Structural insulated panels, or SIPs, are pieces of insulation bonded on each side to (typically) two skins of oriented strand board (OSB). These blank panels start their life in a 1.2m-wide format and come in various standardised heights from 2.4m to 7.45m.
The panels are then cut to size, timber edge pieces added and any openings formed. The same panel is used for the external wall, any internal loadbearing walls and the roof panels.
- SIPs now accounts for 8% of all UK self build construction systems
- The average SIPs house spend is £1,220/m2
- Self builders specifying timber frame spend £1,379/m2
- Render is the main cladding choice for self builders using SIPs, followed by (in order) timber, bricks and stone.
SIPs or Traditional Timber Frame System?
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 dramatically reduces repeated coldbridging in the building.
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.
Part of the construction phase is completed in a safe, dry factory, which has a huge impact on fabrication and build times. A typical two-storey 200m2 house could take just 12 days to factory fabricate and then 12 days to erect on site.
What are the Different Types of SIPs?
There are four main differences between the panels provided by the various suppliers:
There are two main types. 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 type uses an injected fibre-free rigid urethane core. The two types of core affect costs and the thermal performance of the 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.
All of the blank panels are surrounded by solid structural grade timber, giving the panels great 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.
There are two main products. 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 second 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).
What are the Design Benefits of a SIPs Home?
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 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 are linked and stacked on top of each other. Ensure that your design team work closely with your frame and window supplier.
Large open plan, flexible spaces are 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, the most cost-effective solution is to integrate a steel beam, hidden in the floor build-up.
Designing with SIPs in Mind
A SIPs designer will be able to design to the capabilities of the product, making sure you get the most for your money.
Most construction products come in standardised sizes, so it makes sense to set out your design based around these. As the most common SIPS panels come as 1.2m x 2.7m sheets, it also 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. 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.
As you are creating an airtight fabric (aim for an air test value as low as 1.0m3/hour/m2), you need a ventilation strategy such as a mechanical ventilation heat recovery (MVHR) whole house system. The design and pipework integration has to be carefully considered with the floor joist layout.
Make sure that all of the major construction details are worked out, including any areas of repeated or linear coldbridging.
What are the Limitations of SIPs?
SIPs is one of the most expensive timber frame systems. Expect to pay anything from £250 to £500/m2, depending on specification, how efficient your design is and 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.
You need to be able to get your 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 your access is a country lane, you will struggle to get the right crane on to fit the roof.
External and Internal Finishes
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 use 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, a competent DIYer could fit it themselves.
Heavier cladding options such as external skin of brick or stone will need to be built off their own foundations.
Services 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.
How to Build Successfully with SIPs
- Interview various SIPs companies, choosing 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 air leakage is to seal the vertical joints with an airtight tape, where the outside walls connect to each other
- 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
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