How to Design with SIPS
Architect and self-build expert Allan Corfield explains why structural insulated panels (SIPS) are his go-to construction choice for low-energy homes
Architect Allan Corfield explains why structural insulated panels are his go-to construction choice
PROJECT ADVICE
In-depth, need-to-know building and project management advice from leading experts
As an architect I am always trying to find the simplest solution to the challenge of creating low energy, cost-effective homes for self-builders. Structural insulated panels (SIPS) have been used as a construction method in the UK for almost 20 years. Although unknown by most and seen as specialist by some, many self-builders have embraced this construction method — as have I.
For the last six years I have used SIPS as my go-to construction type for creating ‘ fabric first’ low-energy homes. Fabric first is a principle that focuses on getting the forever bit right first – the fabric of the building – then worrying about low-energy eco bling after (if at all).
One of the key benefits over traditional timber frame or modern closed timber frame systems is the spacing of the vertical timber studs within the walls. In a timber frame building these are at 600mm centres, while in a SIPS wall they are at 1,200mm centres. Therefore the amount of poorly performing timber in the wall is halved. This has a dramatic effect on the re- peated coldbridging in the building, which is one of the key principles in a fabric first approach. Factory-made SIPS panels will be more accurate than on-site cut timber frames; and this helps to limit gaps in construction. This along with the inherent airtightness of oriented strand boards (OSB) and a rigid insulation core, helps to limit air leakage in the building, making SIPS a great fit for Passivhaus projects. The other main benefit of a fac- tory-made product is that part of the construction phase is completed in a safe, dry environment (unlike a lot of sites). This has a huge impact on the time to fabricate and then build. A typical two-storey 200m2 house could take as little as 12 days to factory fabricate and then 12 days to erect on site.
Before looking at the pros and cons of using SIPS, here is a brief description of the different types of SIPS. There are four main differences be-
tween the panels provided by the various suppliers (and there are many):
Type of insulation core There are
● two main types of insulation used for the core of a SIPS panel. The first is a solid rigid board made from EPS (expanded polystyrene). These boards are made up of thousands of small foam beads. The second method uses an injected fibre-free rigid urethane core. The two types of cores affect two main areas when considering which supplier to go with: the cost of the panel and the thermal performance of the panel.
Depth of panel The EPS panels
● range from 95mm to 195mm (but can go much deeper, if required). The injected urethane core has a slightly better thermal performance, and therefore tend to be smaller — with the larger panels being 172mm. Always go for the deeper panel if you are trying to create a low-energy home, with achievable U values going down as low as 0.13W/M2K.
Panel joining method All of the
● blank panels are surrounded by solid structural grade timber, giving the panels a huge amount of stability and strength. Most manufacturers use a male to female timber stud to connect the panels to each other. However, some manufacturers use a unique jointing system, which is made up from a mini SIPS panel. This can help with lowering the overall wall U value and minimise air leakage at the connection joints.
Intermediate floor construction
● There are two main products used by the various suppliers to provide your intermediate floor deck. The first is a timber I-joist: these have a vertical web made from OSB and a top and bottom flange of solid timber. The sec- ond is a posi-joist: this has the same solid top and bottom flange but with a metal vertical web. Both products range from 200mm to 400mm deep (depending on the span) and are topped with a structural floor deck, giving rigid squeak-free floors up to 5m in width (without extra support).
The Benefits of Designing a SIPS Home
When creating your dream self-build home, low-energy principles should be the backbone of the project; however, creating amazing and uplifting spaces for your family to grow in should be at the heart. As discussed, the low-energy benefits are huge when choosing SIPS as your construction method, although there are more immediately obvious and breathtaking benefits when it comes to design:
Open plan rooms Most self-build
● ers want to live in large open plan, flexible space. This is easy to accommodate within a SIPS home, due to the long spans of the floor joists and roof panels. The I-joist floor system can efficiently span 5m (300mm deep joists at 600mm centres). If you want a wider span then the most cost-effective solution is to integrate a steel beam, hidden in the floor build-up.
“One of my favourite design features of a SIPS home is the possibility of vast vaulted ceilings”
Vaulted ceilings One of my favourite
● design features of a SIPS home is the possibility of vast vaulted ceilings. As the roof panels are structural there is no need for traditional roof trusses — the panels can easily span 4.8m from eaves to ridge and sit on a boxed-in steel beam or a feature glulam timber beam. This can transform the top floor from a standard 2.4m floor-to-ceiling height, up to a double height of 3.6m.
Glazed gables This design feature
● goes hand in hand with vaulted ceilings, and brings the outside in to your large double-height vaulted space. Glazed gables look like a high-end design feature, but if designed efficiently can cost around £6,000 (including the window). The large opening is usually created by a steel frame that is fitted within the SIPS frame around the opening, then standard double or triple-glazed units (timber or aluminium) are linked and stacked on top of each other. Be careful to make sure that your design team work closely with your frame and window supplier.
