"RISING FOAM"
Author : Gerald Cole
Consulting editor, Self Build & Design
Synopsis
Sprayed polyurethane foam is best known in Britain as a means of restoring failing roofs or for insulating hot water cylinders. But, thanks to imminent changes to the Building Regulations, its remarkable effectiveness as an insulating material is set to become much more widely know.
Article
President Bush may be dismissive of global warming and the energy saving measures to fight it, but it's an increasingly hot topic for the UK construction industry.
In April, as part of its commitment to the 1997 Kyoto agreement to limit greenhouse gas emissions, the British government is enacting a far-reaching amendment to the Building Regulations - the technical requirements all builders are legally obliged to follow. Part L aims to dramatically improve the energy efficiency of all new buildings - particularly in the use of insulation. Other changes relate to the efficiency of boilers, the use of low-energy light fittings and airtightness - something not previously considered, even though draughts are a major cause of heat loss.
All well and good, you may say. If you're planning a new house, office or factory, building in improved energy efficiency not only helps to save the planet but promises useful future savings for space and water heating. Whatever the future holds, energy costs are expected to rise.
But, as with many things, the devil is in the detail, and the implications are especially profound for house building - not unreasonably since Britain's homes account for around 30 per cent of the country's total energy bill.
Under the current building regulations, new houses and conversions are already required to meet energy efficiency standards. Various methods of compliance are accepted, but the simplest is the Elemental Method, which specifies maximum U values for different parts of a house. A U value is a measure of heat loss - actually, the energy loss in watts per square metre of any given part of a building for every degree difference in centigrade between the inside and outside - written W/m2K.
At the moment, the regulations require a maximum U value of 0.25 W/m2K for pitched roof insulation and 0.45 for external walls and ground floors. In the roof this is most commonly met by 150mm of glass fibre or mineral wool - the cheapest form of insulation - laid between the ceiling joists of the loft and 65mm inserted into the cavity of a double wall; 50 mm-thick expanded polystyrene or polyurethane sheets are widely used beneath a solid ground floor. The new regulations, however, will require a maximum U value of 0.35 for the walls, 0.16 for a pitched roof and 0.25 for a ground floor.
The obvious response is simply to beef up the insulation. But that presents problems for wall construction. Ninety per cent of Britain's homes are built with an inner wall of concrete blocks, a 50-75mm wide cavity and an outer wall of brick. But increasing the width still further is likely to threaten the stability of the walls and create problems with deeper window and door openings.
The new requirements seem less problematic for roof insulation - simply a matter of deepening the mineral wool in the loft. That, however, presumes the loft is dead space, inaccessible because insulation covers the ceiling joists. Now more and more developers, and existing homeowners, are reluctant to abandon the attic - and the opportunity of turning a three-four bedroom house into a four/five bedroom.
Building Research Establishment demonstrate conclusively that by eliminating energy wasteful roof space ventilation, buildings can be warmer, dryer and more environmentally friendly.
Polyurethane foam can be sprayed onto walls and floors, leaving a surface that can be screeded, plastered over or painted. Or it can be injected into a wall cavity; creating a high level of insulation for existing brick and block walls. Cavity widths can stay exactly as they are.
For the past 20 years the insulating properties of the material have largely been confined to the industrial sector, particularly refrigeration and agriculture. For housing it's been marketed on its strength and adhesive qualities as a means of stabilising failing roofs and cavity walls whose wall ties have corroded.
But, on the continent, spray foam has been common in house insulation for many years. An estimate 30,000 tons are applied every year in Spain alone, while throughout Europe it is routinely used to insulate ground floors.
The rising need for energy efficiency looks set to give spray polyurethane foam a marked boost as an insulator, but, like most building materials, it has both its downside and its detractors. One difficulty is that the gas, which forms the bubbles, was originally a CFC - the gas that damages the ozone layer. It has now been replaced with less damaging HCFCs, while even more ozone-friendly HFCs are promised in the next couple of years, but the environmental worry remains.
Ironically, it's the high thermal efficiency of these very gases that make polyurethane foam such an effective insulator. It's a question of balancing limited ozone layer damage against overall reductions in greenhouse gas emissions. Recent European studies indicate that polyurethane foams are actually more effective at reducing overall greenhouse gas emissions than standard mineral fibre.
The other major concern in the building industry is that the foam is applied directly to roofing timbers. This flies in the face of long practice, which insists roof timber should be fully ventilated to remove any dampness in the roof space and so avoid rot.
The sprayed foam industry argues that where dampness does reach timbers - either from below where rafters are left exposed, or from above where breathing paper is absent - it simply evaporates out the way it came in. By its very nature the warm roof that foam provides prevents moisture condensing on the rafters. In fact, insulating with conventional mineral fibre at ceiling joist level actively encourages moist air to collect within the loft space, especially when there isn't enough wind outside to draw it out the ventilation gaps.
The spray foam industry claims that in 20 years of roof application no incidences of rot have been reported.
Given the deeply traditional nature of Britain's construction industry, however, the arguments are bound to roll on. But the odds are in favour of sprayed foam - and not just because of its success in Europe and elsewhere.
When the UK government first proposed its Building Regulation amendments last year, the U values quoted were much lower – just 0.30 W/m2K for walls – with a further drop to follow after two more years. The housebuilding industry were deeply alarmed, knowing that 'traditional' brick and block construction couldn't cope - not without drastic, expensive adaptation. Government took note and downgraded Part L accordingly.
But thanks to global warming and the inevitable rise in fuel prices, the demand for energy efficiency is only going to grow. And sprayed polyurethane insulation is one of the most effective ways to meet that demand.
Just as importantly it promises to keep things - at least in terms of 'traditional' housebuilding - pretty much as they are. The average British builder couldn't really ask for more.