Which types of roof insulation are best for your roof?

The types of roof insulation available are all suitable for different applications — put simply, what works for one roof might not necessarily be the best option for another. 

You home insulation should be a major priority, whether you are self building, renovating or constructing an extension — without proper care and attention to this element, your home will not only feel less comfortable to live in, but you will also find your energy bills to be higher. In addition, when building from scratch there are also certain requirements of the Building Regulations to be met. 

Here, we take a look at the options you will be faced with when it comes to choosing roof insulation and explain the pros, cons and price implications of each. 

How do I choose the right types of roof insulation?

"Insulating a roof is perhaps the easiest of all the energy-saving home improvements," says  expert in sustainable building methods and energy efficiency in residential homes Tim Pullen.

Before you can select the best products for your home, there are several considerations to take into account. Ask yourself the following questions before making any decisions — the answers will help you make the right choice. 

• What roof type are you insulating? Flat, pitched, conservatory and so on.
• What is your budget? Are you willing to invest in your insulation?
• What type of roof structure do you have and how deep are the rafters?
• Are you retrofitting or building from scratch?

What are the different types of roof insulation?

Just as when it come to how to insulate your house, you will find that there is not just one type of loft insulation you can use — which can make the decision making process feel a little daunting. 

“The most familiar types of insulation are probably the big rolls of loft quilt you see in DIY stores,” says chartered surveyor and renovator Ian Rock. “But, there are many types of insulation, including stone wool, glass wool, sheep’s wool and wood wool.” 

“There are also plastic foam insulations such as polyisocyanurate (PIR) and spray foam," adds James Francis, product manager GBI at ROCKWOOL® UK. "In addition, you may come across cellulose (recycled newspaper), hemp, polyethylene, cellular glass and ceramic fibre.”

Can fibreglass insulation be used to insulate a roof?

Rolls of insulation (or 'blanket' insulation) are a popular and cost-effective method of insulating a roof — plus this is often a job that can be carried out on a DIY basis.  

This type of insulation is available in a number of different materials, including fibreglass and mineral wool.

Fibreglass insulation is often sold in rolls that are easy to cut to size (if not a little irritating to work with). They are usually fitted directly between the rafters (providing there is a roofing underlay).

Fibreglass insulation, as the name suggests, is made from glass fibres made by melting glass before spinning it quickly to create fibres that are then bound together. Air pockets within the glass fibres act as barriers against heat loss. 

As well as being available in rolls, fibreglass insulation can also come in the form of batts (pre-cut in flat pieces), as loose-fill insulation and also within rigid insulation boards.

What is mineral wool insulation?

Mineral wool insulation is made from raw materials such as stone, volcanic rocks or recycled glass (with added sand, limestone, and soda ash). As with fibreglass insulation, the materials are melted and spun into fibres. It is sold in rolls and batts.

While it is a little more expensive than fibreglass insulation, it is easier to handle, cut and fit, plus it offers good soundproofing qualities. 

On the downside, the fibres can still cause irritation to skin and protective clothing, goggles and a face mask are recommended, as when handling fibreglass insulation. It is also not biodegradable. 

Rolls of mineral wool insulation tend to be much easier for the DIYer to achieve good results with when insulating a roof than rigid boards (which are best left to the professionals.)

"Rigid board insulations need to be cut to size, with their joints tightly sealed, which allows plenty of opportunity for human error," explains knauf Insulation's expert Matthew Prowse. "By comparison, mineral wool insulation is flexible and adapts to fully fill the cavity. Where two rolls or slabs meet, they also ‘knit’ together, minimising gaps and maximising thermal performance. Mineral wool insulation is simply easier to ‘get right’."

Is loose fill insulation a good option?

Loose fill insulation can be made from fibreglass, cellulose, stone wool and even cork granules. Products containing high percentages of recycled materials are a great sustainable option.

Loose fill insulation is often blown into place, making it this is a good option for hard-to-access roof spaces. You can read more on loose-fill cellulose insulation in our guide.

Can rigid foam plastic insulation boards be used for roofs?

Foam plastic (also referred to as plastic foam) insulation can be used to insulate many areas of the home (including roofs) and is a popular product. 

“Foam plastic is a term used to describe insulation materials derived from petrochemicals,” explains James Francis. “These oil-based chemicals are used to create various insulation products, which are commonly formed into rigid slabs."

There is not just one type of foam plastic insulation and the most popular products for insulating a roof tend to be spray foams and rigid insulation boards, which can made from a number of different types of foam plastic, including phenolic foam (PF) insulation and Expanded Polystyrene (EPS) insulation.

The way in which you insulate your roof will depend on its structure, but commonly rigid boards will be fitted between the rafters as well as on gable walls, party walls and chimneys in the loft space. 

"With vaulted ceilings or when insulating a loft which will become a habitable room, the ceiling may need dropping to allow rigid foam insulation to be installed between the rafters (leaving an air gap between the insulation and the underside of the roof), says Tim Pullen. "The thickness of the insulation will vary depending on the depth of the rafters and the headroom available." 

