Insulation Board

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Insulation Board Buyers Guide

You can use Insulation boards for walls, floors or roofs.

Rigid insulation board is one of the most effective insulation materials around.

It is a very good insulator for such a thin material. Other materials such as fiberglass need to be much thicker to give the same level of insulation.

This means that:

a) your electricity and or heating bills will be lower and,

b) your property will be much warmer and more comfortable,

All without taking up as much of your valuable living space as some of the other insulation options.

The boards are usually faced with a wide range of materials. Such as paper, bitumen, glass fleece, or plasterboard. But will be most likely faced with aluminium foil.

There are a lot of different options out there when it comes to choosing insulation board.

In this expert buying guide, we’ll take you through:

1. the different insulation board material options,
2. the different facing options,
3. the different areas it can be installed
4. other things to consider

1. What types of insulation board insulation material are available?

There are many different types of insulation board available. Some of the more popular types include the following:

PIR insulation board - Great for most applications such as floors, walls, and roofs

Polyisocyanurate, (also referred to as PIR or Polyiso) has great thermal efficiency. (50% better than polystyrene). And since it is more mainstream it tends to be cheaper than some of the alternatives.

The production process doesn’t produce any harmful materials. This is good for the environment.

The material is tested for performance and is fire proof. It meets all the current building regulations.

Examples of PIR board include Celotex GA4000 and Ecotherm Eco-versal

Rigid phenolic insulation. - Perfect for when high performance and saving space are important

Phenolic insulation is a rigid foam sandwiched between two flexible layers

It is more efficient and has lower U values than the PIR boards for the same level of thickness.

This is particularly significant where space saving is important.

It is popular in highly efficient low energy design such as passive house. It meets the new Building Regulation standards without increasing the thickness.   

The downside is, they are generally more expensive than PIR boards of the same size .

Examples include the Kingspan Kooltherm range.

Polystyrene boards

Polystyrene is also known as Styrofoam or Aeroboard.

It's been around a long time and has a wide range of uses.

It is one of the cheapest forms of insulation.

It comes in two types; extruded (xps) and expanded (eps)

They are both made of polystyrene but they have very different performance properties.

The major issue with these boards is that they are flammable. This rules them out as a viable option in a most applications.

Extruded polystyrene (XPS) - Used for specialist applications

XPS is strong when compressed under load.

This is perfect for specialist applications like under car parks and commercial floors.

Expanded Polystyrene (EPS) Foam Insulation - Used for specialist applications

EPS is a good cheap insulator. It works especially well for basement walls and areas where space isn't at a premium.

2. What facing options are available?

Some Insulation board comes bonded to a range of different materials. Here are the most common:

- Insulated Plasterboard

The perfect solution for internal solid wall upgrades. The plasterboard is bonded directly to the insulation board.

You can install the insulation and plasterboard at the same time. This cuts down on time and labour costs.

Examples include the Celotex PL4000 range.

- Foil faced Insulation

Choosing insulation board with a foil facing reflects heat back into the building.  

Adding a foil facing to insulation significantly improves the U-value.

This enables a thinner insulation to be used to achieve the same U-value.

You can create an effective vapour barrier by taping the seams with  foil tape.

- Glass tissue (external wall)

This is an excellent option for reducing heat loss from your walls if you do not have cavity walls.

It’s a rigid insulation board that is fitted to the exterior walls of your house. It's used behind both render and dry cladding. And it provides excellent thermal performance.

You can give an old house a fresh look by applying external wall insulation to the exterior. It can transform the look of the property.

One thing to consider is that it is more pricey than EPS insulation. Examples include Kingspan K5

- PIR Plywood Laminate

Plywood laminate is insulation board bonded to plywood.

It is designed for flat roof systems which will by covered with a waterproofing felt layer.

It cuts down on installation time and cost and also reduces the risk of condensation.

- Bituminous waterproof facing

Insulation board attached to a bitumonus layer for use in warm flat roofs.

It's used under torch-on, bituminous waterproofing systems.

Again, it cuts down on installation time and labour costs.

3. Where can insulation board be used?

- Walls

With insulation board you get high u-values using the thinnest amount of insulation.

