125 Slab Edge Insulation

Henry Edney, General Manager - Sales at MAXRaft
Henry Edney, General Manager – Sales at MAXRaft

Why is slab edge insulation so important? Surely it’s the roof we should all be worried about, right? Doesn’t heat go up?

This is true, but the ground is cold. More importantly, the air right near the ground is really cold in winter. And this really cold air is right up close to one of the thermal weak spots of most houses.

MAXRaft is a fully insulated concrete foundation system.
MAXRaft is a fully insulated concrete foundation system.


MAXRaft first appeared on the Home Style Green podcast in episode 20, way back in February 2013. Since then the company has grown, along with the increasing demand for better homes. That growth has been in part due to the efforts of Henry Edney who came on board over a year ago in a sales and marketing role.

Thermal modelling clearly shows why slab edge insulation is important.
A surface temperature of less than 12 degrees in most parts of New Zealand means a risk of condensation. Thermal modelling clearly shows why slab edge insulation is important.

Improvements in Slab Edge Insulation

When I saw Henry again at the recent BuildNZ expo here in Auckland, he filled me in on a couple of exciting innovations they’ve been working on including an XPS option and a rigid board protective edge for their full slab insulation solution.

I’m very pleased to have MAXRaft back on the show to share this update.

The option of more robust, denser extruded polystyrene (XPS) around the edge, is a recent enhancement of the MAXRaft system.
The option of more robust, denser extruded polystyrene (XPS) around the edge, is a recent enhancement of the MAXRaft system.

A Fully Insulated Slab

If you’re at all interested in building a better place to live, you need to start with good foundations. There are other solutions out there, but MAXRaft offers the only complete system here in New Zealand that is fully insulated.

The resulting slab has an impressive R-value of up 4.5, about three times the standard. For an investment of only about 10% above the standard slab price, this is well worth the comfort and energy savings.

To find out more, check out the MAXRaft website, and let them know that I sent you!

Is This an Advertisement for MAXRaft?

Does this post, and this podcast sound like an advertisement for MAXRaft? Feel free to leave a comment below. I’m interested in your thoughts.

Yes, this is a sponsored post. Having sponsors for the show helps me to get the message out about ways to make homes better. But I wouldn’t have just anyone on the show, only products that I believe in and know will do a good job. If you disagree with this strategy or have suggestions for other products you’d like me to look into, then please let me know.

Wherever you're building, you should probably have a fully Insulated foundation.
Wherever you’re building, you should probably have a fully Insulated foundation. Click the image to find out how (sponsored link).


Sponsored Link
Sponsored Link

  • The facts on Polystyrene here are bassackwards, but you are on the other side of the world. That’s all I will say about that.

    • mcutlerwelsh

      Feel free to enlighten and correct us. There’s some confusion out there about polystyrene. I’m interested to know what is backwards.

      • Hello Mathew,

        My name is Fabio Parodi, I am a CPEng Structural Engineer with an academic
        background in green design. I am cofounder of Cresco Group (www.cresco.co.nz), an international engineering firm with projects on 32 countries.

        There are several drawbacks in using Expanded Polystyrene (EPS).


        (1) especially when used on building sites (where cuts are required) EPS produces small particles very difficult to collect and dispose

        (2) In New Zealand* and Australia EPS still might contain a dangerous chemical called HexaBromoCycloDodecane


        (3) EPS is uneconomical to recycle (resulting one of the less reclaimed plastic material in many countries – http://www.reclaim.co.nz/productitems.php?id=anytime4908294936a39) and, when contained in a waffle slab, it is virtually impossible to be separated from the other debris in case of demolishing.

        (4) EPS carbon footprint is affected by fuel consumption in freights (typically for a slab with a 180 m2 floor area a 40ft container of EPS is needed).


        (1) Insulation: Long term EPS (and XPS) insulation performance tends to degrade. The two primary mechanisms for the loss of R-value of insulations are water absorption and loss of blowing agents.

        (2) Mechanical:
        One of the reasons (maybe the primary reason) of the EPS success is that it is cheap compared to other options. High density EPS and XPS have acceptable (or good) creep performance and therefore they are suitable to bear loads without incurring in long term deflections. Unfortunately, high density EPS and XPS are more expensive than conventional types and only a fraction of EPS used to insulate slabs is high density type…

        (*) New Zealand has a notice of non-acceptance in place for HBCD, which was listed under the Stockholm Convention in 2013. This means that New Zealand will face no obligations towards
        HBCD-containing material under the Stockholm Convention until it chooses to remove that notice.
        There is a voluntary industry accord in place for HBCD which represents an understanding between signatory companies to move away from HBCD towards alternatives. NZ Ministry for the environment is not a party to the accord and does not have the ability to enforce its terms. The NZ Ministry for the Environment is continuing to work with industry to confirm supply settings are right before regulating HBCD which we anticipate will be late 2016 or early 2017. This would include HBCD in building products.

        There are better (and not necessary more expensive) alternatives to EPS. I am happy to provide more information if you like.

  • Pingback: Concrete Slab Edge Insulation Update()

  • Pingback: Thermal Mass()

  • Kara

    12.6C are the minimum to avoid mould growth at 20C indoor air temperature and 50% relative humidity, as this will lead to water activity at surfaces exceeding 80%, which is all that many mould fungi need to grow (in addition to ubiquitous substrate). But: if your indoor relative humidity is >56%, the same is true for 14.4C. So: 14.4C is certainly better than 3.6C, but by no means safe. To achieve safe temperatures at the edge, you either need to put insulation on top of the slab, or cantilever the wall to allow for more insulation around the perimeter. So: Maxraft is on the right track – but there’s still some way to go.

    • mcutlerwelsh

      I think that’s a good summary of (parts of) the NZ building industry Kara: “on the right track – but there’s still some way to go”!

  • Robert Potts

    Floor Heating
    If the correct planning is not followed for floor heating design and installation then it should be omitted from the project.
    The building will provide the owner with poor personal comfort and associated high running costs.
    Key points of correct design are:
    1. A structural slab is poured
    2. Walls are built and the roof is constructed
    3. Insulation and thermal perimeter insulation is fitted to all inside/outside walls.
    4. R1.9 insulation is installed on the structural floor. This is required by the NZBC when heating floors
    5. Pipes are laid in circuits for each room
    6. Small aggregate concrete of 90mm is poured over the top
    7. Floor coverings are important. Concrete, tiles and vinyl have 100% efficiency. Timber has a 60% efficiency and
    carpet is rated at 50% efficiency.
    8. Energy source must have weather compensation ability for the modulation of the water temperature pumped
    through the floor. This prevents over/under heating and increases the efficiency of the energy appliance.
    9. This will give the house owner a system that will give them a comfortable house with low running costs.
    10. This method is a European Standard.

    There is a standard EN that we install our floor heating systems to which is a fully thermally broken floor on a room by room basis. Only when this is done the NZBC of R1.9 is acheived.
    It makes no sense that when one room has reached temperature and turns of that the room/s adjacent carry on heating. Thius causes heat transferance