Page 81 - Build 151

Modelling results
Figures 1 and 2 show the modelling results for a square 100 m2 oor 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 xed at 600 mm. Including insulation underneath the oor slab provided a signi cant 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 ine ective, 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 e ective (see Figure 2).
In practice, it is often di cult 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 eld 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 oor slab monitored in Christchurch (see Figures 3 and 4) provided results for a heated slab in a climate with a signi cant di erence in temperature between the interior oor surface and the outside air and ground temperature.
The oor slab system was a wa e-pod style incorporating 220 mm high EPS pods.
Because the insulation needed to be retro tted, only a short section of the Christchurch oor 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 signi cant 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 oor 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
10
9 8 7 6 5 4 3 2 1 0
without perimeter insulation with perimeter insulation
Figure 3: Insulation and monitoring system retro tted to sections of the Christchurch oor slab.
May June July
August
September
Figure 4: Heated residential oor slab in Christchurch.
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 di erence 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 di erence in heat loss of approximately 900 kWh.
Savings recover cost relatively quickly
The estimated cost for the perimeter insulation, including tting to a new oor 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 oor 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!
For more A BRANZ study report containing full results of this research will be freely available shortly from www.branz.co.nz/shop.
Build 151 — December 2015/January 2016 — 79
Perimeter heat loss (W/m)