Passive Solar Design Principles - External Walls

How do external walls contribute to thermal comfort?

External walls allow almost as much heat exchange between the interior of your house and the exterior as the roof does—often as much as 20% of the total when uninsulated. 
As well as acting as a primary insulation layer, the inner part of an external wall can also be used to store energy, but this can only be achieved with compound wall construction where the outside cold wall is separated from an inside warm wall by convective and (sometimes) radiant insulation.

External walls fall into three basic categories:

Because the primary thermal task of an external wall is to provide the vertical insulation layer, low-mass walls can function perfectly well in a passive solar house provided the thermal inertia is provided by internal wall, floor or ceiling elements.
Materials like Hebel block or limestone have characteristics of both an insulator and a thermal mass, but do not perform to a high level in either capacity.  These intermediate materials rely on greater wall thickness to achieve good levels of performance.  They can, however, be used in a single-skin (without cavity) format, which makes construction simpler, and their air sealing characteristics are very good.

Wall Construction Details

CONVENTIONAL BRICK VENEER is the most common domestic wall construction for our region.  It consists of a loadbearing timber frame wall, a cavity of 50mm width, and an external, non-loadbearing cladding wall made of brick.

The brick skin does not significantly contribute to the thermal performance of the wall, thus it behaves in much the same way as a weatherboard wall.  Relatively high convective insulation values are possible, in the range of R2.0– R3.5, depending on the wall thickness and radiant/convective insulation combinations used.


INSULATED CAVITY BRICK uses waterproof rigid insulation boards, usually extruded styrene, to achieve a good level of convective insulation external to the inner skin of the masonry wall. The outer skin of the wall doesn’t perform a significant thermal role. 

Insulation is generally mechanically fixed to the inner skin of the wall leaving a small cavity between the styrene and the outer skin.  Insulation value depends on the width of the cavity and the thickness of the styrene boards.  (range of R1.7—R3.4)  Clay brick has a volumetric heat capacity (VHC)of 1350-1800 KJ/m3C0


INSULATED PRECAST CONCRETE consists of two skins of cast concrete sandwiched around a core of rigid extruded styrene insulation.  While the concrete is dense and has no real convective insulation capacity, the styrene core solves this problem, giving the complete construction both capacitive and convective insulation value.

200mm thick cast concrete in two skins as shown has a convective insulation value of around 0.15 and 50mm of extruded styrene adds a further R1.7,  giving a total value of 1.85 m2w/C0   VHC = 2060KJ/m3C0


AERATED AUTOCLAVED CONCRETE (commonly known by the trade name HEBEL) has been used in Australia for about 20 years, but has a much longer use-history in Europe.  In block form, it is intended to be used as a single skin of masonry, and provides both a moderate level of convective insulation and a moderate level of thermal inertia.  A panel form of the material has been developed to improve compatibility with Australian framed construction conventions. R-Value for a 250 thick wall plus gyprock  = 2.1m2w/C0   
VHC = 550KJ/m3C0


CORE-FILLED POLYSTYRENE BLOCK is a widely-used wall construction in Europe and North America and is becoming more common in Australia.  It is available in various forms, basically consisting of prefabricated EPS blocks or formwork walls which lock together during the initial laying operation.  The form-blocks are dry laid, after which, concrete is pumped into the blocks to complete the construction.  It can be a very rapid form of construction and produces both high insulation values and high thermal mass.  Total R-Value for a 230 thick wall is
VHC = 2060KJ/m3C0


EXTERNALLY INSULATED CONCRETE can be made using tilt-slabs or various systems approaches to poured concrete walls.  Thermally, EIC walls work in a similar way to concrete sandwich panels, the only major difference being that the external surface is provided by a thin render coat or lightweight sheet cladding material.  Convective insulation value is limited only by the thickness of the polystyrene (or other) insulation layer fixed to the outside of the concrete.  R Value =  2.0—4.0 m2w/C0   (typical range)
VHC = 2060KJ/m3C0
Concrete thickness is usually 200mm.


REVERSE MASONRY VENEER is a very effective wall construction for creating both convective insulation and thermal mass.  In principal, it works the same as externally insulated concrete, however a cavity is formed between the inner and outer walls allowing both convective and radiant insulation to be placed and thus improving summer-time cooling performance. Convective insulation value is variable with thickness used. Construction method is comparable to conventional brick veneer. Thermal capacitance will vary, depending on the density of the particular masonry as indicated below.
R  = 3.5—5.0 m2w/C0   
VHC = 1350—1800 KJ/m3C0


STRAW BALE WALLS have a relatively short use-history in Australia but have been used in the USA for around 150 years.  Their main advantages come from extremely high convective insulation values and good vapour permeability allowing the interior to breath without the effects of drafts from air leakage.  The walls need to be covered in thick earth-based render in order to be structurally sound, and this adds a moderate amount of thermal storage capacity to the system.
Total R-Value for a 500 thick wall is AROUND 6.0 m2w/C0   
VHC = 1500KJ/M3c0 (for render)



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