How does insulation work?

✔ Insulation prevents, or slows, the flow of heat in and out of a building

✔ Insulation can lower your energy bills and reduce your carbon footprint

✔ Common types of insulation include loft, floor, and wall insulation

Before you consider the price of insulating your home (such as spray foam insulation costs), it’s important to understand how insulation actually works and why it’s beneficial.

Insulation helps keep your home warmer in winter and cooler in summer, reducing the need for you to use artificial heating and cooling.

If you’re considering installing insulation in your home, this guide will tell you everything you need to know, including the different types of insulation available, what each offers, and the insulation costs involved.

Already familiar with how insulation works and just want to find out how much it might cost you? Fill in this simple form and our trusted insulation installers will get back to you with free quotes for you to compare.

How does insulation work?

Heat or thermal energy tends to move naturally from a high-temperature region or system to a low-temperature one. This phenomenon — which can occur in one of three main ways; conduction, convection, or radiation — is known as heat transfer.

What this means for your home is that in winter, heat will flow from the interior (which is warmer) to the outdoors (which is colder). In summer, the reverse will occur, i.e., heat will flow from the outdoors into the house’s interior.

This is where insulation comes in.

Essentially, insulation works by providing a thermally resistant barrier that restricts heat transfer into and out of your home. As a result, your heating and cooling systems don’t have to work as hard to achieve the desired temperatures in each room. This can lower your energy bills.

The key areas of a home that facilitate heat transfer the most and that you, therefore, need to prioritise when it comes to insulation are attics, floors, basements, walls, and crawl spaces.

What are the main types of insulation?

Insulation can be classified into several groups according to the area insulated and the type of insulating material used. The main types are as follows.

• Loft insulation: Installed in your loft or attic space.
• Cavity wall insulation: Installed inside the gap (cavity) between an internal and external wall.
• Floor insulation: Installed below or above the floor surface.
• Fibreglass insulation: A type of insulating material made of plastic reinforced by tiny glass fibres.
• Foil insulation: A type of insulation incorporating a reflective foil layer (usually aluminium) to reflect radiant heat
• Wall insulation: Applied over the inner or outer surface of a solid wall
• Roof insulation: Installed between and over the roof rafters, and sometimes on top of the roof.

What is insulation R-value?

The R-value is a number assigned to an insulating material to quantify its resistance to the passage of heat. It’s typically expressed in terms of resistance per inch of depth. The higher the R-value, the more the material prevents heat transfer and thus the higher the level of insulation.

For example, fiberglass batts typically have R-values ranging from 2.9 to 4.3 per inch.

Rigid insulation boards typically have R-values of between 4 and 7 per inch.  Spray polyurethane foam has the highest R-value of any insulation material, at between 5.6  and 8 per inch.

Worth noting, however, is that R-values are additive.

So, a material with a lower R-value can provide the same or even more heat passage restriction than one with a higher R-value if you use a greater amount of it.

For example, one inch of spray polyurethane foam insulation with an R-value of 7, is equivalent to two inches of fiberglass insulation with an R-value of 3.5.

What is insulation U-value?

Insulation U-value measures the rate of heat transfer through a building component like a roof, a wall, or a floor. The value includes adjustments for any air gaps or fixings.

U-value shows how much heat (in Watts) will flow per square metre of area for every degree difference (K) in temperature between each side of the building component. The lower the U-value, the better the heat efficiency or insulation of the component.

Typical U-values for common building components before insulation are as follows:

• Solid brick wall – 2 W/(M²K)
• Cavity wall – 1.5 W /(M²K)
• Solid floor – 2.1 W/(M²K)

Current regulations require building elements to have a specific minimum U-value. For example, when it comes to external walls, that value is 0.18 W/(m²K). Insulation can help you achieve the required value.

How does loft insulation work?

Loft insulation is installed in the attic or loft space of a building to prevent heat escape.

Here, a thermally efficient barrier that can be made of mineral wool, sheep’s wool, or rigid insulation boards is fitted between and over the loft joists — that is, the horizontal beams that run across the floor of the loft. The barrier traps heat before it can escape from the living spaces below and out through the roof.

A home equipped with loft insulation can help keep up to 25% more heat inside and can save you up to £590 per year on energy bills, according to the Energy Saving Trust. Additionally, it can reduce carbon dioxide emissions by up to one tonne per year, depending on the size of your home.

