Entering waffle pod slabs into PHPP

19 October 2021 by Jason Quinn

Entering slab-on-grade floors into PHPP is easy when continuous underslab insulation is the main core of the slab. The insulation has a constant thickness; simply enter that value and the thermal conductivity into PHPP on the U-values sheet. Then correct for the slab edge and slab thickening thermal bridges by entering their PSI values in the Areas sheet of PHPP.

Figure 1: Enter slab on-grade with area and PSI values. Image based on photo courtesy of Josh Chapman

Waffle pod slabs are different

Not completely different, but the main core of a waffle pod slab is a field of … waffle pods, plus concrete ribs. The ribs exist to stiffen the slab in the case of poor ground conditions. The heat transfer through these waffle pods and ribs is a fully three-dimensional (3D) heat transfer problem and it needs to be simplified before it can be entered into PHPP.

Figure 2: Waffle pod slab ready for pouring concrete. Photo courtesy of Josh Chapman of jkbuild.co.nz/ – YouTube.

Figure 3: Waffle pod slab on-grade made up of a concrete slab on top with concrete ribs (1) with waffle pods (2) below. Passive House projects often use another layer of insulation on top of the concrete or below the ribs to increase the performance.

There are three types of waffle pod slabs commonly used in New Zealand.

    1. Most common is a moulded EPS pocket pod (220mm or 300mm high versions), 1100×1100 mm wide with either 64 or 81 symmetric air pockets. Manufactured by injecting EPS foam into a mould. Air pockets reducing both cost and the amount of EPS.
    2. Solid EPS pods are the same dimensions but hot wire cut from a larger block of foam and have no air pockets.
    3. Moulded plastic pod without EPS foam. (Examples are

Here’s how we recommend simplifying the 3D heat transfer problem. 3D model a symmetric section of a waffle pod slab of infinite size and determine the equivalent thermal conductivity of a layer that could replace the waffle pods and concrete ribs while producing the same heat transfer result. This is described in the High-Performance Construction Details Handbook we wrote and a summary of calculations is presented in Element J, where equivalent thermal conductivity of a generic air pocket EPS waffle pod and a solid EPS waffle pod were provided. These calculations are for the specific geometry and EPS thermal conductivity used. If you want to use these values in your certified Passive House project, check with your certifier if they will accept the use of these values. The Sustainable Engineering Ltd certifiers will accept these values.

In PHPP on the U-values sheet, replace the layer of foam pod and concrete ribs with the noted equivalent thermal conductivity from Table 1 below. The U-values sheet will calculate the floor construction U-value. This neglects the impacts of the ground, but the Ground sheet uses the floor construction U-value to calculate the ground temperatures and the heat transfer to/from the ground.


Table 1: Equivalent thermal conductivity calculated in accordance with ISO10211:2017 for a generic waffle pod with 100mm concrete ribs at 1200mm centres in the field of the slab are assumed. Note these numbers have been updated since the V1 release of Quinn, Jason E., “PHINZ High-Performance Construction Details Handbook,” 2021.

waffle pod type Equivalent thermal conductivity
220mm moulded EPS pocket pod λequiv = 0.40 W/(mK) over 220mm height
220mm solid EPS pod λequiv = 0.32 W/(mK) over 220mm height
300mm moulded EPS pocket pod λequiv = 0.46 W/(mK) over 300mm height
300mm solid EPS pod λequiv = 0.25 W/(mK) over 300mm height

Figure 4: Example of a waffle pod slab. Note the concrete rib in this CAD drawing. Credit: Sustainable Engineering Ltd, High Performance Construction Details Handbook.

Figure 5: PHPP U-value calculation for example waffle pod slab.

Figure 6: Example of a waffle pod slab edge PSI calculation. Note the missing concrete rib. The rib is missing because the thermal impact of the rib is included in the material (220mm MouldedPodOnGround_EQ) below the slab in the core.