SIPS - What You Need To Know

The main attraction of SIPs building is that it provides a fantastic insulation level built into the fabric. This is particularly attractive for sloping roofs, which have become cumbersome and awkward to insulate using traditional methods.

• SIPs also offer very good air-tightness levels, seen as an increasingly important aspect of low-energy housebuilding.
• SIPs companies often advertise the speed benefits of using SIPs. However, there are only marginal speed advantages in using SIPS over the more common factory-built timber frame.
• SIPs are easily adaptable on site. You can cut holes anything up to 1,200mm wide in the panels without any form of structural support. However, this is not the way to take advantage of factory-built panel sys­tems, and it is also extremely wasteful.

Even more than timber frame, SIPs buildings require accurate foundations, ideally ±5mm in level and squareness. If the base is inaccurate, then the panels never fit snugly together and you risk building a loose structure that will not be airtight and may be at risk from water penetration. Employ groundworkers who know what they are doing and who know what the issues are.

Watch out for competing claims from SIPs manufacturers. In particular there is a debate about which is the best form of insulation with which to fill the core. The two basic choices are expanded polystyrene, which is much cheaper but requires glue to make it stick to the outer boards and also requires additives to become fireproof, and polyurethane, which is innately adhesive and doesn’t require any further treatment, but is considerably more expensive as it has to be blown into the space between the outer boards. Polyurethanes are better insulants and therefore don’t require such thick panels, which can be a plus point on smaller homes.

Watch out for certification issues. The whole concept of SIPs lies well outside the scope of the building regulations and conse­quently warranty and mortgage pro­viders like to see some form of third party certification to show that the pro­duct is fit for its purpose. A BBA approval is the best recognised but many other forms of certification exist. The key factor to look for is to see if the war­ranty providers, such as the NHBC or Zurich, have already worked with the particular panel system you propose to use and whether they will be happy to accept it on your scheme. If they will play ball, then mortgages and insur­ances should follow without any problem.

One key point to watch out for is the detailing around the inside and outside of a SIPs panel. Just as with timber frame, it is vital to keep some form of ventilation gap or cavity between the panels and the outer cladding. This enables any penetrating rain to evaporate harmlessly. This requirement is no different to conven­tional timber frame. However, on the inside, you have to make allowance for service ducting because you cannot run cabling or pipework through the SIPs panels (though some American systems do just this). This service void adds some expense to the overall design and also nibbles away at your floor space. However, it is arguably a much better construc­tion detail than normally employed on timber frame houses where the cables are laid within the actual wall and the insulation is often disturbed to accommodate it.

Does a SIPs panel house require a vapour barrier? The short answer seems to be no. Provided the wall/roof is genuinely airtight, then there should be no problem with moisture penetration from the inside. Neither polys­tyrene nor polyurethane absorb or transport water. However, some more cautious pract­itioners prefer to install a vapour barrier, just to be on the safe side. This would normally be placed in the service void.

Whilst many of the early SIPs panel homes survive in good condition, they don’t have a track record stretching back hundreds of years and you will still hear sceptics suggesting that such a structure won’t stand the test of time. From what has been learned to date, the one thing that causes problems for SIPs is the one that that causes problems for all other types of building — water penetration. If a building is well designed and well built, then it really shouldn’t suffer any problems. If by chance it isn’t, then repairs may have to be carried out after a number of years, but the risk is essentially no different to any other form of building.

SIPs are designed to make up external walls and roofs. As such, they are similarly priced to timber frame walls and rafters. However, if you continue to use SIPs for the internal room-dividing walls, you run into a cost penalty because you are effectively over-engineering the walls. You can decide to switch to a cheaper wall form, typically timber studwork, or stick with the same external wall profile, if only because of the good acoustic properties of the panels.

Most SIPs suppliers recommend that you should install a whole-house mechanical ventilation and heat-recovery system. Whilst airtight houses ensure good energy efficiency levels, they also require some form of managed ventilation. This draws stale air from the so-called wet rooms, – the kitchen and the bathrooms – takes the heat out of them and transfers it into fresh air, drawn in from the outside and blown into the dry rooms. Mechanical ventilation needs to be designed well and integrated into the structure otherwise it can be noisy and ineffective. It’s an expensive addition to a house, adding perhaps £3,000 to the overall budget, but it probably makes sense with a SIPs panel house. However, the regulations don’t require mechanical ventilation and the standard, cheaper choices are an option.

SIPs Step by Step

1. The foundations for an individual home built using SIPs are constructed in the same way as for any other type of construction.

2. The panels, with window and door openings already cut out, are erected to first floor level.

3. Roof panels can also be constructed using SIPs.

4. The house is then weatherproofed and finished as per usual construction techniques.

Further Reading:

• SIPs examined