The “Weetabix” insulation

The jigsaw of wood fibre insulation on the loft wall

Insulation is an important part of any eco-friendly build. Done right, it saves on energy bills in the winter and also prevents overheating on hot summer days. We were looking to do two things: insulate heavily with a material with a high “thermal mass” (so it’s slow to heat up or cool down) and create a space that was airtight but breathable (a bit like Goretex but for houses, breathable membrane lets you conserve heat but avoid condensation).

Want to know more about the innovative, eco-friendly insulation we used? Click on read more...

Our architect found two products from Natural Building Technologies (NBT) called Pavatherm and Pavaflex (from £11.80 per square metre) that fitted the bill perfectly. The insulation is made from waste wood fibre, so it’s using material that would have otherwise been destined for landfill. It has a high thermal mass and isn’t full of nasty chemicals.

When the delivery came, there was literally a lorry-load of insulation boards and batts with our name on. The large-but-light batts looked like something a giant would breakfast on and were soon nicknamed “Weetabix” by our builders, who enjoyed having a laugh at our choice of hippy materials. I’m sure they called them a few other things too, because they were a pain to retrofit to an old Victorian house. On a new build it would be a doddle, but they weren't quite the right size to fit in the gaps between our rafters, studs and joists, so every piece had to be cut to fit. There wasn't much waste, but it was a giant jigsaw puzzle.

You can see the resulting jigsaw in the picture above. Each piece was cut to fit and carefully numbered so the builders knew where to put them. Then they took them off and literally buttered them with special lime plaster before sticking them back on again.

Pete the plasterer 'butters' the insulation

Once the insulation was in, there was no more cursing it. The builders, slaving away in the loft through the summer, appreciated it immediately. Even on a hot summer's day, the space didn't heat up the way you'd expect in any other loft. The pleasant working temperature meant that they soon forgot the hassle of installing it and swore by the Weetabix, so much that they said they'd be keen to use it on other jobs.

The loft sits on top of a not-terribly-well-insulated Victorian house (we do our best, but we can’t afford the cost or disruption of insulating our solid walls or replacing our century-old single-glazed windows) but the loft has been designed as a standalone space. This is ideal because we're planning to use it as an office. On work days, we can close the door to the rest of the house and use the heat pump (see “No it's not air con”) to heat just the loft very efficiently. This will save us a fortune on winter central heating bills, compared with heating the whole house just so that it's warm in the office.