Wall thickness was a priority on this narrow site. The Kingspan TEK® Building System was sourced from SIPs@Clays

The debate over which is the best construction system continues to rumble on, with no real prospect of agreement or resolution. However, even once you’ve opted for a particular system – be it masonry, timber frame, ICF (insulated concrete formwork) or structural insulated panels (SIPs), for instance – there are still choices and decisions to be made. Here, we look at SIPs in detail and explore the different options available.

What are SIPs?

Essentially a SIP is a panel consisting of a rigid foam insulating core bonded between two structural facings, typically oriented strand board (OSB) but this can also be plywood or metal. The bonding of the insulation to the facing material creates a stress that allows the whole panel to withstand structural loads.

The idea of a foam core panel originated in the 1930s in Wisconsin, US, but did not really get going until the 1970s. Now there are millions of SIPs buildings around the world. However, we still tend to think of SIPs as a new, unproven technology, when the reality is that even such luminaries as Frank Lloyd Wright have successfully used SIPs.

  • The key differences between SIPs systems is the material used for the core – this could be polyurethane (PUR), polyisocyanurate (PIR) or polystyrene (PS) insulation – and the method used to connect panel to panel
  • Panels with a PUR core achieve the same U value over a thinner cross section than panels with a PS core
  • Attention to detail and quality workmanship on site is key to ensuring a SIPs build remains airtight

How SIPs Differ

There are two distinct differences between different SIPs products: the insulation material which makes up the core, and the method of connecting the panels. To be considered a genuine SIP the insulation can only be a rigid foam and therefore polyurethane (PUR), polyisocyanurate (PIR) or polystyrene (PS).

The latter also comes in two forms:

  • expanded (EPS) which is the white, bead-like form we are used
  • extruded (XPS) which tends to be colours other than white (grey and blue are more typical, but XPS can be orange or even green).

There is another ongoing debate about which type of PS is best, but in terms of insulation quality there is really little to choose between the two. A paper entitled The Way To Go published by Harvard University in 2008 essentially suggested that polystyrene, on a global scale, is a negative thing from an environmental perspective; so maybe which is the best insulator is a bit of a moot point.

Just as important is the structural stability of SIPs, which comes from connecting panels. There are three options for connecting panel to panel:

  • OSB thin-spline
  • mini-SIP spline
  • dimensional timber spline
SIPs jointing type illustrations

A paper published by Prathan Rungthonkit and Jian Yang at the School of Civil Engineering at the University of Birmingham (Behaviour of Structural Insulated Panels (SIPS) Under Short-Term and Long-Term Loadings) compares the three options. Laboratory testing shows that a dimensional timber spline (which consists of a 50x100mm softwood between joints) performs best in structural terms, i.e. it supports a greater load before failure and deflects less under loads.

It’s worth noting that there are also panels sometimes referred to as SIPs that use sustainable insulation materials — hemp, hemp lime, straw, etc. While using sustainable insulation may be a good thing, these panels need dimensional timber on all four sides and the insulation is not bonded to the sheet material being used as an infill. So these are, in fact, closed panel timber frame systems, not SIPs.

Key Issues

From a technical perspective, choosing between SIPs systems comes down to a decision on the insulation (PUR or PS) and the panel jointing method — these are, after all, the only features that really differentiate the various options. Here we look at the principal issues surrounding these two points.

Cost

None of the main suppliers will commit on this point. “Unfortunately it is very difficult to give you an idea of costs, as this is very much dependent on the complexity of the building design,” says Andrew Porter of SIPS@Clays. Other suppliers say broadly the same.

In the same way that a SIPs system forms only a part of the cost of the build, the panels themselves are only a part of the cost of the system. Design, engineering, delivery, and erection will all be added to the cost, as will the charge rate and profitability of that supplier.

Longevity

Again, all the major players say a similar thing: they all work to a 60-year design life, but state that this is the minimum standard. There are a few buildings in the US dating from the 1930s and 1940s, but the technology only really got going in the 1970s, so we only have around a 40-year history to work with. That history shows that SIPs buildings continue to perform well, and that different systems or core insulation materials have no impact on the life of the building.

Having said that, it is recognised that SIPs do not perform well in flood situations; be that internally or externally caused. The sealed nature of a SIPs wall means that it takes a long time to dry out. This can lead to the OSB or plywood delaminating (i.e. falling to bits), which tends to happen at low level and will have a serious impact on a panel’s stability.

Insulation quality

Here there is inarguable difference. PUR has an R value (this is a measure of thermal resistance) of 0.021W/mK, while the R value of PS is 0.035W/mK. What this means is that PUR transmits about 40 per cent less heat for a given thickness, so PUR SIPs can achieve the same U value in a wall with a thinner cross section. Constructing a wall with a U value of 0.2W/m2 will make it around 212mm thick overall. Achieving the same U value with EPS will mean the wall will be 262mm thick. The question then is does that loss of 50mm internal floor space matter to you?

