Exclusive app offer: free delivery on orders over £200*Terms & conditions apply
Who are we
Insulation buying guide

Insulation buying guide

Jennifer, Self-taught DIY enthusiast, Manchester

Guide written by:

Jennifer, Self-taught DIY enthusiast, Manchester

21 guides

High-quality insulation can deliver comfort and savings. But there are plenty of options out there from glass wool, rock wool and synthetic insulation to insulating underlay and foil insulation. Whether you're looking to insulate internally or externally, read on to find the perfect insulation for your home.

Important features

  • Mineral
  • Synthetic
  • Insulation foil
  • Natural
  • Insulating power
  • Thermal conductivity
Shop our insulation

Insulation comes in a variety of forms but all forms of insulation have the same purpose – to avoid the warmth generated by your heating system flying out the window or through the roof! Among the most common options are insulation boards, rolls, blow-in insulation or even expanding foam for gaps and other hard-to-reach spots. Your insulation may also be backed up by products like pipe lagging or window seals. There are five main groups of insulating materials.

Mineral insulation

Mineral insulation is the most common option as it provides good thermal insulation and soundproofing properties. Included in this category are materials like glass wool, rock wool and cellular glass.

Synthetic insulation

Synthetic insulation includes materials like expanded polystyrene, extruded polystyrene, polyurethane, polyisocyanurate and phenolic. These materials are what is known as foamed plastics.

Insulated building materials

Concrete block insulation involves using blocks filled with insulating materials. Thermal plasterboard is based on a similar concept. These forms of insulation can only be installed in new constructions or as part of renovation work.

Natural insulation

Natural insulation is made of animal-based fibres such as sheep wool or duck down, or plant-based fibres such as hemp, cellulose wadding or wood wool.

Insulation foil

Insulation foil is known by a number of different names such as multifoil insulation, radiant barriers, reflective insulation, and so on. This type of insulation is made up of aluminium sheets or aluminium polyethylene film. Insulation foil is often crticised due to its debatable performance and price.

Insulation can be applied to the outside of your home or the inside. It can cover any surface from floor to ceiling, including walls, doors, windows, roofs and loft spaces. It's possible to install insulation foil both during construction and post-build.

The insulating power of materials, or their thermal resistance, is measured by their R value in units of m² K/W. The higher the R value, the more effective the insulation; R = 0 is the lowest value while R ≥ 7.5 equates to effective insulation. In short, you need a high R value to ensure your insulation performs as expected. Whether you're insulating a new or older property, take care to ensure a minimum R value is reached.

Insulating new builds

Got a new house in construction? Go for insulation with a thermal resistance of R 8m² K/W for the roof and R 4m² K/W for the walls, roof gables and ground floor.

Insulating an older property

If you're hoping to make your home more energy efficient, improving the quality of your insulation is a must. Here are a few guidelines to help you out:

  • R 3m² K/W for basement or cellar ceilings and open passageways;
  • R 3.7m² K/W for walls;
  • R 6m² K/W for loft spaces and attic ceilings;
  • R7m² K/W for any ceiling in contact with an uninsulated loft space.

Of course these are just guidelines – be sure to follow the advice of industry experts when it comes to minimum R values.

It's also important to think about the flammability of insulating materials which is governed by the fire classification standard EN 13501-1. The classifications are as follows:

  • A1 – Non-combustible materials. No contribution to fire.
  • A2 – Non-combustible materials. No noticeable contribution to fire.
  • B – Combustible materials. Little or no contribution to fire.
  • C – Combustible materials. Limited contribution to fire.
  • D – Combustible materials. Contributes to fire.
  • E / F – Combustible materials. Major contribution to fire.

Be sure to check the legislation for each room you have to insulate. The rules may change depending on whether you're dealing with a private home or a public space.

The lambda value (or λ-value) measures the quantity of heat that can be conducted through a material within a certain amount of time. Thermal conductivity is measured in W m/K. But why exactly do we need to talk about thermal conductivity? The answer is very simple – it's all about the thickness of your insulation. Bear in mind that the lower the lambda value (or λ) the thinner the material and the higher the performance. To give you a quick idea, a λ-value of 0.06 is considered quite poor while insulation with a λ-value of 0.02 is considered effective. A low lambda value signifies low thermal conductivity meaning better insulation.

On the opposite end of the scale, the higher the thermal resistance (or R value), the more effective the insulation. If this is all sounding a bit complicated, don't worry – all these details will be provided in the product specifications. If you're looking to compare two types of insulation with the same thickness, you just need to divide the thickness of the material by the R value to find the lambda value:

Lambda λ = Thickness / R value

  • Example n° 1: 120 mm rock wool with an R value of 3.4 = 0.12/3.4 = 0.035. Therefore the λ-value = 0.035.
  • Example n° 2:  100 mm glass wool with an R value of 3.1 = 0.1/3.1 = 0.032. Therefore, the λ-value = 0.032.
In summary, the most insulating material will have a high R value and a low lambda value. It's up to you to find the right the right solution by weighing up factors like budget, space and surface type!

