Concrete Slab Edge Insulation Update


Updated: See my updated list at the end of this article for currently available products for slab edge, perimeter insulation.

BRANZ recently released their fifth edition of the House Insulation Guide. Changes in this edition include ‘waffle pod’ concrete slab systems, and a concrete slab edge insulation update.

I discussed the importance of insulating the edge of concrete slabs in this video (just moved from it’s previous location here). Since recording this video, I’ve been amazing at three things:

  1. How common it still is to construct totally uninsulated slabs here in New Zealand.
  2. How weird this is for people coming from overseas.
  3. How little extra R-value is added with waffle pod systems (about R 0.2)

Check out Ian Cox-Smith’s summary of the update here.

Why Insulate the Edge of a Concrete Slab?

When you go camping, one of the most important pieces of equipment is a mattress, something to protect your warm body from the cold ground. It doesn’t have to be very thick, but that thin layer of insulation can be critical for keeping you warm.

Why then, are we allowed to build houses without any insulation under the concrete slab? Keep in mind that concrete is actually a very poor thermal insulator (not to be confused with its good thermal mass properties).

The edge is important because that’s where most of the heat (up to 80% according to BRANZ), escapes.

Two other cases where you definitely want edge insulation are:

  • If you want to achieve a Homestar rating above 4.
  • If you’re planning on using in-slab heating.

But I would recommend it all cases.

Two Main options for Concrete Slab Edge Insulation

1. Thermal break

BRANZ now recommend using 10 mm of extruded polystyrene (XPS), where previously they recommend 45 mm of timber.

BRANZ previously recommended timber as a thermal break for edge insulation.
BRANZ previously recommended timber as a thermal break for edge insulation. As well as being tricky to construct, this is now deemed to be seismically risky. Image Source: BRANZ Build 134 – February/March 2013
BRANZ now recommend using 10 mm of XPS as a thermal break for slab-edge insulation.
BRANZ now recommend using 10 mm of XPS as a thermal break for slab-edge insulation. Image source: BRANZ Build 142 – June/July 2014.

2. Insulation on the exterior:

Sticking something on the outside of the slab, or forming the perimeter foundation within an insulating material.

NUDURA perimeter insulation
NUDURA ICF can be used to form the perimeter foundation. A tapered version should allow for a standard 90 mm stud thickness (140 mm shown here)

Product Solutions for Concrete Slab Edge Insulation

There are probably others out there. What option would be your preferred choice? Leave a comment below or on Facebook to let me know.


  • Very good episode Matthew, you do a good job on the solo-casts. And for this episode Nudura was a good match with sponsorship!

    It does make me sad to think just how far the mainstream in NZ is behind in these areas. I have sometimes heard the comment “heat rises” which is possibly why people don’t bother to insulate under the floor – not realising that while hot air rises, heat transfers in all directions to achieve equilibrium.

    As you probably know, insulating on the outside of the concrete (even the very thin bit of Nudura) is going to be more effective than the BRANZ detail. The reinforcing passing through the “thermal break” discounts a large portion of the break!

    And why is there treated timber under the slab edge? To allow for movement?

    • mcutlerwelsh

      Elrond, thanks very much for your comment. I really appreciate your feedback. Always a bit daunting ‘go solo’, but I’m pretty passionate about this topic!

      Good question about the treated timber. I’m not sure. I’ll have to ask the BRANZ boffins.

      • No problem. It’s a good subject to be passionate about since there are easy to understand and implement solutions to what is a major problem!

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  • Foundation Slabs are best when the concrete is on the interior of the structure. The Europeans have nicely solved this with pre-manufactured systems, but you can also make your own from EPS slabs.

    • mcutlerwelsh

      Ah, a fully insulated envelope! Beautiful.

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  • sdemler

    Matthew – Ian Cox’s update mentions a new product is being dveloped that allows an EPS foam layer with plaster preapplied to be slipped into the boxing. Are you aware of this product and how I could get in touch. To my mind this seems like it would be the most cost effective and least disruptive product to most builder’s current approach.

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  • Duncan

    Hi Mathew, great video thanks!
    One thing that I don’t understand, is that if the qualities of XPS are known (and relatively constant), why couldn’t a fully insulated slab be a part of NZS3604, rather than requiring specific engineering? I imagine that something like this will be standard here one day, I hope it is sooner rather than later!

    • mcutlerwelsh

      I hope so too. Until then, the powers that be deem R 1.3 to be adequate for a floor slab and with the appropriate area:perimeter ratio, you don’t need edge insulation to achieve this usually.