Best Types of Loft Insulation




best types of loft insulation image shows insulation and boards

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There are so many types and brands of loft insulation available it’s hard to know which is best. You need to insulate to save on energy costs and to reduce noise. But which do you choose? Loose fill, blanket, foil – the list goes on. Each type of insulation has its good and bad points.

Whether you’re converting the loft, laying down boarding, or renewing the existing insulation you need to choose your new insulation material wisely. If you don’t you could be seriously out of pocket and faced with mounting energy bills.

The spec of the best types of loft insulation

When looking for insulation it should be remembered that most loft conversions involve upgrading the existing roof space by fitting the insulation material above or sometimes below the rafters.

Because the building regulations are so strict on the U-values that roofs have to meet using the most efficient insulation material is essential.

As a note it should be stated that the U-value of the roof must be 0.20 W/m2k.

In recent years the process involved in insulating pitched roofs has changed in that creating a ‘warm roof’ is now sometimes used. A warm roof is created by installing the insulation material above the rafters. This allows extra thickness and creates a kind of insulated shell.

But often a warm roof isn’t practical or required and the insulation material can alternatively be placed below or between the rafters. And of course, that is the option you would take with a basic conversion or when you are renewing your current insulation material.

Insulation above the rafters

Sometimes circumstances dictate that a mixture of the two kinds of rafter insulation is used.

The disadvantage with having to install the insulation below the rafters is of course that the available headroom will be reduced.

The roof space will be further reduced by the need to include a vapour control layer to prevent condensation.

There are also some issues with placing the insulation boards between the rafters the major one being that a 50mm gap for ventilation must be left above the insulation material. The common solution is to use between and below insulation.

How thick should loft insulation be?

Probably the most difficult question to answer when looking at the best loft insulation for loft conversions is ‘how thick does the insulation need to be’?

The usual answer was always around 150mm but different building control officers will have different interpretations but most are open to being convinced that a lesser amount can sometimes be used.

Rafters are usually 100mm in depth but are then built up with a 20mm timber batten.

This allows the 50mm ventilation gap to be left along with the bulk of the insulation being fitted between the rafters with perhaps just 50mm below the rafters.

But, whichever method of installing the insulation is used we still need to find the best material for the job.

Well, in the opinion of many in the trade, the best loft insulation material is foil faced Polyisocyanurate boards.

These boards have a remarkable 0.017 heat conductivity value which makes it the most efficient material available. Not only that but the foil facing acts as a vapour control layer making them much more space efficient than other materials.

What are the different types of loft insulation?

There is a wide variety of insulation that can be used in loft conversions and all have different strengths. We’ll look at each and decide which is the best loft insulation shortly.

But, don’t forget that if you are converting the loft then it isn’t just the new loft floor that will need to be insulated but also the walls and the roof space itself.

The technology used in loft insulation has advanced incredibly in recent years.

Formerly the usual solution when thinking about the flooring was to use the traditional fibreglass blanket which, as most of us will know, is an absolutely horrible material to work with.

Thankfully it is now more common to use rigid insulation boards and there are different varieties to use in the floor, the walls and the roof.

These boards are the most efficient in term of insulating performance, are simple to install and are very thin. This later point is of course very important in loft conversions where the available headroom may already be at a premium.

But the technology in the rigid boards has also advanced and the white expanded polystyrene boards that were previously used have now being replaced by thinner and better boards such as those made from polyisocyanurate.

Types of insulation

We’ll look at each type in much more detail later in the article but for now here’s a list of the most popular types of loft insulation. Let’s look at all the varieties of insulation both for flooring and walls / roofs.

Loose Fill

The first replacement for the fibreglass blanket loose fill insulation is easier and cleaner to use.


A very useful material with a controversial past but vermiculite insulation is very effective.


Hugely versatile and a very high-performance rigid board insulation material. Its use is recommended whenever possible.

Foil Insulation

Once the great hope for energy conservation in loft conversions this material isn’t now in favour with many builders.


A volcanic glass perlite is a very popular and effective insulation material.

Roof Foam

Roof foam insulation is so necessary nowadays and easy to use.

Thermal Envelope

The thermal envelope is something that can’t be ignored when if you are converting the loft. Your loft conversion must be energy efficient. The building regulations are very strict about the energy efficient levels that your insulation must meet.

