The Quick Read
- SIPs are exact, engineered, very fast to build on site, and easily achieve good U values and excellent airtightness
- SIPs panels are around 10–15 per cent more expensive than conventional timber frame, but as the frame accounts for perhaps 30–40 per cent of the overall budget, its total impact is 3–5 per cent
- They are also used as roof panels, particularly when wanting to create vaulted roofs without a complicated structure
Structural insulated panels, or SIPs, are sometimes described as being like timber frame without the timber. SIPs are made by gluing insulation between two sheets of oriented strand board (OSB) to form a tremendously strong wall or roof section, which requires little, if any, further support — in fact, the panels are so strong that they can be used to build structures much bigger than homes. It sounds miraculous, but similar concepts are employed in widely different applications, such as manufacturing aircraft wings, where a body gets its strength from knitting different materials together.
SIPs are not a new idea: they have a long and colourful history. The original experiments with honeycomb panel construction went on in the 1930s in the USA, working to some designs by the architect Frank Lloyd Wright. It was a student of Wright, Alden B Dow, who first placed extruded polystyrene (made by his family’s company, Dow Chemicals) inside a SIPs sandwich and used it to build a series of houses in Wisconsin in the early 1950s.
Dow and others proved that the concept worked and produced durable, long-lasting homes that were test-built across many climate zones. But early SIPs houses proved expensive to build and SIPs remained something of a curio until the 1980s, when a number of small homebuilders in the USA and Canada started using them, chiefly because they valued the thermal efficiency, the airtightness, and the accuracy of assembly. Manufacturing in Britain began around 15 years ago, after a number of SIPs homes had been imported into the UK from North America.
How They’re Made
Commercially, the most significant UK-based manufacturer is the Kingspan group, which runs the only continuous laminated SIPs production line in Europe, at a plant in Selby, North Yorkshire. Kingspan is best known for making polyisocyanurate (PIR) insulation, which is produced by injecting foam between two sheets, where it expands to fill the void.
SIPs production using PIR insulation is a very similar process and, therefore, it was a relatively small step for Kingspan to move into SIPs. At Selby, it produces 1.2m-wide SIPs panels at two depths: 142mm and 172mm.
One of the key factors with SIPs is to minimise the waste, because there is not an obvious destination for the offcuts. Kingspan aims to keep the waste down to between 4–7 per cent of total production, and what waste there is is recycled into other products, thus making the whole process extremely efficient. Kingspan sells these panels, known as TEK, via its own channels, including the Kingspan Potton group, and it also supplies a number of independent specialists such as SIPS@Clays and Lowfield.
Not all SIPs panels are made like this. Some manufacturers, such as the Fife-based SIPs Industries, use expanded polystyrene as the insulant. This is a little cheaper than PIR, but it can’t be blown in the same way and has to be laminated a sheet at a time. Other significant producers include Hemsec (based on Merseyside) and the Dorset-based Build It Green.
The SIPs Niche
15 years on from the rather tentative start that SIPs made in the UK, the SIPs industry is still small in terms of volume, but it’s thriving and winning a lot of work from across the housing spectrum. As insulation standards have increased over the years, the price premium for using SIPs has eroded, but as a rule they are still a more expensive structural system than the two mainstream alternatives — namely masonry blockwork and traditional timber frame.
Paul Newman of Kingspan Potton is in an ideal position to comment because the company produces both SIPs and timber frame, and offers many of its house designs in a choice of either system. “Despite the seemingly ever-increasing demand for greater energy efficiency, SIPs remain 10–15 per cent more expensive than our standard timber frame solutions, and they’re likely to remain at a premium price even after the next round of changes to the Building Regulations in 2016. But having said that, 10–15 per cent on the frame price only represents 3–5 per cent on the overall project costs because the frame is typically a third of the final cost. That’s not such a great difference, and we actually have no trouble selling SIPs to developers who are looking, for whatever reason, to build to higher than what Building Regulations standards require.”
