Wall insulation – a brief overview

Installing insulation is an important part of keeping your house warm and securing it from mould if the insulation is done properly. As important floor and roof insulation is, what can significantly reduce the cost of heating are insulated walls.

According to Building Regulations the thermal conductivity of floors, roof and walls must be the same – the value cannot be higher than 0,30 W/m²K. This is a minimal value; for new builds it is 0.20W/m²K. You should however aim to make it lower.

There are two values that are most important when it comes to insulation. Thermal conductivity is measured by U value, which is the rate at which heat passes through fabric; the lower the value, the better the insulation and less heat escapes through the wall. Thermal resistance of a material is measured by R value, which is related to the thickness of the fabric; the higher the value, the better the insulation.

We can divide insulation into two types:

– breathable

– non-breathable

Breathable insulation is made from natural material and allows moisture, air, toxins to go through the wall from inside of the house. It helps to prevent internal condensation and mould growth. Breathable materials used for insulation are hemp, sheep’s wool, cotton, wood fibre, cellulose, and flax. Breathable materials are also a better solution if you care about the environment – they are sustainable and some of them can sequester CO2. However, to create a breathable house, the entire wall must be constructed from breathable materials. Achieving the same U value with breathable insulation requires it to be thicker.

Non-breathable insulation does not let anything to go through. It increases the chance of damp but improves thermal conductivity. Non-breathable materials are mineral wool, expanded polystyrene, polyurethane, polyisocyanurate. You can achieve a better U value with non-breathable insulation by using thinner insulation, which impacts the cost – it makes things cheaper.

The types of insulation are:

rigid insulation (insulation foams): rigid polyurethane (PUR), polyisocyanurate (PIR), phenolic foam. They all offer similar level of insulation, PIR and phenolic foam are used most often.

semi rigid: mineral wool, wood fibre, hemp; they include resin bonding, polyester and other fibres which give stiffness to the insulation. They have higher thermal conductivity than rigid insulation.

flexible: sheep wool, straw, cotton; same as semi rigid but without stiffening, it needs support to stand on its own, they are good to install on a floor or a roof, but it is hard to prevent them from sagging even with some support if installed on a wall; flexible insulation has a poor U value so to achieve the satisfactory level of thermal conductivity twice as much insulation must be used.

Insulating already existing builds is harder than insulating new builds, since walls are already up, and you must work with what you have. For solid-walled house, cavity wall insulation cannot be done, so you are left with two choices – internal or external insulation. You must choose the option suitable for your house – both types have their advantages and disadvantages. Internal insulation will inevitably affect the interior of the house, forcing you to remove things like radiator, switches etc. from the room before insulating and it will eventually make the space smaller. It is however cheaper to do internal insulation. The most common way to install it, is to create a new stud wall and then add the insulation to the wall. By installing external insulation, you would avoid it, however not all houses can be externally insulated. External insulation obviously changes the appearance of the house and in Conservation Areas you will most certainly not receive the permit to do so. In most areas it will require a planning consent. Despite the fact that installation of internal insulation is easier and cheaper, external insulation seems to have more advantages, which are for example not interfering with inside of the house, not reducing the floor area, improving waterproofing and soundproofing as well as improving the lifespan of the wall. It significantly increases thermal resistance of the house.

Installing cavity insulation is more complicated. It requires careful research as many times it is not possible to insulate it due to the way such cavity walls were built – some are just not suitable for insulation, for example if the brickwork is poor or if the cavity is not clean. The way to find out is to visually inspect it, usually by inserting a small camera, which has to be done by a competent insulation installer. Also weather conditions can become an obstacle when it comes to insulating a wall cavity. Building Regulations Part C refers to a map of UK that shows levels of the risk of driving rain based on the region. If the risk is high, the cavity insulation in new builds is prohibited in those places.

As you can see, many things can affect the choice of right wall insulation for your house and the decision must be made carefully, by a specialist, in accordance to local regulations. Any mistakes may later lead to water condensation, which causes damp and mould, harmful not only for the structure of your house but also your health.

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