Summary
• ModCell is a sister company to the White Design architectural practice.
• Straw is low carbon and provides good thermal insulation.
• Panel assembly takes place in “flying factories”.
• Structural support is provided by the glulam frame.

We learn the tale of the three little pigs at our mother’s knee so we all know what can happen if you build your house out of straw.

But that first little pig may have been right to choose straw after all. He just should have been thinking prefab.

At least that is the view of ModCell, the prefabricated straw bale construction system specialist set up five years ago as a sister company to the White Design architectural practice.

“We developed the product after recognising straw’s excellent sustainability credentials,” said Finlay White, ModCell’s operations and marketing director.

“It’s low carbon and provides high levels of thermal insulation. And about three million tonnes of it are grown in the UK annually.

The key to commercialising the system was to apply the principles of modern methods of construction and to deliver a factory-built, fully-enclosed modular system.

“We can deliver an enclosed panel complete with two coats of lime render,” said Mr White. “It’s an almost completely finished product with just a skim finishing coat needed on site, so it can be installed very rapidly.”

Market opportunities

Modcell says the system has a very real market in the housebuilding sector and some exciting prospects in retail construction, particularly with supermarket chains, but the current emphasis is on educational and commercial facilities.

One of its success stories is Renewable Schools, a fast turnkey solution providing teaching spaces within existing school grounds.

The glulam frame, which holds the straw and which provides the structural support for the building, is manufactured to 0.5mm tolerances by Austrian supply chain partner Mayr-Melnhof Kaufmann Holz according to cutting schedules produced by ModCell’s in-house-developed parametric software.

Back in the UK, assembly of the panels takes place offsite at “flying factories”, temporary manufacturing facilities set up, typically, in a farmer’s barn, close to site and within easy reach of the straw.

Assembly is pretty straightforward as the panel frames arrive pre-cut and pre-drilled. The only tools that are required at this point are a hedge trimmer, for tidying up the straw bales, a drill, a spanner and a render gun.

The panels are filled with straw bales, compressed to avoid settlement and a base coat of lime render is applied directly to the straw using a render gun. A further skim coat is applied by hand.

Services such as first fix electrics can be incorporated into the panel at the flying factory, or can be routed between the panel joints or around the roof.

The panels are then delivered to site, manoeuvred onto the sole plate and screwed together.

ModCell will supply just the straw bale wall panels, but where it is providing the full shell it will deliver partition walls, roof cassettes and cross-laminated timber floors.

Dimensions

Panel dimensions tend to be determined by, but not limited to, the size of the straw bale – hence the most common size is 3.2m wide by 3m high, but they are bespoke and ModCell has produced them up to 4.5m wide and 5m high. “Our limitation is what we can get on a truck,” said Craig White, director of White Design.

At 480mm the walls are unashamedly thick. “We’re fat and proud,” said Mr White. “And our view is that everyone else is having to put on weight in order to meet Building Regulations anyway.”

Straw has a thermal conductivity of 0.03W/m²K and, in tests, the ModCell straw bale panels achieved a U-value of 0.18W/m²K, improving to 0.13W/m²K if drylining is used.

Passivhaus requirements

“We’ve also just done the modelling to take the whole system up to Passivhaus,” said Mr White. “This requires the outer panel to be a SteicoWall-type render board and takes the U-value to 0.1W/m²K.

“And because of the way we build the system out of prefabricated highly-toleranced timber, the whole thing is designed for airtightness,” he added. “We can achieve airtightness of 0.8/hr @50 Pa, which is more than 10 times better than Building Regulations and can achieve the Passivhaus airtightness standard when requested.”

The fire risk is designed out by oversizing the glulam to its char rate, which is 0.3mm per minute. The char rate of straw is about 0.7mm per minute, but the panels achieve good fire ratings because they are closed with lime render and the straw is very compressed, so they hold little oxygen. They have a fire rating certificate of two hours and 15 minutes, which is about one hour and 15 minutes longer than it has to be.

As well as providing functional teaching spaces, Renewable Schools have found a place in the curriculum with the build process and the rationale behind the different materials followed in lessons. And the wider community is involved too, both at open days on completion of the project, and at the assembly stage.