Want to get a better idea of how much this will cost you? Head to our Guide to Loft Insulation Costs or our Best Types of Loft Insulation pages for more information.

How does cavity wall insulation work?

Cavity wall insulation is a type of insulation for properties whose walls consist of two layers with a gap in between them. The insulation process involves filling this gap with insulation material, thus preventing the movement of heat.

The most commonly used insulating material (and the most environmentally friendly) for cavity walls is mineral wool. However, polystyrene granules and polyethene foams (PUR) are also viable options. Polystyrene granules are the least expensive. PUR is the priciest material but offers the highest level of effectiveness.

The Energy Saving Trust says that this type of insulation can lead to energy savings of up to £690 per year.

Cavity wall insulation requires professional installation. The suitability of the cavity also needs assessment before the installation. If not properly installed, condensation problems can occur and the insulation might not perform as desired.

How does floor insulation work?

Floor insulation involves putting insulation material, such as mineral wool, rigid foam boards, or expanded polystyrene, beneath or above the surface of your floor. Insulating your floor will keep your home warm and minimise draughts. It can help you save between £74 and £195 per year on your energy bills.

However, floor insulation can be disruptive, especially if you have a suspended type of floor that isn’t accessible from below. To insulate a suspended floor, you’ll need to install the insulation from above, specifically by lifting up the floorboards first.

You must also address any dampness before you install floor insulation to avoid moisture problems in the future.

How does fibreglass insulation work?

Fibreglass, which consists of very fine glass fibres, is one of the most popular insulation materials. The material contains tiny air pockets within its structure that trap air thereby disrupting the flow of heat.

As an insulating material, fibreglass is more affordable than spray foam insulation (though the latter is more effective). It’s also quite versatile, coming in various forms such as batts, rolls, or loose fill. This versatility makes fibreglass ideal for use in different parts of the house, including walls, floors, and ceilings.

That said, adequate safety precautions are necessary when installing this type of insulation as fibreglass can irritate the skin and respiratory system. Additionally, the material can settle and sag after installation, which will reduce its R-value over time.

How does foil insulation work?

Foil insulation consists of layers of wadding laid in between aluminium foil sheeting. It works by radiating heat back towards the source.

So, in winter, it prevents heat from leaving your home by reflecting it back inside, while in summer, it reflects the sun’s radiant heat away from the building, keeping it cooler.

Foil insulation has the advantage of being thin and lightweight, making it suitable for areas where space is an issue. It’s also versatile — you can install it in your attic, walls, and ceilings.

To work properly, foil insulation needs to be free of dust and debris.

How does wall insulation work?

A solid wall comprises a single series of bricks or stones. This type of wall is common in homes in the UK built before the 1920s. There’s no cavity in which you can put in insulating material, so insulation is typically attached to either the interior or the exterior side of the wall.

Solid wall insulation can help reduce heat loss by up to 60% and reduce your carbon emissions by up to 1.6 tonnes per year.  It can also save you up to £930 per year on your energy bills.

Wall insulation comes with the extra benefit of sound insulation. Additionally, insulating the outer walls of a building can improve its appearance and weather resistance. What’s more, a government study further shows that external wall insulation can boost your home’s value by up to £8,000 in a normal housing market.

Read more about the advantages and disadvantages of external wall insulation on our page.

However, when it comes to insulating interior walls, one big disadvantage is that it can shrink the space in rooms. Also, the overall costs of insulating solid walls tend to run higher than those of cavity wall insulation.

How does roof insulation work?

Roof insulation involves fitting insulation between and over the top of your rafters, i.e., the sloping beams that make up the main framework of your roof. The insulation material can be rigid insulation boards cut to size or foam insulation sprayed between the rafters.

If you have a flat roof, you can also insulate from above by first attaching a layer of rigid insulation board on top of the roof’s weatherproof layer, and then adding a new weatherproof layer on top of the insulation.

Roof insulation is usually a bigger project than loft insulation and therefore tends to cost more. It’s not a DIY job and requires professional installation.

Most homeowners will normally only insulate the roof if they are using the loft as living space. Otherwise, they’ll just insulate the loft and leave the roof as it is.

Next steps

A properly insulated home can provide year-round comfort, lower your energy bills and help you reduce your carbon footprint. Insulation could even increase the resale value of your home.

If you’re ready to take the plunge and want to know how much insulating your home could cost you, simply pop in your details in this form and we’ll put you in touch with our reliable insulation installers for quotes.

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