External insulation

High-quality external insulation allows you to save energy and money. What's more, it's good for the planet! If you are in the process of building a new house, external insulation is a great call. This type of insulation is particularly beneficial if you have limited space inside the house. Your choice of insulating materials in this case will be:

  • glass wool or rock wool;
  • extruded polystyrene or polyurethane boards;
  • wood fibre, expanded cork or hemp (for the most environmentally conscious);
  • rolls of insulation can be installed on any surface and work particularly well for roofs.

As you may have guessed, you'll need a range of equipment to install external insulation including:

  • tracks for installing the insulation panels;
  • corner beads (which can be fitted with mesh);
  • fibreglass rolls to reinforce render;
  • insulation fixings to secure your insulation rolls/panels;
  • a render base coat and/or adhesive to apply before insulation;
  • a primer to apply to polystyrene boards to make sure they last. This will also help to preserve the colour and finish of your finishing coat.
  • a special type of finishing render to apply over the polystyrene insulation. 

It's also possible to invest in waterproofing products to make your walls and roof more moisture- and mould-resistant – or else you might find yourself having to insulate the inside of your home, too!

Internal insulation can be flexible, semi rigid, rigid or foam-based. It can be installed using adhesive or fixings, depending on the type of insulation you choose. A vapour barrier should be fitted over your insulation before your final wall covering is installed whether this be plasterboard or wood panelling. Let's take a quick run through of the most common insulating materials used indoors.

Insulation type 

Interior insulation comes in the following formats:

  • flexible rolls: for walls or sloping walls (thin insulation);
  • semi-rigid insulation: for walls, partition walls, ceilings and lofts. Rock wood, hemp and flax wool, flexible wood fibre (the insulation is then covered with a layer of plasterboard, wood panel, etc.);
  • rigid panels: for walls, floors and ceilings (expanded polystyrene and polyurethane). Two-in-one products are particularly effective (e.g. plasterboard on one side and insulation on the other);
  • loose-fill (or blow-in) insulation or foam insulation: for hard-to-reach areas. Blow-in glass wool insulation and cellulose wadding are particularly effective for loft spaces, insulating between floors and work to eliminate thermal bridges.

As a reminder, thermal bridging is the movement of heat across an object that is more conductive than the materials around it. It happens in building where there is a direct connection between the inside and outside.

Locating thermal bridges 

Thermal bridging usually occurs:

  • around doors and windows;
  • on hot water pipes;
  • the spaces surrounding pipes or cables;
  • garage doors;
  • anywhere where floors, walls or ceilings meet;
  • roofs.

How to prevent thermal bridging  

The location of your thermal bridges will determine how to deal with them:

  • insulation foil for garage doors;
  • expanding foam to fill in spaces and gaps in the roof, doors or windows or to fill in the spaces surrounding pipes or electric cables;
  • adhesive draught seals for window and door frames. These seals usually come in rolls and can be made of polyester, thermoplastic, rubber or even foam rubber.
  • draught seals designed for the base of doors can be screwed, glued or slid into place and can be made of aluminium, rubber or PVC.

If you have the choice between internal and external insulation, go for external insulation. This option is more expensive and has to be installed before your final exterior wall covering. However, it is highly effective at eliminating thermal bridges and doesn't take up any room indoors. Internal insulation is the cheaper solution. It can be installed in any type of home and the range of insulating materials on offer is huge. The downside of internal insulation is that you lose part of your interior space and it is trickier to tackle thermal bridging.

No matter which type of insulation you choose, it is highly recommended to cut the material as precisely as possible to prevent heat loss at any point in the walls, ceilings or between floors.

In addition to providing thermal insulation, most insulating materials also offer some acoustic insulation though this does vary. If you think that the material you have chosen is not providing enough soundproofing, double up with floor or wall underlay. In terms of regulations, it's always best to read up on materials before you invest!

Shop our insulation

Guide written by:

Jennifer, Self-taught DIY enthusiast, Manchester, 21 guides

Jennifer, Self-taught DIY enthusiast, Manchester

I didn’t receive any special training, I learned everything on the job. And what a joy it is to be able to do little jobs around the house that we love so much. That is, until the moment we decided to move and had to do everything; from the floor to the ceiling, from the kitchen to the bathroom...In short, you become as good as a pro. So today, my friends don’t hesitate to call me when they need help. And when you dip your toe in, there’s no turning back. It’s a true passion that drives us to take on the challenges, to have an idea in mind and see it come alive with just a few tools. And a passion is even better when you can share it. So, whenever I can give you a little advice, it’s with great pleasure.

  • Millions of products

  • Delivery to your home or click & collect

  • Hundreds of dedicated experts online