This makes the best use of the space you have available.

You can reduce other construction costs like using smaller fasteners and timber studs.

Solid walls can also be insulated with insulation board.

On the inside of the solid wall, insulated plasterboard can be used.

And on the outside of the solid wall, insulation board with an external facing render can be used.

It is also more likely to perform well over the entire life of the building. It doesn’t slump or sag over time like glass type insulation can.

There is no risk of gaps appearing. Which would cause thermal bridging and reduced performance

The gaps between the boards can be sealed with for even better airtightness and thermal performance.

There are two types of insulation board that can be attached to the outside of the home;
1. Expanded polystyrene (EPS) and
2. Phenolic.

The expanded polystyrene is the most popular since it is the most cost effective.

Phenolic insulation is best when space is at a premium. You can use less insulation to achieve the required u-value.


In most cases, homeowners get in a professional to install solid wall insulation. It is not normally considered a DIY job.

This is because the insulating process involves covering the original brickwork. The process transforms the outside of the property and is labour intensive.

The same goes for internal wall insulation, it is advised to get someone who knows what they are doing to install it.

- Pitched roofs

Insulation boards are commonly used in pitched roofs.

It can be used between the rafters, over the rafters and under the rafters depending on the application.

Insulation board allows you to insulate to the current standards without sacrificing valuable headroom.


If you are installing rigid boards you can either install them;

1. at the joist level, which is above the ceiling or
2. at the rafter level, which is below the roof.

You first need to find out if you working with a cold or warm roof layout. This will determine where you put the rigid boards.

If you are going to install them at joist level and you intend to walk on the boards, we recommend putting a layer of chipboard on top to help spread your weight across the joists.

Try to use boards which have a foil coating on one or both sides. This will help reflect heat back into the space you are trying to insulate.

If the foil is only on one side, and you want to use these boards to insulate your rafters, you need to ensure the foiled side is pointing towards you when you install it. This ensures the heat is being reflected back into the home.

You will need to make sure that when you cut the material that it fits tightly in loft joist / rafter space.

You can use a combination of cutting tools from a Stanley knife, a fine-toothed saw or a rigid board cutter.

Measuring it key and getting it right first time is important as you don’t want to be throwing material away.

Also when cutting, ensure you handle the tools with care.

- Flat roofs

Insulation boards are the most common form of insulation in flat roofs.

There are a few reasons for it’s popularity:

The boards have good compressive strength and are able to withstand foot traffic. This is an important consideration in modern roofs with single ply construction. With other materials there is the risk of compression which could cause water to pool and the roof to leak.

They can meet or exceed the current requirements without needing as much space as other forms of insulation.

Rigid insulation is just easy to work with. It’s light, easy to cut and easy to handle which is important in roof applications.

Installation: Homeowners should hire a professional to look after insulating a flat roof. There is a lot involved to make sure it meets regulations.

- Floors

Floor insulation is an area that is often overlooked as an area that can bring significant reductions in energy consumption.

The thermal performance of an uninsulated domestic floor is usually fairly poor. The greatest heat loss through an uninsulated floor is from the edges. But if you look at the floor dimensions of a typical house, complete floor insulation has a big advantage over perimeter insulation.

The best way to insulate a suspended timber floor is to install the insulation between the joists.

If you are using a fibrous material would mean supporting it underneath with netting. The risk of sagging, and possible deterioration of performance because of air movement or moisture makes this an uncertain solution.


To insulate a suspended timber floor, you place insulation between the timber joists and under the floorboards . This may or may not be easy, depending on the space below and the difficulty in removing floorboards to get in.

Older floorboards are butted together tightly.

If there is enough void space underneath it may be possible to install insulation by removing a few floorboards at the side of the room. This would allow access to the void space.

If it's a relatively new floor, i.e. less than thirty years old, the floorboards may be tongue-and-groove board. In this case they will all have to be lifted .

The latter option will generally be uneconomical unless you need to lift the floor to carry out some other work also.

- Cavity Walls

Cavity wall construction still remains the predominant build method within the UK.