Why do we need loft conversion insulation?

Much of a house’s heat is lost through the roof, so when you carry out a loft conversion the choice and installation of insulation is obviously very important.

Extreme changes in temperature occur in the loft space throughout the year and that affects the comfort of the rest of the house.

Proper insulation in the loft space will deal with that.

Good insulation will also reduce the noise during the construction of the loft conversion.

And insulation is an important component of the building regulations.

In order to comply with the building regulations, the type of insulation to be used must be specified.

So, it is in your own best interests to be aware of the minimum standards the regs demand before putting in your building regulations application.

Urethane or Polyisocyanurate roof insulation is often used for insulation in a loft conversion. It comes as a rigid foam and is covered in foil.

The foam is a very effective insulating material. Just 30mm of this substance is a better insulator than a brick wall.

However, it is recommended that the insulation should be a minimum of 150mm to 200mm thick. That will reduce heat loss by a fifth.

Apart from the statutory requirement to insulate your loft conversion, the reduction in heat loss, and subsequent reduction in heating bill, will eventually more than recompense your initial outlay in cash.

A more commonly used material is the fibreglass blanket.

Fibreglass blankets come in rolls and are relatively easy to fit Fibreglass is a very useful insulation material.

It can also be used as insulation for the cavity walls in the loft.

However, it throws off a lot of dust and fibres, so when working with fibreglass you should wear a face mask, protective goggles and gloves.

You should also keep your arms covered to avoid skin irritation.

Another form of insulation for your loft conversion is loose fill.

Using loose fill insulation is simply a matter of pouring the material in the space between the joists and floor of your loft.

Loft conversion insulation is not only a building regulation requirement, it also makes sense.

It will help reduce the heat-loss through your roof, keeping everyone in the house snug and warm in the winter and during those cold nights.

The best loft insulation material

When looking to insulate the loft it’s important to remember that the lowest priced material may end up costing you much more in the long run.

The role of insulation is of course to conserve heat and energy whilst reducing condensation and ensuring proper ventilation of the roof space.

And the insulation has to do all that whilst keeping the thickness of the material to the minimum.

Considering the above the cheapest loft insulation may not provide the best performance.

And the result of the insulation not performing to the maximum level will result in energy being lost and energy bills rising.

The role of insulation is of course to conserve heat and energy whilst reducing condensation and ensuring proper ventilation of the roof space.

And the insulation has to do all that whilst keeping the thickness of the material to the minimum.

Considering the above the cheapest loft insulation may not provide the best performance.

And the result of the insulation not performing to the maximum level will result in energy being lost and energy bills rising.

This of course will cost the householder much more in the long run than the extra cost of installing high performance insulation in the first place.

The effectiveness of insulation is measured by its heat conductivity value (the lower the better) and the best value material combines a low conductivity value with the minimum possible thickness.

It is important that the insulation is as thin as possible to conserve space.

And, as an example, for polystyrene boards to match the performance of their Polyisocyanurate counterparts it would be necessary to use twice as much material which is not viable for the majority of modern loft conversions.

Another factor against choosing cheap loft insulation material is the fact that a vapour control layer will need to be added above the insulation material which will of course add to the thickness of material.

Alternatively, using good quality foil insulation boards removes the need for an extra vapour control layer and is much more efficient.

In conclusion it is recommended that when looking around for a quote that more factors than simply the cost of the material is taken into consideration.

The performance and efficiency of the insulation should be the determining factor when making the purchasing decision.

Loose fill insulation

Using loose fill as your insulating material is a viable alternative to other types, such as fibreglass rolls, and is very easy to lay down, or fit, into your loft.

It is a much cleaner and more convenient product to use and, in material such as vermiculite, is a completely natural and environmentally friendly product.

Whereas traditional material such as a fibreglass blanket is supplied in rolls this material comes in bags or sacks which are simply emptied and raked into the gaps between the joists and the ceiling / loft floor.

Laying lose fill insulation

Although the material is simply poured between the joists in the roof space there are a few important points to note.

The first is pretty obvious – make sure there are no cracks or holes in the ceiling!

The last thing you want is someone in the bedroom below being showered by a cloud of insulation falling through cracks in the ceiling. Needless to say, any cracks should be patched before the insulation is put down.

