R-Values RibRaft XPod vs polystyrene RibRaft


We have seen a recent evolution of concrete floors from the NZS3604 traditional slabs-on-grade through to the polystyrene waffle slabs to the recent RibRaft Xpod.Fabio Parodi





Throughout the last 10 years the thermal efficiency of houses has become increasingly important and considerable improvements have been made through better performance in the design of windows, walls, roofs and foundations.
New Zealanders are adopting timber floor houses for concrete floor houses and this has seen the most significant results for foundations with added thermal efficiency.
We have seen a recent evolution of concrete floors from the NZS3604 traditional slabs-on-grade through to the polystyrene waffle slabs to the recent RibRaft Xpod.
With this article I will answer our most commonly asked questions:
What is the difference in thermal insulation of the new RibRaft Xpod when compared to a traditional polystyrene RibRaft?”
The answer is:
“The thermal insulation delivered by a RibRaft Xpod are similar to a polystyrene waffle slab.
To be able to understand this statement here is some information from the New Zealand Building Code for R-value calculations.
In building and construction, the R-value is a measure of how well an object, per unit of its exposed area, resists conductive flow of heat: the greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the object. (1)
The minimum R-Value in New Zealand required for floors are R1.3 for light timber frame construction, and for masonry construction typically R1.5. R is needed values of R1.3 can be used for masonry construction if glazing with greater insulation is used (refer NZBC, Clause H1). If in-floor heating is used the minimum required R-Value is increased to

The fourth edition of H1 (amendment 3, January 2017) states that “Concrete slab-on-ground floors are deemed to achieve a construction R-value of 1.3, unless a higher R-value is justified by calculation or physical testing.
For new houses with double glazing using a concrete foundation even without underfloor heating your R-value will be in accordance with the NZBC.
In other circumstances, there are other methods available for a designer to calculate the R-value, these however give quite different R-values.
NZBC clause H1 prescribes that an “Acceptable methods for determining the thermal resistance (R-values) of building elements are contained in NZS4214.”
If the R-value calculation must comply with the Building Code, then the NZS4214 calculation is probably the best alternative due to its explanation in
H1. This method appears to have been used to calibrate the minimum R-values stipulated in H1.
In my opinion, the correct road to demonstrate compliance with the limits set out in the building code is to follow the NZS4214 although it has been proven the results obtained with its methodology are often considered inaccurate.

This table shows that a polystyrene RibRaft and a RibRaft Xpod have similar thermal efficiency in terms of compliance to the NZBC.
It also demonstrates that, in some circumstances, the compliance to NZBC for buildings with underfloor insulation can be achieved only by introducing additional insulation (e.g. perimeter insulation, underfloor insulation. See Is it worth insulating a concrete foundation?).
If in New Zealand is that they don’t want to demonstrate compliance to the NZBC but is to determine more accurate R-Values, then more technical in- depth calculations, such as two-dimensional thermal modelling, should be used.
Table 2 – R-Values calculated with two-dimensional thermal modelling for a conventional polystyrene RibRaft, a RibRaft Xpod and a NZS3604 slab on grade used in combination with a 90 mm wall framing.
From the above table it appears that, in terms of its thermal properties, the polystyrene RibRaft and RibRaft Xpod should be considered to be similar (although slightly better) than a traditional concrete slab on ground.
It doesn’t matter if within a foundation polystyrene pods, air pockets or nothing is used: as its thermal properties will end up being similar.
Both for a polystyrene RibRaft and for a RibRaft Xpod, a large portion of the heat flow happens at the thermal bridges where there are ribs and at the edge footing locations where they are in direct contact with the ground and the elements.
There is a simple way to explain such a counter-intuitive result:
You can put your hand 10cm away from a burning candle without getting burnt. This is possible because of the excellent thermal properties of air that insulate your hand from the flame.
But what happens if you hold a piece of steel onto the flame? Very soon you’ll get burnt!

In this configuration the insulation provided by the air is compromised by the piece of steel that acts as a thermal bridge transferring most of the heat flow to your hand.
The ribs and the edge beams of the foundations act like the piece of steel conducting a large portion of heat regardless the insulation offered by the polystyrene pods or the air pockets.
There are other ways to improve the insulation of a foundation but the solutions are not always worth the investment and in some cases they might be detrimental to the structural integrity of your slab (See Is it worth insulating a concrete foundation?)
However only a fraction of the heat loss occurs at the foundation location (because the soil itself provides some insulation) and only when underfloor heating is used the introduction of an additional insulation may be necessary.

Fabio Parodi
CPEng (NZ_AU) MIPENZ Dott.Ing(ITA) M.Eng(Hons)
CEO and Founding Director of Cresco 

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