Does External Wall Thermal Insulation work?

Putting thermal insulation on the outside of a solid wall will not significantly reduce the heat escaping from your home. Claims that upto 30% of the heat loss is through the walls may be true, but externally insulating will not achieve much.

The reason; the insulation will be on the wrong side of the wall.

Let's look at the actual science

We will start with what heat actually is:

That is the basis of heat production

It is only the product of electrons colliding that produces heat.

No impacts = no heat.

All atoms have / contain some energy otherwise they cannot move = no impacts = no heat = absolute zero temperature = 0 Kelvin.

Every impact uses energy. The biggest drain on energy is gravity. The Earth is a giant magnet containing countless billions of atoms. Atoms contain a nucleus of protons and neutrons. Protons have a positive charge (just like a bar magnet). Neutrons have no charge and orbiting the nucleus are electrons. They have a negative charge.

Most atoms have the same number of protons as electrons therefore they are neutral. However, there are plenty that have more protons which makes the Earth a positive pole. That is gravity. The positive pole of the Earth is constantly trying to attract the electrons of every atom (Atomic attraction). In effect constantly slowing every electron down very slightly.

The electrons whizz around the nucleus at speeds approaching the speed of light. When they collide with the electrons of another atom the speed at impact produces heat.

Thermal insulation - What exactly is it?

Thermal insulation is just a method of reducing collisions between electrons.

There are different methods such as:

• Reduce the number of atoms in a given volume (reduce the density). Low density materials like expanded polystyrene and mineral wool. Sheeps wool, cork, loose paper pieces. Use a different density gas.

• Reflect the magnetic energy back in the direction it came from. Metallic foil such as polished aluminium foil, aluminised plastic, atomised metal coatings on glass. 

Why external thermal insulation doesn't save you energy

You heat the air in your room by increasing the energy input to the atoms. If you don't add energy, the atoms (electrons) will slow down = less speed, less impacts = less heat. Increasing the energy levels will enable the atoms to travel faster, bounce off other with more impact = more thermal output = more heat.

The electrons slow down. Energy has been converted into heat (thermal energy) and used for momentum. The attraction between atoms caused by bonding means they become closer together. Gravity has more effect. Warm air rises (less dense), whilst cooler air (more dense) is pulled down by gravity.

Evidence:

Humidity is the vapour / gas state of H₂O. The molecule has plenty of energy and can travel between the other air atoms (Mainly Nitrogen and Oxygen).

When the molecules slow down they attract other H₂O molecules to form a liquid state. The density has increased as the atoms are closer together. Gravity has more effect and will pull the molecules down as rain for example.

The warm air in the room will pass energy to everything it is in contact with. When the other atoms and molecules have the same amount of energy. The term is equilibrium. High energy atoms will pass some of their energy to less energised atoms until they are both at the same energy level. That is how heat will pass between atoms and molecules.

Evidence:

If you have an infra-red thermometer handy, check the temperature of anything at the same level in a room. Be it soft furnishing, metal legs of a chair, wooden wardrobe or the wall. The temperature should be the same. (If it isn't there are other factors to be considered - moisture content and air flow).

Whilst you have the IR thermometer handy check the temperatures of the wall at floor level, about half way up and at ceiling level. Then try the corner of any external wall and then about half way across the wall at the same height. They should all be different. - WHY?

At floor level the air atoms will have the least energy. That is why you may have cold feet when it is very cold. About half way up the wall it is generally the room temperature you feel. Thermostats (room stat) should be at that level.

At ceiling level the wall will be at its warmest.

Why is the corner of a room with outside walls cooler than the rest of the room? It is because there are a lot more molecules to heat up and the air doesn't move as much in the room corners (air flow). That is why condensation occurs in rooms near the floor especially if furniture is close to the corner.

If the warm air cannot circulate in the corner there is nothing to pass energy to the wall. The wall will naturally have some energy that has been conducted from other materials or substances.

A RECAP

• Heat is the product of collisions

• The greater the impact and the number of impacts the more heat is produced

• Thermal energy is based on the number of impacts

Back to:

Why putting thermal insulation on the outside of a building will NOT significantly reduce your heating bill

You heat the air in the room so that you feel comfortable. The energy used is transferred to everything in the room until all the atoms and molecules have the same amount of energy. That is why it takes time for the air in a room to become comfortable as it is having to raise the energy content of billions of atoms. All the furniture, the fabric of the room, air movement.

Each collision between the atoms uses energy. Gravity is trying to pull the electrons down and effectively slowing them.