In a similar vein the gaps between the ends of the joists under the eaves must be sealed. Simply screwing a piece of plywood between each pair of joists (take care to leave a gap behind the board for ventilation) should be more than sufficient.

Before laying the material make sure that the space between the joists is relatively dust-free and that the depth of the joists will be sufficient to hold the amount of loose fill insulation necessary to give an acceptable level of insulation.

As most joists have a depth of four inches or so it will be necessary to raise the height of the joists by adding wooden strips along its length.

It is important that any electrical cables are kept above the insulation so they may need to be re-routed by an electrician.

Actually, laying the material is simplicity itself. Simply pour out of the bag and brush or rake between the joists ensuring the fill is kept consistently level.

For the loft hatch it is best to use blanket material contained either by plastic or an old sheet.

If you are simply insulating the loft and not putting any boarding over the loose fill insulation, make sure you keep a check on the level of the material during the winter months as high winds can unsettle the material and blow it around the roof space.

Foil insulation

Reflective foil is one of the new breeds of cleaner, greener and more effective insulation materials available today.

Made from layers of aluminium foil, wadding and air bubbles the material works by reflecting radiant heat when used with an air-space, whereas traditional materials such as fibreglass blankets conducts heat.

Confused? You’re not the only one. But let’s try to lift the mist.

Traditional materials like fibreglass reduce heat transfer by trapping air in the mass of the blanket but this does not reduce radiant heat transfer.

Foil insulation on the other hand does reduces radiant heat transfer and does so by up to an incredible 97%.

What this means is that it produces much better results with far less material than would be the case with fibreglass and other kinds of insulation.

And, because, of the high performance of the material, foil insulation reduces vapour and moisture condensation which is usually such a big problem with products like fibreglass.

Like all products there are variations of foil insulation. It is important to check that the material meets the requirements of Section L of the building regulations, some of the products, such as that used in France, will not meet the required standard though it could be used in conjunction with other material.

The best products are made from multiple layers of foil and soft wadding and can contain up to 14 layers of material which amounts to a thickness of around 25mm.

The Benefits

  • Unlike fibreglass there are no health risks when using the material as there are no fibres to irritate the eyes or particles to inhale.
  • The material can be fitted without the need to wear protective clothing such as masks and gloves.
  • The material can be very easily and cleanly cut simply by using ordinary household scissors.
  • Because it does not have the density of fibreglass, foil insulation does not absorb and retain heat.
  • It provides a very effective vapour barrier reducing moisture build up and condensation.
  • Can be fitted to rafters by using staples, nails or screws.
  • The material is non-toxic and environmentally safe and, in most cases, can even be recycled.
  • Ultra-high performance.

Using Foil Insulation

The material must be used with an open-air space otherwise all the benefits listed above will be nullified.

To install the material properly and to maximise its moisture barrier the seams of the insulation must be taped.

This kind of insulation should only be used in single sheets. Doubling the thickness of the material by using two or more layers will not improve performance.

It can be used as a standalone material or as part of a complete radiant barrier system by being used on top of mass insulation material in the roof or floor.


The good news is that the material can be very easily and quickly fitted using the minimum of tools. The material is supplied in rolls which are very easy to handle even when they are removed from their protective packaging.

As with most tasks of this nature it is best to work left-to-right and top-to-bottom. Staple the material to the first rafter or truss and unroll the length of the material stapling to each rafter or truss.

Some manufacturers recommend over-lapping each joint by around 50mm (both vertical and horizontal).

When you have finished the top layer begin the second (lower) layer, again working from left-to-right, remembering to overlap the joint slightly. Excess material can be easily trimmed with either scissors or a knife.

When all the material is fitted all the joints lust be taped with foil tape to control vapour. Counter battens can then be applied (which will provide the necessary air gap) before fixing the plasterboard and finishing off.

Some of the foil-based insulation products are designed, or best suited, to be used with other products rather than as a standalone solution so do check before using the material.

Perlite Roof Insulation

It isn’t surprising that perlite is becoming more well known and popular with householders.

Because of the escalating price of energy and the need to reduce carbon emissions the need to have adequate insulation in the roof space is as important as ever.

And perlite is an excellent insulation material.

Perlite is a volcanic glass. When heated it expands up to 20 times its original volume. Perlite is used in industrial, horticultural and construction applications.

