Which Alternative Materials Could I Use to Insulate My Extension?
Hay, wool, even paper… There are many eco-friendly ways to keep your home toasty. Check out this expert guide
The good news is there are plenty of eco ideas out there. Your professional will advise you, but it’s worth understanding the many choices you have and that these will not only affect the temperature and ventilation in your new room, but also the extension’s final aesthetic and its sustainability credentials.
Professional advice from: Nimi Attanayake of Nimtim Architects; Lu Bai of Matthew Giles Architects; Phil Wontner-Smith of PWS Architecture + Design
Why the focus on extensions? Well, for one, it’s a new construction and, as such, all insulation options – from industry standard to niche – can be explored with your professional.
Extensions also come with quite specific insulation requirements, mostly because they tend to have more glazing than other parts of the property. Phil Wontner-Smith advises increasing insulation in the room itself to mitigate the heat ingress and loss through the windows. “Triple glazing is also a good start,” he adds.
“You should insulate the new elements – walls, floor, roof – of an extension so they meet the minimum U-values set out in current Building Regulations,” Nimi Attanayake says. (U-values are used to measure how effective elements of a building’s fabric are as insulators – the better the insulation, the lower the U-value.) “Most insulation suppliers will be able to assist you in calculating the relevant U-values,” she adds.
Below are just some examples of eco-friendly insulation.
Hempcrete is a biocomposite material that contains a mixture of hemp, hurds – the coarse part of hemp or flax – and lime (the mineral not the fruit!).
Nimi explains that it acts as an insulator and moisture regulator, and lacks the brittleness of concrete, so no expansion joints are required. She further describes the material as fairly lightweight. “This can dramatically reduce the energy used to transport the blocks,” she explains.
“As we begin to add more insulation to buildings to lower their energy requirements, the volume of insulating material we use is going to rise dramatically,” Nimi continues. “So it makes ecological and financial sense to fill this volume with materials that are renewable, low-impact and, ideally, sourced from waste streams or by-products from other processes.”
Nimi says she and the Nimtim team are very excited by hempcrete. “It meets all of these important criteria, and compares favourably with conventional insulation materials in many ways,” she says.
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Perhaps the most visually appealing way to boost your extension’s insulation is by growing vegetation on top of it. There are various ways to do this, depending on how much weight your structure (and budget) can support.
“A green roof has excellent insulating values and is great for preventing water runoff from your property,” Lu Bai says. “We always try to propose a green roof for the new extensions we design [if the roof is flat enough].”
Is it also an expensive option? “There are very competitive products out there now and it’s definitely much more affordable than it used to be,” Lu says. There’s also the potential for retrofitting a green roof in certain situations.
More: How Can I Install a Green Roof?
Cork may conjure up images of peeling 1970s floor tiles, but the material has come a long way and is back in favour in many applications – including as insulation. It’s natural, incredibly durable, and can be fully recycled. Nimi adds that it’s breathable and free from chemicals, synthetic resins and carcinogenic materials, too. As such, it creates a healthy environment inside the house.
This extension, for example, was built with walls constructed from breeze blocks and clad internally and externally with thick cork.
“We used cork as an insulating material and exposed it internally,’ Nimi says. “This was a cost-effective approach, as it meant we didn’t need to plasterboard, skim or paint. It also has damp-proof qualities and performs well acoustically.”
Nimi adds that cork cladding may need treating for fire spread, depending on where it’s used in the building and how much of it there is. “This is something that should be reviewed and approved by the Building Control authority/officer before it’s installed.”
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Phil advocates for the benefits of one of the oldest, most natural materials around: wool. “Wool is surprisingly readily available and there are lots of companies supplying it,” he says.
As well as providing good insulation, it’s also breathable – in other words, it lets moisture in and out. And ventilation is key to creating a healthy internal space. “[This works] as long as you have a breathable external surface – say, lime mortar, rather than cement – on the other side,” Phil adds.
Internally, wool insulation can be faced with a breathable alternative to plasterboard, such as wood wool boards, to provide a surface on which to decorate. Phil adds that if traditional plasterboard/insulating plasterboard and gypsum instead of the alternative boards mentioned above are used, these would typically up the U-value, because they provide greater air-tightness. However, this choice would cancel out the breathability of the wool, as these do not share this quality.