Reducing the number of atoms in a substance / material will reduce the number of impacts. Less energy will be used so the electrons can continue their extremely high velocities.

Putting the low density thermal insulation on the outside will make very little difference to the air atoms having to heat up the room. That has not changed.

The same amount of energy is required to heat up wall 'A' as wall 'B'. The only differences will be the speed that the energy is used up. The outer surface will remain warmer for longer.

We will now look at humidity.

The other factor - Humidity

Yes - the current method for calculating thermal movement in buildings is the U Value. It doesn't matter what order the materials / layers are or emissivity is used. The result is the same.

Humidity refers to how much H₂O is in the air. Air is mostly Nirogen (about 78%) and Oxygen (about 21%). There are also small amounts of 'trace' gases (about 1%). There is space between each of the atoms. Absolutely nothing there. That space is where the H₂O molecules will take up. It is referred to as a relative percentage of the 'dry' air - Relative Humidity (%RH).

It requires a lot of energy to increase the temperature of water. If a wall or soft furnishing is damp it will take a lot more energy to increase the temperature. Wet clothes are an example. Your body has to produce a lot more energy to raise the temperature to body temperature.

(N) Nitrogen has 7 electrons - they go around in pairs therefore 14 electrons

(O) Oxygen has 8 electrons - they go around in pairs therefore 16 electrons

(H₂O) Hydrogen has 1 electron x 2 + a single oxygen 8 = 10 electrons

Let's say the air has a relatively high humidity at 60%RH. The temperature makes a lot of difference to how much humidity the air can hold. An example: air temperature of 25°C with an RH of 60%. The amount of energy required to maintain those figure is a lot. The air pressure is trying to expand but the walls, ceiling and floor will not allow it to (the room 'fabric').

The pressure increases pushing the atoms and molecules against the fabric. The H₂O is in its gasious state (vapour). It will be pushed into the fabric such as plaster, plasterboard, blocks, carpets etc. as they all have pores. The molecules will be assisted by polar attraction. That is where a molecule is polar like very small bar magnets. A positive side and a negative side.

H₂O is polar and will be attracted into any polar material. As the energy decreases the molecules slow down and become attracted to each other. The volume of the molecules condenses. That is condensation. The change from vapour to liquid. In a material the change is referred to as the 'dew point'. That is exactly the same as dew occuring outside. The temperature falls, less energy for the molecules to move around so they become attracted to each other. As they do, they change from vapour to fluid (humidity to water).

Humidity is H₂O in vapour (gas) form. As gas it can pass through very small gaps termed 'pores'. Your skin is a good example. When your body temperature becomes too high it naturally pushes out H₂O in vapour form through pores in your skin. The energy in the atoms reduces and the gas changes to a liquid (sweat - all right, for the girls 'glow'). When you go swimming the water cannot go into your body through the pores as the molecules are grouped together, too big.

Now consider the wall. The vapour form of H₂O can be pushed through a porous material helped by atomic attraction (polar attraction). As the energy is converted into heat the molecules slow down and become cooler.

As the H₂O molecules slow down the polar attraction (atomic attraction) enables the molecules to bond together forming a liquid state. The liquid state (water) cannot pass through very small pores (termed micro-pores) so they build up and the material becomes damp.

When the Sun shines on the surface very high levels of electromagnetic radiation energise all the atoms it collides with. The water in the material becomes vapour once more and the pressure greatly increases. The vapour can then pass through the micro-pores and the material can dry out.

Now consider the wall with the thermal insulation on the outside. It is on the cold side of the wall. The wall has allowed the humidity in the room to enter the pores in gas form. It has lost energy by gravity slowing the electrons down. The humidity has changed from a gas state to a liquid state on the wall side of the insulation. (Particularly occurs when the heating in the room is turned off).

The Sun shines (solar radiation) on the external surface of the thermal insulation. By its design, the energy will not easily pass through the insulation. That means the surface touching the wall will not be energised by the solar radiation. The moisture ('dew point' moisture) in the wall is in liquid form and cannot pass through the external non-polar thermal insulation = damp wall.

Damp walls require a lot more energy to become in equilibrium with the energy in the air. Your body will find it difficult to heat wet clothes for the same reason.

Conclusion:

• Putting thermal insulation on the outside of a building does not save you energy

• Humidity in your rooms cannot escape. It will condense (dew point) within the wall making it damp

• Damp walls require a lot more energy to become warm

• Mould needs moisture - condensation increases as the humidity cannot escape