Designing with SIPS in Mind
From the outset if you are considering a specific build method or plan for a low-energy home, then I would suggest working with professionals that specialise in these areas. You will be surprised how many architects and engineers have no idea about SIPS, let alone how to detail the various structural connections. A SIPS designer will be able to design to the capabilities of the product, making sure you get the most for your money.
Most (or all) construction products come in standardised sizes, therefore it makes sense to set out your design based around these. As the most common panels come as 1.2m x 2.7m sheets, it makes sense to utilise a 1.2m floorplan grid. That way you can lay out your walls along a grid, which will minimise the amount of cut panels. Remember, every time the fabricator has to cut a section of a panel you will be charged for the labour time and for the disposal of the excess material!
The most common ground floor ceiling height is 2.4m, which can be created by using a 2.7m high panel with a 300mm floor joist hung from the top — giving 2.4m, which is also the size of a piece of plasterboard.
One of the most important areas to consider when designing a low-energy SIPS kit home is making sure that all of the major construction details are worked out. If you want to create an airtight energy-efficient structure, any areas of repeated or linear coldbridging must be considered.
As you are creating an airtight fabric (you should be aiming for an air test value as low as 1.0m3/hour/m2), you must have a ventilation strategy. I would suggest a mechanical ventilation heat recovery (MVHR) whole house system. This uses warmed moist air within the home to warm fresh air from outside, via a heat exchanger and pipe network. The design and pipework integration has to be carefully considered with the floor joist layout.
The Limitations of SIPS
Like all other construction methods SIPS has its limitations and draw-
SIPS and Mortgages SIPS Lenders accepted is classed (CML) by most as and a lenders. modern as long method as it has of the construction appropriate by BBA The certificates, Council of should Mortgage be One thing that you would not expect to make a difference to your self-build mortgage is the choice of external cladding! If you pair your kit with a traditional brick or render on blockwork then you don’t have to worry. However, if you choose a lightweight material such as timber cladding, cement-based weatherboards or a render board system — take note. Some lenders do prefer the timber cladding element to be less than 50% of the external area of the property, so this could have a big impact on the look of your home.
backs. It is worth exploring and researching all of the various construction methods before you commit to one structure.
Tight budget SIPS is one of the most
● expensive timber frame systems, but it is also, in my opinion, one of the best. Expect to pay anything from £250 to £500/m2, depending on specification, how efficient your design is and also which company you choose. Compare this to a lower specification timber frame with a trussed roof, which could come in as low as £150/m2. If budget is more important than energy efficiency then maybe SIPS is not for you!
Tight site One of the main selling ● points for SIPS is speed of erection. This can only happen if you can get the panels on site, along with the right plant to fit them. A telescopic handler will be used to manoeuvre the wall panels around, while a crane is required to lift the heavy, long roof panels. If you can’t get a bin lorry up your country lane, then you will struggle to get the right crane on to fit the roof.
External and Internal Finishes
External As the panels provide the
● insulation and structure, there is no need for the external finish to provide any structural support — other than to support itself. This means that you can utilise modern, lightweight cladding systems, including timber, render board, rainscreen cladding systems or even brick/stone slips. The chosen façade treatment is fitted via 50mm timber battens (required for ventilation) fixed to the outside of the kit, which will have been left with a breathable membrane. If you are going for a simple horizontal timber boarding, then there is no reason why a competent DIYER can’t fit it themselves.
If adding an external skin of brick or stone, then bear in mind that these heavier cladding options will need to be built off their own foundations.
Internal One of the most common
● questions I get asked about SIPS is: “Where do my services go; do they go in the panel?” And the answer is no (unless you are building in the United States). Instead they go in a service void, created with a 25mm x 50mm timber batten. This service void is wide enough to house your heating and electrical services, with larger pipes (such as MVHR or soil waste) located in internal timber stud walls.
My Top Tips for a Successful SIPS Build
Interview various SIPS companies, ● choose the company that gives a balance of value for money, quality product and assistance throughout the project. Try to work with a company that provides a total package: design, fabrication and site erection.
Work with design professionals who ● are used to working with SIPS and can maximise the benefits and help reduce the costs.
Design efficiently, by working with ● standard panel widths and heights — this will give you the maximum building for your money.
A great way to limit all chances of ● air leakage is to seal the vertical joints with an airtight tape, where the outside walls connect to each other. I would always suggest that the client does this; it’s a great way to check the workmanship of all of the connections.
Before the breathable building paper ● is fitted to the outside of the kit, get your architect to ‘ snag’ the building prior to releasing the final payment. This way any issues with the fabric can be identified and then corrected. H