Insulating with Hemcrete

Old buildings expert Marianne Suhr revisits Watts Barn to see how an eco building material has been used to make new walls, creating a shell with excellent insulation that is sympathetic to the original architecture.

From the outset, it was clear Watts Barn was going to be no ordinary project. Richard Cain, the project surveyor, visited me at my house just as he was starting to design the conversion of this ancient timber-framed barn into a family home.

The standard modern solution for insulating walls in a building like this is to use some sort of modern insulation material, a lightweight foam board, or mineral wool quilt, with plasterboard on the inside. But Richard wasn’t happy with this, predicting that the inside of the space would look rather flat and characterless as a result.

ABOVE (left-right): Mixing the Hemcrete in the industrial-sized pan mixer is a full-time job for Lionel, one of the builders; The wet Hemcrete is poured between the shuttering; The battens are fixed to the Hemcrete ready for the weatherboarding to go up.

Insulation uppermost
As luck would have it, I was repairing the ancient timber frame of my own house at the time, and replacing modern infill panels with Hemcrete. As soon as Richard saw it, he knew that this would be the perfect material for the walls of Watts Barn, just on a rather bigger scale.

Hemcrete is basically hydraulic lime mixed with ‘hemp shiv’, the waste material from the hemp plant, chopped into small lengths. When mixed with lime and water it becomes a lightweight gooey mix and, when it sets, it looks just like Weetabix. For my own house, I had been using a standard cement mixer to mix the hemp and lime in gradually just a few bucketfuls at a time. But Watts Barn was going to prove rather more of a logistical challenge.

Gates Builders hired an industrial-sized pan mixer and acres of specially designed plastic ‘shuttering’ to create a temporary mould for the Hemcrete while it was being poured.

Richard designed the walls so that the main timbers were visible on the inside face, but wrapped around with Hemcrete to form a 12in wide wall. This provided a semi structural, breathable wall, with fantastic eco credentials and great insulation properties.

ABOVE: The shuttering is removed after the Hemcrete is set solid.

Design integrity
But the real beauty of the Hemcrete was the ability to follow the wobbly lines of the timber frame, and then plaster the undulating inside face with lime. This system seemed so much more compatible to an historic structure than plasterboard and modern insulation, Richard told me. Within a couple of days the Hemcrete had set enough to remove the shuttering and allow the air to start drying out the thick walls. Timber battens were screwed to the outside face of the Hemcrete, ready for fixing the new bark edged timber boarding. Several tonnes of Hemcrete and three months later and the walls were complete. Finally, the structure was starting to resemble a house.

ABOVE: The new boarding is attached to the walls.

Marianne's checklist: Preserving and replacing traditional boarding

If you are stripping old boards from the outside of your building, do try to salvage them for reuse. If they are old, they will probably be elm, which is now extremely difficult to source. Elm is, however, rather susceptible to woodworm so is often in poor condition. If you are lucky it may be possible to salvage enough boarding to complete one elevation.

If replacing the boarding, here are a few general pointers to remember:

• Oak looks beautiful but is very expensive. An alternative is larch, which is a durable native softwood, ideal for external cladding.

• When possible, source your boarding from the local saw mill to cut the cost.

• If treating the boarding, make sure you use a clear preservative, rather than standard tanalised treatment which has an ugly green tinge. Suppliers are reluctant to use clear preservative, so be persistent.

• If staining the timber, apply it to the boards before they are fixed; otherwise they will shrink and reveal a line of unstained wood.

• For a traditional finish, use ‘waney edged’ boarding – boards with bark edge.

• Specify a board that is minimum 1in thick and 12in wide. Each board should lap the one below by 3in, leaving 9in visible.

• Fix the linings around window and door openings, as well as lead flashings, before the boarding is fixed.

• Attach the boards to tanalised timber battens, using hidden galvanised nails.

• Use a closing batten at the base of the boarding to prevent wasps and vermin from getting up behind.

Read more from Marianne on draught-proofing and insulation...

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WORDS AND PHOTOGRAPHS MARIANNE SUHR
Featured in the May 2011 issue of Period Living