Although insulated plasterboard is not a natural product, Phil points out that, when used correctly, it can mitigate the carbon used in its creation. He explains that the board is a highly effective insulator – which should reduce fossil fuel use for heating – as well as extremely long-lasting, meaning it shouldn’t need replacing. The choice of breathable or non-breathable board will depend on the building and project priorities.
“The only downside to using wool is that you have to use a thicker amount,” Phil says. Bearing this in mind, a good place to use it is when insulating a roof with deep timbers of 200mm to 300mm. “Then you can pack out that cavity with wool insulation,” he says.
Wool could in theory be used in place of mineral wool, which is made from glass or stone and takes a lot of energy to produce, from the quarrying to the heat required to be able to spin the raw material into the wool-like texture it becomes.
Also waving the flag for minimally processed, breathable materials is hay. What could be more natural than dried grass? The idea is that hay is used in bale form, making for a very solid building material.
A benefit of using hay bales to insulate, Phil says, is that – with the addition of a mesh product – you can plaster directly onto it. “As long as you’re using lime,” he adds, “because it flexes to stop it cracking.” The bales are packed into wooden frames.
Hay is a much more specialist option, and not generally suitable for a small, urban extension. “You’d need some serious space to use hay, as the bales can be half a metre thick,” Phil says.
We’re not talking about your scrunched-up weekend papers here. This specialist waste material comes in the form of a powder treated with flame retardant. Like wool, it’s added to wall cavities between studs.
“It’s also readily available,” Phil says, “but the method of getting it into the area to be insulated is a bit less common; you need specialist equipment to hose it in.”
SIP stands for structural insulated panel and it’s not only great for keeping heat inside a building, it’s also strong and thick enough for building walls.
Phil has a very visual way of describing the boards: “This new-ish technology is like an ice-cream sandwich, but with insulation material instead of ice-cream and OSB [oriented strand board, similar to particle board] instead of cake,” he says.
“The OSB part of SIPs is made from wood chippings, a waste product that would otherwise be landfilled or burned,” Phil adds. It’s also typically made from fast-growing wood from carefully managed forests.
The foam, according to the Structural Insulated Panel Association (SIPA), is composed mainly of air and just 2% plastic, and takes less energy to produce than a fibreglass equivalent.
“It’s really good for quick building,” Phil says. “It’s structural, and comes in the form of what is pretty much a ready-made wall, factory-cut and drilled to the precise size needed, with holes in the right places.” This precision results in minimal waste during production.
SIP is insulation in itself, but you can also add insulation over it, should that be necessary. The material’s biggest pro, Phil says, is its energy-efficiency. “It creates an airtight space, so it’s great for people who want to use an air recovery system or build a passivhaus,” he explains.
Bear in mind that when an airtight space is built, ventilation is even more important to consider. “Ventilation is crucial, as the most well-insulated house can still condensate when there’s bad ventilation,” Lu explains. “Always make sure the new glazing comes with trickle vents, incorporate plenty of openable windows into the design, or default to a mechanical system if really necessary.”
Can an extension be retrofitted with insulation? “Definitely,” Lu says. “We tend to specify insulated plasterboards when dealing with existing parts of the house.”
As already mentioned, insulated plasterboard in itself is not an ‘eco’ material. However, its results can be very green, as it’s such an effective insulator, helping to conserve energy. Equally, there are breathable alternatives to plasterboard available. If this idea appeals, it’s worth asking two or three professionals for their thoughts on this, its availability and its suitability for your project.
Do remember that any additional internal insulation is likely to thicken your walls and, therefore, shrink your room. External insulation can also be retrofitted in some cases, such as insulated render, and this may resolve the issue. As cork can be used externally to insulate walls, this may be suitable, too. Nimi says, “The cork [in the previously mentioned Nimtim project] was used as a cladding to the structural blockwork walls, but it could be bonded onto brickwork if clients are looking to retrofit. It can also be used with timber construction.”
It’s not just walls that can be retro-insulated, and many areas may lend themselves to the use of the alternative materials mentioned in this piece. “Some of the best places to insulate retrospectively are roofs, as heat rises, but the floor is another great place where possible,” Phil explains. “If it’s timber, you can remove the floorboards to insulate; if it’s concrete, it’s not impossible, but it’s likely to be expensive.”
More: How to Retrofit Insulation in an Existing Property
Have you used or are you tempted to use one of these insulation materials? Let us know your thoughts in the Comments.