Topping out

Gerald Cole

Foaming at the mouth

Choose innovations with care

Let’s start with a confession. I can get quite excited by bright, shiny innovations that solve an awkward building problem simply, quickly and economically.

This is why, 17 years ago, when I first encountered an intriguing new form of attic insulation I became instantly fascinated. Though new to me, sprayfoam insulation had been around since the 1970s. It’s a liquid form of polyurethane, which previously I’d only known in the form of rigid foil-backed boards. They provide highly effective insulation - pricier but much better than polyurethane’s rigid rival, polystyrene.

Polyurethane boards are excellent for insulating solid ground floors and walls. Using them in attic roofs, however, is more awkward since the boards have to be cut to fit the often irregular spaces between the rafters. A tight fit isn’t always possible. But even a gap as small as a millimetre can reduce the effectiveness of the insulation by up to 40 per cent.

My attic of 17 years ago was insulated with rolls of mineral wool covering the floor to the depth of the floor joists, rather less than the 200mm depth recommended at the time. But it meant I could easily board out the floor for storage, which is what I wanted.

It also meant, however, that the house wasn’t as warm as it could be, and the attic itself was freezing. It might also be vulnerable to damp because the roof was over 40 years old and the sarking - fabric designed to lie beneath the tiles and prevent windblown rain from entering – hung in tatters from the rafters. Not great for stored books or papers.

But here sprayfoam insulation scored again. Its advertising at the time stressed its value as a way of extending the life of aging roofs as much as insulation. So, a triple solution. I could make the house warmer, create a dry attic and postpone replacing the roof for a good 10 years – the warranty term offered by the sprayfoam company of my choice. The process took about three days.

It works by combining two different chemicals under pressure in a nozzle. As the mixture emerges, it instantly expands, filling every nook and cranny of the space it’s applied to. It quickly dries to a hard surface, creating a roof-wide shell which locks the tiles in position.

I chose a ‘closed cell’ form which is impermeable to air and moisture. An ‘open cell’ option is also available, which allows moisture to pass through, though the insulation remains flexible.

For the next six years all was fine. The house felt warmer, the attic was dry and the heating bills stayed affordable. Then along came something else new, bright and shiny (in a glossy black sort of way): rooftop photovoltaic panels. In other words, free, home-produced electricity. What’s not to like?

Well, for a start, the price. At the time PV panels were double their current cost, extending their payback time far into the future. But then the government introduced a remarkably generous Feed-in Tariff on each kilowatt produced, guaranteed for 25 years.

"Overnight PVs became an excellent long-term investment, easily outgunning the current most generous saving accounts."

Not straightforward

Fitting them to a sprayfoamed roof, however, was not straightforward. PV panels typically sit on parallel rails which, in turn, are supported by slim metal roof hooks attached to the rafters. Accessing the rafters, however, means lifting the tiles then replacing them. Not an easy task when they’re stuck down.

My installers succeeded, but made good the roof with proprietary spray foam available in canisters. The PV panels worked splendidly, but heavy rain produced occasional drips in the attic. To their credit the sprayfoamers returned to do what they could, but were unable to trace the source of the leaks.

Which brings me to my next roof problem. I’m tempted to remortgage to finance even more bright and shiny improvements. No problem, brokers assure me, until they learn I have sprayfoam insulation.

Lenders, I’m quickly informed, will not touch sprayfoamed properties. Their argument, I gather, is that because the roof timbers are encased in foam they can’t be inspected for leaks, rot or other damage. The fact that roof timbers hidden behind sheet insulation, vapour barriers, tape and plasterboard can’t be inspected either cuts no ice.

So now, I’ve discovered, there is a vibrant sprayfoam insulation removal sector, sometimes charging almost as much as the original sprayfoaming to remove it. After which you may well need to renew the roof since not all the tiles and battens are likely to survive the process.

The moral of this sorry tale? Firstly, be sensibly suspicious of bright, shiny innovations. Which, given the need to decarbonise and dramatically reduce our energy needs, is not going to be easy. For example, the government’s current bright, shiny innovation is heat pumps for all. But will that be the case by the time your project is completed?

The second moral, then, is to put fabric first. A well constructed, well-insulated, well-ventilated house will need very little energy, whatever form it takes. And it will create an even better longterm investment than the government’s original PV Feed-in Tariff (which is no longer available for new applications).

So, if I have to replace my roof, the replacement should be a warm roof. Here all the insulation lies on top of the rafters with a continuous vapour barrier immediately below. The roof timbers are now within the insulated envelope of the house, protecting them from leaks or rot. All I have to do now is pay for it.