Its insulating properties sees perlite being used for under-floor insulation as well as roof insulation boards in loft conversions.

Perlite insulation is very effective on flat roofs. The insulating properties are provided by the air cells contained within the perlite.

This type of insulation board is provided by combining the perlite with fibres and binders. The board is then treated to prevent the insulation absorbing any asphalt. It is designed to be fitted over structural roof decks.

Perlite concrete also provides first class roof insulation. If used in conjunction with polystyrene insulation boarding it produces high thermal resistance. It also offers superior wind and fire resistance compared with other roof insulation materials.

Perlite concrete insulation is also ideal for reroofing. The polystyrene insulation board is fixed to the substrate with a perlite concrete mixture. The board is then covered with the perlite concrete insulation.

The use of perlite roof insulation seems to be assured for the long term. World reserves are currently assessed at 700 million tonnes. It also remains relatively cheap.

Other factors which make the substance so popular as an insulation material are that it is easy to handle, and install.

The great things about Perlite are that it does not shrink or warp, does not retain moisture, is fireproof and meets the fire regulations. It also does not rot and because it is inorganic never becomes infested.

Polyisocyanurate Roof Insulation

Polyisocyanurate is one the most efficient and high-performance insulation materials available for loft conversions and is becoming more and more popular with builders.

This material is most commonly supplied as foil faced rigid boards.

Supplied in a variety of thicknesses from 20mm up to 100mm and sized 2400 x 1200mm these boards can be used in both pitch and flat roofs as well as for wall insulation.

Polyisocyanurate (PIR) is one of the three most common multi-use rigid insulation boards along with phenolic foam (PF) and polyurethane (PUR) but is a more fire-resistant material than its competitors.

The fire safety aspect of polyisocyanurate insulation material is hugely important for loft conversions.

With the building regulations so rigid when it comes to fire resistance the reduced combustibility of PIR makes it very attractive to planners and builders.

Although polyisocyanurate roof insulation is made in a similar way to polyurethane and, although the same elements are used in the manufacture of the two products, the higher intensity of materials used in polyisocyanurate gives it greater fire resistance and forms less smoke.

The material uses a blowing agent, often pentane, which expands the foam and enhances the thermal resistance of the insulation.

The performance of insulation material can be measure by its conductivity value.

And foil faced polyisocyanurate has the best (lowest) figure.

Typical values are:

Foil faced polyisocyanurate 0.017
Foil faced phenolic 0.021
Foil faced polyurethane 0.021

The great benefit to builders and planners in using multi-use rigid insulation boards such as foil faced polyisocyanurate is that only half the thickness is needed compared to other more traditional boards such as expanded polystyrene which has a conductivity value of 0.033.

Roof Foam Insulation

Insulation is of course an essential ‘must have’ for the homeowner because much of a property’s heat can be lost through the roof space.

Indeed, as we shall see, there are strict rules on insulation covering new builds which includes loft conversions.

Typical foam insulation is produced by combining two liquid components. When sprayed they produce a foam which expands. Within five minutes it solidifies to become a material with exceptional insulation properties.

Indeed, the foam serves a double purpose. When sprayed on the underside of roof tiles the foam not only provides insulation, but fixes the tiles in place. So, it is particularly useful for older roofs and is sometimes used in loft conversions.

Installing this kind of insulation can also be carried out as a DIY project. Kits are available for those wanting to take on the challenge.

his type of sprayed material offers several advantages over other insulation and can maintain the properties thermal envelope. The foam adheres to all surfaces and is easy to apply.

Anyone used to spraying paint should have no problems when working with foam insulation. It dries quickly and does not drip.

The manufactures of the spray foam insulation kits assure the public that experts are not required to carry out the work. It is just a case of reading and keeping to the instructions.

They also give assurance that the foam will not rot the roof timbers. However, if rot is already present it should be dealt with before any spraying can commence. Also, the foam insulation in the roof will not absorb moisture.

As for the risk of fire the material contains fire retardants. Also, current building regulations demand that any roof insulation foam should be covered by a fire-resistant structure.

Roof foam insulation, as well as saving on heating bills, offers other benefits. It helps cut down on noise pollution and keeps the property cooler during the warmer weather. It also helps improve the stability of the roof itself.

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