Which Type of Loft Insulation Should I Choose?
Looking to update your roof insulation? Let our experts talk you through the options
However, there’s more than one type of loft insulation, and there are pros and cons to each – including limitations on how and where you can best use it. We asked three experts to talk through the key loft insulation options to help you decide which might work best in your home this winter.
Professional advice from: Anthony Brown of Dwell Design; Angus Eitel of fiftypointeight Architecture + Interiors; energy efficiency expert David Hilton of Heat and Energy
In simple terms, insulation is a physical substance that stops heat escaping through your roof and walls. This not only means your home will feel warmer during the colder months, it’s also good for the planet, as you won’t need to use as much energy to keep it warm.
Your loft is particularly important when it comes to insulating your home. According to the Energy Saving Trust, a quarter of heat is lost through the roof in an uninsulated building. The trust also says that, installed correctly, loft insulation will pay for itself many times over during a 40-year lifetime.
“The choices of insulation on the market can be quite bewildering,” Anthony Brown says. “As well as cost, think about where the insulation is being used, how much waste material might be generated by using one type over another, the possible future use of your space, and how it’s accessed,” he advises.
There are also a few terms it may be helpful to understand. “U-value and R-value are both used to identify the efficiency of insulations, but in directly opposing ways,” Anthony explains. “The U-value is the conductivity of insulation [measured in W/m2K], whereas the R-value is the resistance [measured in m2K/W], and they are the inverse of each other.” So while a high R number is more effective, the lower the U-value, the better.
You might also hear references to “warm”, “cold” or “hybrid” roofs. “A ‘warm’ roof has the insulation in the pitch of the roof,” David Hilton says – so fitted between rafters, within the timbers that make up the roof, rather than the floor joists below.
“A ‘cold’ pitched or flat roof installation has air flowing beneath its waterproofing layer of tiles or roofing membrane,” Anthony says. “And a ‘hybrid’ installation will have insulation against this waterproofing layer, but also between the structure.”
More: How to Retrofit Insulation in an Existing Property
You need a dry space for insulation to work effectively, so any damp issues should be dealt with first.
Remember also that a more thermally efficient home can cause condensation to form on cold surfaces, which ultimately leads to mould and rot, so ventilation is crucial. “Older properties might have a non-breathable membrane under the roof tiles,” David says. “If you suddenly start packing insulation in, it can suffocate your timbers and trap moisture.”
You also need good draught-proofing. “If you live in a wind tunnel and heat is escaping through windows and under doors, your insulation will be wasted,” David says.
Your insulation choice will also inevitably be affected by whether you have a loft conversion and whether your loft space is boarded and used for storage. It’s a good idea to get professional advice on the best type of loft insulation for your space if you’re uncertain.
With all of this in mind, let’s consider the options…
This is the classic insulation you see in either thick rolls or sheets in the DIY superstore. “There’s not much difference between an insulation batt and a quilt,” Angus Eitel says. “The quilt comes in a roll; the batt is just a sheet of the same product held together more firmly.
“Blanket insulation is available in a variety of materials, including natural sheep’s wool, mineral or rock wool, and fibreglass wool, from a variety of manufacturers,” he continues. “It comes in a few thicknesses, usually 100mm or 200mm.”
Fibreglass or glass wool is made from glass and sand, melted and spun into fibres resembling wool. Mineral wool is made from raw materials such as stone and glass, similarly melted at high temperatures and spun into a wool-like substance. It usually contains more recycled materials than glass wool, and tends to be more expensive and denser, with a higher R-value.
In a loft, the recommended depth of blanket insulation (fibreglass or mineral wool) to meet Building Regulations is 270mm.
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It can easily be trimmed to fit different spaces. “If your loft space is not required for storage, then you can use the cheapest quilt insulation possible, as thickly as possible, while making sure air can flow over the insulation,” Anthony says.
If you’re concerned about noise, it’s also good for acoustic insulation, plus it’s versatile. “It can be used in cavity and stud wall construction,” Anthony says.
Of the different options available, “mineral wool doesn’t burn, is natural and is cheap,” David says.
Sheep’s wool is an all-natural option that’s highly effective and itch-free. “It’s likely to be more sustainably sourced, as it’s a natural by-product rather than a fabricated material,” Anthony says.
Cons Although sheep’s wool is natural, it’s more expensive than other blanket options, and it’s often (though not always) treated with chemicals to disinfect it.
“Blanket insulation is bulky and there are certain limitations when using it around lights and electrics, so take advice from an electrician before fitting it,” Anthony says.
Fibreglass blanket insulation can get damp if a space isn’t well-ventilated. If it gets wet, it may need to be removed. Mineral wool, on the other hand, is water-repellent.
If you want to lay boards over your joists for storage, you may need to raise the level of the floor (by using timber battens or plastic legs, for example) to fit in the right thickness of insulation.
The fibres from fibreglass or mineral wool insulation can irritate eyes, noses and mouths. Wear a mask if you’re fitting it, and take care not to breathe in the particles. This type of insulation shouldn’t be disturbed once it’s laid for the same reason.