Most modern buildings have two walls. An outer wall and an inner wall and there is a gap in between them to stop the wind and the rain.

If there is nothing between the inner and outer walls the heat will simply radiate through. To stop that from happening you need cavity wall insulation.

When cavity walls were first designed heat retention wasn’t a priority, so the vast majority of all homes weren’t built with adequate insulation. Blow-in insulation is most commonly used to insulate existing cavity walls.

Nowadays in all newly built properties, cavity wall insulation is added as part of the building process. Using insulation board can be a great option for new builds.

With new builds you have two options when it comes to using insulation board in cavity walls:

Partial fill cavity wall insulation

A partial fill cavity wall application is designed to provide an effective barrier to rain penetration by allowing the traditional cavity wall to be maintained. The insulation is fixed to the inner leaf of the wall construction maintaining a clear cavity. Cavity closers should be used at window and door openings to prevent cold bridging at junction details. Examples include the Kingspan Kooltherm k8 range and the celotex cw4000 range

Full fill cavity wall insulation

New innovations in insulation board such as the Celotex CF5000 range allow the full cavity to be insulated with rigid board. The are specifically designed with a rebated edge to eliminate the passage of moisture. This maximises thermal performance without the requirement to widen the cavity.

3. What else should I consider?

Plan your job to get your materials on time.

It is important to plan your job correctly. Ask for delivery 2-3 days in advance of when you will need it. This gives you a bit of breathing space in case of unforeseen circumstances.

If you don’t get the delivery on time, you are left with men on site waiting around. This costs time and money. Make sure you plan correctly and go with a supplier who has good on time delivery and good reviews.

If you need accessories make sure you take this into account. We supply all the accessories you might need such as bonding, foil tape, mushroom fixings, knives and saws.

Be careful to buy new insulation and avoid buying “seconds”

“Seconds” are the boards that didn’t make it through the manufacturer's quality control process. They are a gamble, some boards come with air bubbles the size of your fist but you have no way of knowing just by looking at them.

That might be ok on a small project like a shed or a dog house but you should definitely avoid it on a new build. Seconds can lead to problems like thermal bridging and condensation spots.

If your building contractor is supplying the insulation then you should check for receipts to make sure you are getting good quality new insulation boards.

- What thickness of board do you need?

Rigid boards are sold with different thicknesses. They start at 20mm and go up in 10-15mm increments up to 200mm. The thicker the board the better the thermal performance.

A good benchmark for the level of insulation you should be looking to achieve is 0.21 W/m2K.

However, in order to achieve that standard using the standard insulated dry-lining panels, you would require a panel of the order of 100mm thick. This includes the plasterboard that is bonded to the insulation. This can be an issue in terms of losing space in rooms or in corridors/stairways.

If your walls currently have little or no insulation, you can still significantly reduce heat loss through the walls significantly by internally insulating.

- What u-value do I need?

U values are important to understand when it comes to selecting the right insulation board. The lower the U value that is achieved post insulation the better the heat retention in the property. For example when using a typical rigid board of 50mm, it is equivalent to a U value of around 0.4, 100mm is equivalent to 0.25  and 150mm and thicker is a U value of 0.2 or lower.

- Where can I buy rigid boards?

Rigid boards can be bought at your local DIY store or they can be sourced from a specialist supplier online. Whether you are buying the product online or at the store you still need to get the measurements right so you know how much of the stuff you need to buy. You can visit the DIY store and have a look at how the product looks like. When you are happy, you can go back online and buy it as very often this will give you a further saving.

- Features and benefits of insulation boards

Low thermal conductivity

- space and resource saving eg. for timber frame buildings, no need for increased studs

Dimensional stability

- No danger of slumping and sagging

Closed cell structure

- unaffected by air movement or moisture.

- longevity, majority of installations will retain thermal qualities for at least 50 years

- eliminates risk of water penetration

No hazardous and non fibrous

- no need for personal protective equipment

- non irritant

- Disadvantages of insulation boards

Insulation boards are more expensive than fibreglass. They can be up to ten times more expensive for twice the performance. They are also more easily damaged on site and can break in the wind.


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