Is spray insulation for roofs a good idea?

In the case of spray foam insulation, a polyurethane or latex spray should be applied directly to the underside of the roof deck, slates and tiles. This will not only insulate the space, but will also seal up cracks and prevent moisture from getting in.

However, do be aware that spray foam insulations should only be used on roofs that are in good condition.

"Some companies may offer to fix your damaged or leaking roof by spraying foam insulation directly onto the underside of the roof without fixing the problem first. We don’t recommend that you do this," warn Energy Saving Trust. "As with any type of insulation, you need to make sure that the roof is dry and in good condition before adding any insulation."

What natural insulation can be used for roofs?

There are several natural insulation types suitable for roofs. 

"Natural insulation is made from naturally-occurring sources with thermal properties," explains Tim Pullen. "Generally, these come from two main sources: animal-based insulators (mostly sheep wool insulation) and plant-based insulators."

Eco insulation comes in the form of loose fill, rolls and solid boards, with the materials used varying wood fibre and cork is often supplied in board form.

"Wood fibre insulation can be much harder than cork and is available in weatherproof grades for insulating a roof from companies like Pavatex, Gutex and Steico," advises Tim. 

Rolls of flexible natural insulation often incorporate materials such as sheep’s wool, flax and cotton, while cellulose is the most common loose natural insulation.

"It's mostly a smart term for recycled newspaper, although in some cases wood pulp is used," says Tim. "It is available loose in bags for DIY applications and can also be blown into a cavity (typically a timber frame), or wet sprayed onto a timber frame."

What insulation is best for flat roofs?

A common question is how to insulate a flat roof, given that in the past flat roofs had a bad reputation. 

"Flat roofs are a major weak point in the thermal defences of our homes because so many were originally built with little or no insulation," explains Ian Rock. 

"Some older roofs may have a layer of mineral wool quilt or a similar material laid between the joists above the ceiling, like in conventional lofts," continues Ian. "But in many cases the insulation will be well below the current stipulated minimum 270mm depth."

In the case of 'cold roofs', where the insulation is located between the joists, it is wise to upgrade the insulation by placing thick layers of insulation above the joists (effectively turning it into a 'warm roof'.)

"If the roof structure is in good condition, it’s a fairly straightforward task to place a new layer of insulation (e.g. 150mm rigid PIR board) over a sheet of vapour barrier laid on top of the deck," explains Ian. "To finish, a new waterproof membrane surface covering will need to be laid (e.g. EPDM rubber or a high-performance mineral felt). Depending on the type of insulation material used, a new timber upper deck may first need to be placed over the insulation before the new surface covering is applied."

If you don't want to raise the height of your flat roof when insulating, it is possible to work from below, taking down the ceiling. 

"Insulation can be inserted between the joists (such as rigid PIR boards or mineral wool)," advises Ian. "It’s important, however, to leave a clear ventilation space under the deck with a flow of air channelled from air vents, e.g. in fascias.  Once insulated, a new plasterboard ceiling can then be fitted over a vapour barrier sheet.

"Another solution is to leave them intact and simply line the existing ceiling with new layers of insulated plasterboard (e.g. 55mm thick) placed over a vapour barrier."

What are the building regulations for roof insulation?

“As you might reasonably expect, most heat loss occurs through walls and roofs, but all parts of the design’s thermal envelope have to meet minimum U value targets,” says Ian Rock. “The U values tell you the maximum rate of heat loss (in Watts) that’s allowed to leak out of your home per square metre of wall or roof etc. The lower the figure the better, so for example, 0.28W/m2 K is better than 0.30W/m2K.”

"For insulating a roof the Building Regulations require a minimum of 270mm of mineral wool (or similar natural insulation)," says Tim Pullen. Working out how much loft insulation do I need in most cases will not be a problem in an existing house. If there is some insulation already in place it may be that this can be topped up but if the old material breaks up easily in the hand then it is probably not doing a lot of work and needs replacing.

Which is the cheapest type of roof insulation?

The cheapest form of roof insulation tends to be rolls of fibreglass insulation or loose fill insulation, while spray foams and natural insulation lie at the top of the price scale.  

"To achieve the desired U value, natural insulation products are generally £18 to £22/m2 at 100mm thick, while for the same performance mineral wool is £5 to £8/m2 and rigid foam is £10 to £17/m2 at 50mm thickness," explains Tim Pullen.

According to Checkatrade, spray foam insulation costs around £35/m2 for materials only. 

When assessing roof insulation costs, it is important to bear in mind the savings you are likely to make by carrying out the task.

"You should bear in mind that about 25% of the heat in an uninsulated house escapes through the roof. The typical cost of 270mm mineral wool insulation for a four-bedroom house is around £700, which could save around £500 per year," points out Tim.