If you read any of the magazine case studies about high-performance self-builds, Structural Insulated Panels, better known as SIPs, tend to crop up. They’re often associated with new-build houses, but a question that often comes up is whether SIPs are suitable for building an extension.
The short answer is yes, you absolutely can build an extension from SIPs, and on many projects they make a great deal of sense.
However, like most building systems, they come with their own set of rules as well as pros and cons. Build expert, Mark Stevenson, explains why understanding these solves the question of whether SIPs are suitable for your extension or not.
The benefits of using SIPS to build an extension
SIPs are a factory-made composite building system used to form the thermal envelope of a building – the external walls and roof. Each panel is made from a core of rigid insulation, typically expanded polystyrene or polyurethane, bonded between two structural skins of oriented strand board (OSB). Once bonded, the panels become incredibly strong and highly thermally efficient.
But what is the appeal of using SIPs to build an extension?
1. Speed of build process
One of the attractions of using SIPs to build an extension is speed. Because panels are designed and manufactured off-site, the build process on site is much quicker, as its more about assembly, rather than construction. It’s not unusual for the shell of a SIPs extension to be erected and made watertight in a matter of days. Whereas blockwork, which is built piecemeal and requires drying time, can take weeks and increase your extension schedule.
2. Thermal performance is improved
The exceptional thermal performance of SIPs is also appealing for self builders. As well as incorporating high performance insulation, SIPs require less structural timbers and so have reduced thermal bridging compared to traditional timber frames, where repeating studs create thermal bridges – unless there’s an internal liner of insulation.
This makes it much easier to achieve low U-values, and with a unique interlocking panel joint, SIPs have superior levels of airtightness. This all translates into lower heating bills, and an extension that has more comfortable living spaces.
3. SIPs meet eco-friendly needs
For some, sustainability is also an important consideration. SIPs are precision manufactured, so there’s far less waste on site, and embodied carbon is reduced with fewer deliveries. This all reduces the environmental impact of the extension project.
4. SIPs create structural opportunities
When it comes to structural capacity, SIPs are more than capable of handling the structural demands of any extension. The panels themselves easily transfer the loads involved, and vaulted ceiling ideas and complex roof structures are easily formed.
Whilst the same can be said for the alternative building systems, the panelisation provided by SIPs, reduces the need for additional load bearing structures, freeing up internal spaces that components such as trussed rafters would leave cluttered. This means that features such as rooms in the roof, are easier to form.
5. Building with SIPs can be less disruptive
Extending an existing home has a huge impact on daily life, so another factor to consider is disruption. If you’re living on site while the extension is built, a quicker, cleaner build is hugely attractive, especially to your neighbours.
SIPs which come as a kit of parts can be erected rapidly, and don’t need cutting with noisy disc cutters. They’re therefore much easier to live with than building with concrete blocks.
Disadvantages of using SIPs to build an extension
Aside from all these benefits, SIPs are not a like-for-like replacement for traditional systems, such as building with masonry, all of which have their plus points when it comes to building an extension.
1. SIPs require more accuracy
SIPs demand accuracy, and panels are manufactured to exact dimensions. This means that the extension design must be properly resolved before anything is ordered. As well as resolving things like window openings and door positions, the design must also take into account any inaccuracies in the existing building, which is rarely straight and plumb.
Block work on the other hand is really flexible, and small changes can be made on site with little fuss. When it comes to offsite manufactured systems such as SIPs, late design changes can also be more difficult to manage, and could result in delays and additional costs, especially if panels have to be altered or remade.
Precision also extends to the extension foundations, and SIPs, as with most other building systems, can be used on all the common foundation types (concrete strips, raft foundations and piling) but require accuracy in terms of line and level. Whilst SIPs can overcome level problems, traditional methods are just that little bit more flexible and can easily overcome minor problems.
2. Increased need for ventilation schemes
SIPs are a technical product, and the build up of the panel (which incorporates the insulation), makes compliance with the building regs easier to achieve. Warm roofs are a good example of this, where the use of SIPs avoids the complex layering of insulation and vapour control layers, needed to manage condensation risk.
That said, SIPs are a timber-based building product so an effective ventilation strategy is a key part of the building design.
SIPs buildings are inherently airtight. While this is great for energy efficiency, careful thought must be given to the provision of fresh air. This is even more important in extensions that include a kitchen, utility or bathroom, which typically have higher levels of atmospheric moisture that need to be managed.
Without proper ventilation, moisture will build up, leading to condensation and poor indoor air quality. With a SIPs extension, mechanical ventilation and even an MVHR system becomes essential. In comparison, blockwork, which is inherently leaky will ventilate naturally, so in most cases an appropriate extractor fan will be sufficient.
3. Requires knowledge to use them
When it comes to construction, as with many types of novel building systems, SIPs demand respect, and general improvisation is best avoided. The panels should be installed by component installers who understand the detailing, jointing and sealing requirements, and care should be taken to strictly follow the manufacturer's design and installation guidance.
On the other hand, traditional building methods such as masonry are tried and tested, with industry-wide good practice principles that trades and professionals understand.
4. Individual quirks require extra detailing
As a novel building system, there are also some peculiarities with SIPs that shouldn’t be overlooked. For example, internal service distribution needs careful planning. Unlike blockwork, SIPs panels cannot be chased out to accommodate services. Service zones therefore need to be constructed to accommodate the pipes and cables.
Fire integrity is another concern and plasterboard, which provides the fire integrity, must strictly comply with the fire test data and specification provided by the manufacturer. It’s a similar story with other combustible building systems though, so non-combustible concrete has the advantage here.
The cost of using SIPs to build an extension
Extension costs are a key consideration for most, and often leads to the use of one system over another. On paper, SIPs are more expensive than traditional materials, especially when comparing the cost of the materials.
However, this isn’t the whole picture, and because SIPs, as with timber frame, can be built quicker, the overall project cost is much more comparable. For example, shorter build programmes means lower labour costs, along with less time paying for scaffolding, site welfare and project management.
In most cases, the overall build cost is comparable to a traditional extension, and when the superior energy efficiency is considered, over the life of the building there are savings to be had by using SIPs to build an extension.
FAQs
Will using SIPs to build an extension impact planning permission?
When it comes to planning permission for extensions, the use of off-site manufactured technologies such as SIPs has no bearing on whether you’ll receive planning consent or not. From a planning point of view, what matters is appearance, scale and the impact of the extension on the street scene and your neighbours. The construction system, which is hidden behind the external cladding and roof finishes, is irrelevant.
Will using SIPs cause a problem with building regulations approval?
In terms of building control, there should also be no problems. Timber technology is commonplace, and registered building control approvers/building inspectors are familiar with systems such as SIPs. They’ll want to see structural calculations, thermal performance data and general construction details, just as they would with any other build system.
But even with all these virtues, if I'm asked can you use SIPs to build an extension, in my opinion, it’s always best to keep consistency with the existing building structure to avoid complicated discussions with insurance brokers and mortgage providers.
In layman’s language, this means that unless there’s a standout reason to use one technology or another, I’d probably extend a masonry house with masonry and a timber home with either timber frame or SIPs.
Keen to understand what a SIPs build looks like? Take a look at these 12 self build homes that were all constructed using SIPs.
