recover that investment relatively quickly. At a standard rate of 20 cents per unit of electricity, that heat loss equates to a saving of $180.
The only downside is that the plants near the edge of the slab will lose their indirect frost protection!
A BRANZ study report containing full results of this research will be freely available shortly from www.branz.co.nz/shop.
Articles are correct at the time of publication but may have since become outdated.