Does external wall insulation work?
The short answer is NO, external wall insulation doesn't work.
This article explains the science behind why it doesn't work.
Advertised as: 'the ideal thermal insulation for property with solid walls'. Claims: 'it will reduce the risk of mould and improve air tightness'. Plus 'it will LOWER your energy bills'.
It ISN'T TRUE though.
Easy to say, but here is the proof.
The first section is all about the science of heat. What it actually is caused by and why hot always goes to cold unless energy is applied.
If you are not interested in the science, skip down the page to 'Written as tongue in cheek - the story is based on scientific facts'. Sir Richard Head tries putting his woollen combs on the outside of his armour - the same principle as thermal insulation on the outside of a building.
That is followed by an example solid wall: graphics and notes as to why the external insulation will not help thermally.
It's all about Atoms
All substances are made up of Atoms. Gases, liquids and solids are made up of protons, neutrons and electrons. (One exception is Hydrogen that doesn't have any neutrons).
An oxygen atom for example has 8 protons and 8 neutrons forming the 'nucleus' at the centre of the atom. Orbitting the nucleus are 8 electons. They are held in orbit by a magnetic attraction. The protons being positively charged (+) and the electrons negatively charged (-).
The electrons have been energised and travel (velocity) about the speed of light. To stop the electrons from leaving the atom there is an attraction from the protons. Similar to the gravitational pull on Earth keeping the Moon in orbit, but on a micro scale.
Heat is the result of electrons colliding at about the speed of light. All atoms have electrons orbiting a nucleus. When the electrons of one atom collide with the electrons of another atom they bounce off each other. A product of the impact is heat (thermal energy). It is the result of the impact.
When the atoms collide they bounce off each other to their next collision.
Gravity is pulling down all the atoms on Earth and the sky (atmosphere). Effectively slowing the electrons down. Each atom collision produces heat (thermal energy). As the electrons slow down less impacts = less heat output = they are cooling down. That is the Kelvin theory - when the electrons stop moving it is as cold as it can get = absolute zero.
Back to thermal insulation
Air is made up of atoms. Mainly Nitrogen (about 80%) and Oxygen (about 19%), plus a few trace gases. Each atom is floating around in space. Too small to see, even with a microscope, they bounce around colliding with everything.
When atoms are free to move around as in fluids, the more energised atoms will bounce off with more impacts. That causes more heat (thermal energy).
As the atoms have more energy, they can travel further. That makes the fluid (air is a fluid) less dense at the top, more dense below. The temperature is based on how many collisons have taken place, therefore warm air rises. Meaning there are faster, more energised atoms higher up in the air.
In a solid material (substance), the atoms cannot move freely around. However, there is still space between each atom albeit too small to see. Energising those atoms means they can vibrate though. Bashing atoms around them is still impacts = heat output.
Each atomic impact uses some of the energy required for the electrons to whiz round. It isn't 'heat' that is being passed on, it's energy (thermal energy).
When a wall for example is heated up by warm air, the air cools down. Energy is being transferred from fluid atoms to solid atoms.
Back to the wall
If a wall has a very hard, dense surface such as dense plaster there are a lot more atoms to energise. Therefore a lot more energy is required. A lightweight plaster such as a gypsum plaster is less dense = less atoms. That means less energy is required to heat the plaster up.
Energy is a bit like water. Gravity is constantly trying to pull all the atoms down so the surface of water is always parallel to the Earth's surface. The same is happening to the energy going into every atom. Gravity is 'putting the brakes on' to every electron slowing it down.
How does thermal insulation work then?
Materials that have fewer atoms (less dense) require less energy input to produce heat. The heat (thermal energy) passes on to fewer atoms therefore, the energy levels are maintained for longer.
In a similar way to you having 100 £1 coins. If you gave a coin to everyone standing around you, you would run out of coins fast. If there weren't many people around you your coins would last longer. If you want to maintain your wealth and still give out lots of coins you need a bigger constant supply. Exactly the same with thermal insulation. Lots of atoms means the energy goes pretty fast unless you have a lot of energy being supplied.
Written as 'tongue in cheek', the story is actually based on scientific facts
Sir Richard Head was known to his friends as 'Dick'. He had a thought: Why not put my steel armour on, then my chain mail and then my woollen comb's over the top. Those sales people and 'scientists' state it doesn't matter which order the materials go on, it is thermally the same result.
Yes - the current method for calculating thermal movement in
buildings is using a 'U Value' calculation. It doesn't matter what order the
materials or emissivity is used as the result is the same.
The graphic shows how the layers of material would typically be when wearing a suit of armour.
Woolen combs next to the skin is an excellent thermal insulation layer. Not many atoms and plenty of trapped air. The chain mail over the woolen combs. Although dense metal, the wire is relatively thin so there is trapped air between each wire. Not good thermal insulation, but okay. There is still a thin layer of relatively trapped air between the steel armour and the chain mail. Minimising conduction of thermal energy.
This by calculation has the same thermal insulation qualities as the other combination.
Known as a 'U Value', the mathematical calculation indicates it doesn't matter in which order the layers of materials are placed.
Now using logic:
Woollen comb's are thermally very efficient. Plenty of trapped air held in place by irregular natural fibres of wool. That next to your skin will retain your body heat as it will not transfer by conduction, minimal by
radiation and virtually no convection = excellent thermal insulation.
Chain mail - small threads of metal, mainly steel or iron. Effective against some degree of impact. The diameter of the wire is relatively small so minimal conduction. Space between the wire is relatively trapped
therefore minimal convection. Some radiation as it is metal.
Armour - thicker sheets of worked steel. The mass of the metal is very dense. A knight if knocked over or falls from a horse would have found it difficult to stand up again due to the weight of the armour.
An external solid brick or stone wall: Room side heated to about 21°C air temperature. Outside it is cold, about 3°C.
The atoms of the warm air in the room passes energy onto the plaster by continual bombarding until the plaster becomes the same energy level / temperature. Lots of energy is continually required.
Insulation fixed to the outer wall surface and then rendered over provides little to no thermal insulation to the wall. The energy from the air atoms are still trying to 'heat' the inner wall surface to the same extent as having NO insulation.
Putting the insulation on the room side of the wall significantly reduces the amount of energy to heat the room. The energy contained in the air atoms can easily cause the plasterboard to achieve thermal equilibrium
= efficient thermal insulation
For more information about U Values