Wall thickness aside, there are other differences too. The insulation core of PUR panels is autohesively bonded to the OSB facings; this means that the PUR is injected in liquid form into the panel, where it expands. The panels are held at high pressure, making the PUR foam bond to the OSB. It is said that this provides better adhesion than the glues used in panels using PS as the core insulation.

It should also be noted that there is a geographical difference. PS is the insulation of choice across the US, where the use of PUR remains diminishingly small. This is perhaps a legacy issue: SIPs were developed in the country using PS, and when the technology began to take off there in the ’70s and ’80s, PS was virtually the only option. Similarly, PS has many adherents in the big German off-site house manufacturers, where it is used as the core insulation and as an external insulation. The sense there is that it works so don’t change it. Conversely in the UK PUR is the big player, with the Kingspan TEK® Building System dominating the market.

But the quality of insulation is not just about thermal conductivity, it is also about thermal bridges – said to account for 25 per cent of the heat escaping from a house – and airtightness. This strikes at the joints, so the jointing method and the competence of the erection team is key.

The best structural stability comes from a dimensional timber spline joint, but this can leave a thermal bridge. Mini-SIP joints overcome the thermal bridge issue, as do OSB thin-spline joints. However, in the long term, dimensional timber spline joints perform best as they deal with inherent panel creep (more on which later).

Relatively high levels of airtightness are available as standard with all SIPs systems but actually achieving this is reliant on the competence and diligence of the erection team. The best system available can be wrecked by sloppy on-site workmanship. Airtightness is, ultimately, a function of that team.

SIPs installation on a wooden frame

Wall thickness was a priority on this narrow site. The Kingspan TEK® Building System – sourced from SIPS@Clays, who provided the erection team – were specified by the self builder behind this project; the PUR core gives this system a comparatively thinner cross profile

Structural Stability

In addition, airtightness is a function of structural stability; if the building moves it is likely to open cracks, impacting on airtightness. Independent testing shows that SIPs offer greater structural stability than equivalent timber frame systems. Obviously, structural stability is of paramount importance and the building’s designer will ensure it continues to stand up, whatever construction method is used. Achieving it with SIPs should require less material and (notionally, at least) cost less than an equivalent timber frame.

But this is not the whole story. SIPs are subject to ‘creep’. The bonding of the insulation core to the facing material puts stress on that facing. Over time that stress begins to release and the panels ‘creep’. As the aforementioned University of Birmingham paper suggests, this movement can be measured in millimetres, and dimensional timber spline joints perform best in this respect. The major impact of this is on insulation and airtightness and there is an argument that this really only applies to PUR systems due to the autohesivity (the bonding) of the core insulation.

Environmental issues

The manufacture and use of PS has been subject to a lot of regulation in recent years to eliminate the use of Ozone depleting and toxic gases in its manufacture, particularly in the UK and EU. Now, neither PS nor PUR use toxic or Ozone depleting gases; both use gases with low global warming potential.

In terms of recycling, EPS and PUR can be recycled but XPS cannot. Whether EPS or PUR actually are recycled is another question. One major UK SIPs manufacturer using EPS compresses the waste into solid, heavy blocks which are then shipped to China for ‘recycling’. Whether this is motivated by environmental concerns or merely to meet EU regulations is still in question.

Making a Decision

So, in conclusion, the cost of the system itself will not make or break the deal. Longevity is a non-issue if the build is not in a flood plain. Insulation quality favours PUR but structural stability favours EPS. And so, there are pros and cons to weigh up between the different options. If any of these factors are important to you, then it’s a good idea to do your homework; the finer detail regarding what a SIPs manufacturer and/or supplier offers is not always readily available on their marketing materials or website.

TRADA (Timber Research and Development Authority) suggest third-party certification – which means BBA approval – is a good idea. But over and above this, they suggest that the choice will be between cost and availability — the cheapest local supplier perhaps.

“In my own opinion, the competence of the supplier is more important than the type of insulation that forms the core,” adds Andrew Porter of SIPS@Clays, who also recommends taking up references. Always good advice.

So, disappointingly perhaps, we arrive at a position where, technically at least, there is little to choose between the various SIPs options. The key is to find a good supplier at a reasonable price. Good-quality panels, good design and engineering goes almost without saying. But equally important is the quality and diligence of the team erecting the SIPs system — they have the ability to undo all the good work.

Added to all this, doing a thorough credit check on the supplier is a very sensible idea. So too is asking whether the company have a client or escrow account, whereby the money is held in the account until an agreed point, and is kept separate from the company’s business account. Money in a client/escrow account is safe from company insolvency. You will be handing over a large deposit and it will be good to have the assurance that they won’t go bust before your house turns up on site.

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