Rendered Foam Walls

This addendum was added to an original Post from 2014 as there has been a lot of publicity recently (late February 2019) about foam panels and certification has been removed from certain types of panels.

A particular issue for apartment blocks has been related to high speed spread across the surface of the panels to other flats.

On a standard house the render should protect the insulation from external flames. (If flames penetrate the plasterboard, from the inside, it is likely that the occupants will either have already evacuated, or be dead before the insulation ignites) n

Nevertheless you should review whether the potential risks from foam panels are acceptable to you.

Original Article

Rendered Foam walls are becoming much more common, particularly in the upper floor of 2 storey homes. They offer a real advantage in situations where it would be difficult to provide adequate suppport for a heavy brick wall (For example when the upper floor needs to be set back from the ground floor)

If you are worried about strength you need to be aware that the real strength of the house is in the frame. (see: House Construction – The Frame)


  • The Foam boards, which are manufactured with an external mesh face, are fixed to the frame with special galvanised screws that incorporate spreader washers.
  • Joints are sealed with a polyurethane foam and have mesh jointing tape.
  • External corners are reinforced with metal strips.
  • A minimum of 5mm of  acrylic render  is applied, normally in a three layer system.

Polystyrene Foam

There are 2 different types of foam used in this construction method:

  • Expanded polystyrene( EPS) – Good thermal performance but limited impact resistance/structural strength.
  • Extruded polystyrene (XPS) – Similar thermal performance and looks similar  but the production method is different which results in increased impact resistance and structural strength. Higher cost

Insulation values for the various board thicknesses are:

  • 50mm    – R 1.2
  • 75mm    – R 1.8
  • 100mm – R 2.4

Final Thoughts

Although there are some advantages in this system it does require careful detailing and construction otherwise leakage can occur damaging your house.

The advantage of masonry on the lower part of the house is that it is less likely to be damaged by the bumps and bangs of daily life. Once the wall is above head height damage becomes less of an issue and the rendered foam board should be fine.

I’d prefer XPS to EPS.

Although the insulation values are good the builder will most likely want to save the cost of the insulation batts in the frame. If you ask for the wall to include insulation batts you will have an exceptionally well insulated wall at very little extra cost.


For similar posts see Insulation

For more about house design see Choosing a House . . . A new E-book for only $4 to help plan your new house


Render or Brick?

Rendering is becoming fairly popular at the moment……………… However I think “Why pay more for a something that is only going to require painting in the future?”

As my old woodworking teacher once told me “There’s nothing wrong with being lazy as long as you are intelligently lazy. That means getting the job done but saving effort, both now and in the future.”

Rendering does have its place:

  • It gives a good finish if you are building using Hebel blocks or Foam Panels (See this link: Rendered Foam Walls), which you may prefer to use as they have a better thermal performance than bricks.
  • The render, particularly if painted a light colour, will improve the thermal performance of the walls.
  • You need to do it for some of the Home Builder techniques such as straw bale housing, or even if your DIY bricklaying is a bit rough.
  • Its handy if you are renovating a house and the previous builder has painted the bricks.

Apart from the exceptions above here’s why I don’t like render as a final finish:

  • Additional cost at time of construction.
  • Is it really hiding the use of leftover bricks from previous jobs and perhaps poor workmanship by the builder.
  • It can look fairly flat and bland in large areas like the house below:

  • Future time cost and effort in repainting.
  • If you get building movement it really shows up, with a crack across a flat plain wall. It’s then very hard to satisfactorily repair and hide the crack.
  • Render really shows dirt, spiders webs,and water stains.

Bricks are making a comeback as people who have rendered 10 years ago now find that the additional cost was only the start.

Having a house painted every 10-15 years is an expense that basically starts at $10K. Personally I’d prefer a holiday to Europe.

Render costs approximately $15K on a 30Sq home, plus painting, plus more for maintenance……Brick veneer requires a wash, at the most, with a low pressure hose.

Some people say the modern Acrylic renders are better than the cement renders but I remain to be convinced.

What do you think?.


Whichever you choose the most appropriate Brick Dimensions will make the walls easier to build.

More about selecting finishes including 24 pages of Check Lists in the ‘Selection / Pre-Start Guide’