Insulation 101: Interior vs Exterior Insulation

Almost everywhere in Residential constructions in Australia, when they refer to insulation they mostly refer to the Internal Insulation. There is nothing to be suprised if your builder or trade doesn't know about this concept either -- It is however the poor state of standards in Australia today.

In an interior insulation, and we will keep this conversation to the wall, the insulation material lives with the cavity of the frame. This means if the house is a light frame - timber / steel frame house, which is the vast majority of the homes in Australia, the insulation primarily sits between the frames of the house.

The standard frames are about 70-90mm and are spaced 450-600 mm between each other. This allows the what is referred to a Batts to be placed at 90mm depth and available horizontal space between all the areas.

Bear in mind, this same space is also used for services -- meaning the plumbing lines, electrical lines etc. So the insulation will all be placed only to be removed by the next service trade who wants to put his services there.

Overall, the Batts could be made of different materials, there is mineral wool, fiber glass and other organic and inorganic materials that can do the job. For the volume and density of these batts - an ideal case scenario is that you can achieve up to a Rvalue of 4. But this is theorotical, the actual R-Value varies primarily based on that fact that

1. the timber still is a significant surface area and has a higher thermal conductivity compared to the Batts.

2. There are inevitable gaps in the batts due to the fact the services and electrical lines have to use the same cavity.

So, in reality, the effective RValue of the wall reduces significantly.

The Austrlian construction standards doesn't care about the resultant efficiency of the building after construction, there is no way a surveyor or an inspector is to going to test that. The energy efficiency provide NATHERS are bothered only about the theorotical efficiency at the design stage and stick to a result that comes from the diagrams. This is a poor situation. Nevertheless, lets get back to the efficiency of the Internal insulation.

Let's say we found that best trade in the market and they were diligent in installing the Batts and services were nicely done without disturbing the Batts and their efficiency - the question then comes to how much we can achieve using just Batts. Most of the non-rigid insulation cannot go beyond R4 for 90mm cavity. If we use a rigid insulation -- which I have never heard being used in Australian homes, its possible to go up to may be R6. Even in this case, the theorotical insulation might be more than the effective result still due to the same reasons - the cavity is shared by services and there is significant surface area in the form of frame that still is more conductive.

How do we overcome this limitation? Well, interestingly Australian's don't have to invent the wheel on this -- because honestly the rest of the world is well a head in this space. Here is where Exterior insulation plays a role.

An exterior insulation is one that is installed outside of the frame -- steel or wood. This gives two advantages right out of the box

1. It can be continuous -- that is cover the entire building (except the window/door sections), providing more effectiveness - no studs or gaps that could leak

2. Because it is outside the structural space, the size of it, that is thickness of the insulation is more independent to the structure -- we are not limited by the frame thickness. This allows to use more thicker insulation if required with the efficiency of a full coverage.

The best analogy for an exterior insulation is to think about it as wearing a full long jacket.

Now, if exterior insuation has so many benefits, why does Australia not doing it? It's a bit tricky question, but I have to have a go at it, It's a question of economics - supply vs demand -- the industry is not expected to go beyond what it is doing. They don't have a need for invention. No body is asking for it. Neither the government is setting high standards or the consumers are demanding it.

For governement and the industry - there is no excuse - they should lead for excellence -- so it's poor.

For consumers at least there is a good excuse -- they are not informed -- they don't know what is good -- which is exactly the mission of The Monk.. Let's educate Australian consumers on what good is.