The panels come delivered to site ready to fit together — the panels interlock – and window openings are already cut out. The model above shows a typical structural build-up of the panels with associated structural support, e.g. joists. Openings can be cut on site but must be agreed in advance with the SIPs provider and be a minimum 500mm from the top of the panel
This is something that is often driven by local authority planners who are insisting on new housing meeting higher levels of the Code for Sustainable Homes. Whereas it’s entirely feasible to improve the U values of walls and roofs by simply adding more insulation to any building system, it’s much harder to improve the airtightness levels and the cold bridging. This is where SIPs construction wins out in that it’s a very exact, engineered form of building which delivers excellent airtightness.
Paul Newman comments: “Over the past few years, there has been more emphasis on airtightness. It’s just about the only aspect of energy efficiency that is actually measurable on site, so we take great care in monitoring the performance of our building types. We have found that, whereas our timber frame solutions naturally score around 3 to 5 air changes per hour under pressure, the TEK structures are around three times better. By using one of the new air barriers, like Proctor’s Wraptite, the airtightness score falls below 1, down towards the Passivhaus level of 0.6. That’s really challenging to get to without paying close attention right through the design process, but with TEK it’s virtually automatic.”
Another area where SIPs come into their own is on designs using vaulted or open roof spaces. Whereas conventional roof carpentry would create a web of posts and beams to create the roof space, SIPs provide the capability to span from ridge to eaves with, at most, one intervening horizontal beam, known as a purlin. Thus simple roof shapes can easily and effectively be covered over in a matter of hours, supported only by a ridge beam and two purlins. In instances like this, SIPs panels are the most cost-effective method of building. Consequently, SIPs are proving popular on room-in-the roof designs, such as one-and-a-half storey houses and three storey townhouses.
Andy Porter of SIPS@Clays believes there are a number of other factors that draw people to SIPs: “Our business is split pretty much 50:50 between self builders and small developers. They tend to value slightly different things: the developers, many of whom have come back to us many times, like the fact that SIPs give them a very quick, tried and tested solution that never results in any callbacks or snags to resolve after completion. SIPs structures don’t shrink or move after they are up, and that greatly reduces the issues that the follow-on trades sometimes experience. Whereas our self build clients often want to spend a little more to get an accurately engineered home that is incredibly comfortable to live in and cheap to run. It costs maybe £10,000 more to build a home using SIPs, but you could easily spend that on a couple of granite worktops or a home cinema. Self builders look at the sums a little differently.”
Andrew Rowe from Grantham, Lincolnshire, is in some ways a typical SIPs user. He is a professional builder who has built over 50 homes during a 30-year career, always using traditional brick and block construction. This year he chose to build his own house and he decided to use SIPs for the first time. “I am really impressed,” he says. “They’re incredibly quick to build with and have some incredible qualities — so much so that I’m never going back to blockwork again because it’s slow, it’s messy and the process keeps getting interrupted by the weather. Using SIPs on what is a very big house, we’ve gone from groundworks to moving in in just 16 weeks. In addition, the SIPs panels are so accurately made that I have been able to order windows off the plan: they all fitted perfectly.”
SIPs, Passivhaus and Heating Systems
Passivhaus is a building standard originating in Germany that is now held up as the gold standard for low-energy building across the world. It combines super-insulation levels with careful attention to airtightness and thermal bridging details to produce buildings that require minimal heating — less than 15kWh/m2 per year. In fact, the heat requirements are so low that a small heating coil added to the mechanical ventilation heat recovery system is adequate for almost all Passivhaus homes.
Although SIPs were conceived long before the Passivhaus standard became widely known outside Germany, the qualities that mark SIPs out from competing build systems are identical to these benefits. Not surprisingly, SIPs are now being used to produce Passivhaus buildings and are requiring very little modification to meet the standard.
The vast majority of SIPs homebuilders install a mechanical ventilation heat recovery system in order to take advantage of the high airtightness levels. Most SIPs self builders go on to install some form of heating system, typically just putting underfloor heating on the ground floor and heated towel rails in the upstairs bathroom(s). If being warm, airtight and cheap to run are factors that you prioritise, then building with SIPs is something you should closely consider.