While fibreglass and mineral wool insulation is energy-efficient once in place, its production is energy-intensive.
As the name suggests, this kind of insulation is loose rather than packed in, and requires a specialist company to ‘blow’ it in.
“Blown insulation can be used in the loft,” Angus says. “This is a mechanically blown solution using a granular insulation material. This could be mineral wool, wood fibre or other materials.”
The options commonly include cellulose, which is made from recycled paper that’s chemically treated to resist fire, mould and insects.
Pros Loose-fill insulation can be useful if you want to retrofit, as it can simply be blown into a cavity. It’s thought to keep spaces both cool in summer and warm in winter. It has soundproofing benefits and it’s quick and easy to install.
Cons This type of insulation isn’t a DIY job. “It would have to be installed by a specialist,” Angus says.
This isn’t a practical solution if you’re intending to use your loft space. Loose-fill products are often used where access is tricky, such as beneath low-pitched roofs or above gabled dormers. However, Anthony points out that for loft insulation where access is never required, loose insulation is usually the cheapest. “Though it can be very messy,” he adds.
“If you have a windy loft, you don’t want to put in anything that will blow around,” David points out.
Loose insulation may settle over time, leaving some areas less effectively protected. Blown-in fibreglass insulation can be harmful or irritating to airways if the particles are inhaled.
More: How to Choose the Perfect Hallway Flooring.
PIR boards are rigid insulation boards made from plastic that’s been set into foam. They often feature an aluminium foil backing. Common brands include Celotex and Kingspan.
Pros PIR boards tend to be thin, light, fire-retardant and thermally efficient, with a high R-value. “They’re high-performance, airtight and easy to use,” David says. They can also easily be cut to fit.
The structure means they’re less likely to absorb moisture than some options. They come in different thicknesses, so you can usually find something to suit your space.
“Solid sheet insulation has a higher performance than blanket, with the thermal resistance better for the same thickness of material,” Angus says.
PIR boards are ideal in loft conversions and can be cut to fit along a flat or pitched roof. “If the roof is intended to be inhabited, the insulation needs to be placed in the roof plane,” Angus says. “While this can be done with blanket or batt insulation, you’d need so much thickness, you’d lose significant head height.”
Cons “PIR boards are expensive and you can’t always use them where you want to,” David says.
The boards need to be fitted professionally to avoid gaps, and areas around joins may need extra insulation. They are also more expensive than more basic blanket and batt options.
“These would be the least environmentally friendly, as they’re all man-made products,” Angus adds.
For a greener alternative, look into insulated cork boards. These are lightweight and easy to saw, and can be used in roofs and floors. Cork is a renewable resource and is also naturally damp-proof and resistant to rot.
As the name implies, spray polyurethane foam (SPF) is simply sprayed in place.
Pros “It’s quick and easy to install,” David says. “It fits in if you have an awkward-shaped space, and it fills the gaps – for example, under rafters.”
It can be useful in difficult-to-reach areas, it’s very effective and can also offer sound-proofing benefits.
The options include spray foams made partially from recycled materials, such as Lapolla 4G, which are non-toxic and absorb little moisture.
Cons It’s important to get specialist advice before going with spray foam. According to Which?, it shouldn’t be used in listed buildings or houses with thatched roofs, and it can affect the value of a property. It’s one of the more expensive insulation options. Some foams can release harmful fumes when being installed, so it’s recommended to stay away for at least 24 hours afterwards. You also need to be certain it’s what you want. “Once it’s in, it’s incredibly difficult to remove. It’s a forever decision,” David warns.
This kind of insulation is made of numerous layers of foil interspersed with layers of insulating foam and wadding – look for the trademark shiny silver rolls in stores. Examples include Actis Triso-Super 10.
“Multifoil solutions are usually placed over the internal face of rafters,” Angus says.
Pros They’re less bulky than blanket options. “Multifoil options can provide high levels of insulation for a relatively limited thickness,” Angus says. It can be used in walls or floors as well as roofs.
Cons “Multifoil options are very good with certain types of heat, but less so with others, so they might need be used in combination with other materials,” David says. That’s why you might see insulated boards covered with foil. It’s also not the cheapest insulation option around.
This high-tech insulation material is the gold standard in modern insulation. “This thin, high-performance insulation was first used by NASA,” David says. “It’s a gel where all the moisture has been removed, and it contains numerous tiny, non-porous air pockets.”
Pros It’s incredibly thermally efficient, slim and light. Aerogel has the highest insulation value of any material with the lowest thermal conductivity value of any solid. “It’s really high performance and twice as effective as PIR boards, with half the U-value,” David says.
Cons As you might expect, “the drawback is it’s very expensive, around £100 per sq m,” David says. However, he suggests one option is to use it in tandem with other insulation materials – “for example, in tricky areas such as the ‘reveals’ around windows. You don’t have to use it for a whole wall,” he says.
What kind of loft insulation did you choose and are you happy with it? Share your thoughts and advice in the Comments.