Thermal resistance of waffle pod slabs for NCC energy efficiency

What is waffle pod slab?

Waffle slab is mainly a cost-effective structural element in the building that also has some energy cost savings benefits. It consists of edge footings and narrow internal strip footings. It is built on the ground and 170 to 370mm polystyrene waffle pods fill the voids between the strips. The polystyrene in the waffle pods and air pockets built into the pods provides an insulation to the floor which can assist with NCC energy compliance and 6 star energy rating.

What is the thermal benefit of using waffle pod slab in energy rating?

Use of almost 200mm waffle pod and unventilated air pockets in it, provides an insulating effect under the building floor. In heating dominant buildings, the extra insulation effect of the waffle pod slab assist with better energy compliance of the building at lower construction cost.

What is equivalent R-value of waffle pods?

Referring to the research by CSIRO, the following insulation values are recommended for the waffle pod slabs:

Waffle pod height: 170mm – R0.57

Waffle pod height: 230mm – R0.6

Waffle pod height: 300mm – R0.63

Waffle pod height: 370mm – R0.65

Note that in the above, waffle pod is assumed to be composed of a 50mm polystyrene layers and un-ventilated air pocket.Note that increasing the waffle pod height doesn’t add much to the insulation effect. Double the waffle pod height increases the added insulation value of the waffle pod by 14%.

The waffle pod slabs may be modeled by using a concrete top layer at the specified concrete thickness of the waffle pod slab system with an additional insulation layer of R0.57, R0.60, R0.63 and R0.65 for the 170mm, 230mm, 300mm and 370mm pod heights between the concrete top layer and the ground.

The following shows an example of waffle pod in one of our Energy Compliance reports:

Disadvantage of waffle pods in building energy efficiency

Addition of insulation by waffle pods disconnects the ground thermal mass from concrete thermal mass. This means that the use of waffle slab in a cold climate reduces the thermal mass of the floor, hence less energy is stored during the day.

Read more

NatHERS energy Star Rating

Section J-JV3 modelling (Verification Using Reference Building)

 

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