hempcrete extension and boiler house — Hempcrete by William Stanwix

Cherry Tree Cottage hempcrete extension and new boiler house

2014 Oxfordshire

construction designer | main contractor

This was a design and build project. The architect produced the floor plans and elevations, I took these and created the construction drawings and built the extension from foundation to finish.

I also designed and built the boiler house which was the first underground timber building I have ever done.

The render was by a local plastering company who helped us out when we were busy.

I used cork quite a lot in this project. In the floor build up, to insulate the plinth and in hard to reach areas of the external walls.

Project Costs (supply and install) :

Cast Hempcrete £500/cubic metre / Lime Render £65/sqm / Lime Plaster £45/sqm / Timber frame including roof structure and first floor structure £19,500.00

The site when I started.

Finished extension.

Extension and boiler house.

Bedroom after lime plastering. The ceiling was woodwool board and between the rafters was blown cellulose insulation.

The boiler House, it was mostly built from left over materials from the extension build.

The boiler house is wooden box sunken into the ground wrapped in damp proof membrane and insulated with hempcrete blocks.

The internal walls of the boiler was clad with strips of timber cut from all the waste of the build.

That damp proof membrane continues right under the boiler house and up the other side. The stiff box structure resists pressure from the soil on either side.

Timber I-joists are strengthened at the ends with plywood. The oak sprockets are screwed on at the end and cantilever out supporting the eaves.

Oak sprockets or rafter ends are screwed onto the main rafters which end over the wall. You see the oak from underneath and its much neater than a boxed in soffit.

Cast hempcrete.

Hempcrete next to stone.

The flat roof structure linking the old roof to new roof. The top bits of timber are just there to support the temporary tarpaulin.

The rafters were timber I-joists, these are much lighter and easier to handle than solid timber and also create less of a cold bridge.

Compacted hardcore. The hollow blocks are filled with cork granules to create a break between the insulated floor of the living space (to the left) and the un-insulated floor of the larder to the right.

At the rear gable above the party wall with the neighbours I used cork so that I could put it up and render it straight away then not have to return to that tricky area. I was also worried about splash back from the neighbour's roof deteriorating the

I fitted an entire passive stack ventilation system by Ventive.

Hardcore down, note the cork insulation to the perimeter at the plinth. This insulates, holds up some of the hempcrete and acts as an expansion joint for the limecrete floor.

Geotextile membrane.

Geotextile membrane ready to receive foamed glass aggregate.

Rob installing the waste pipe.

Compacted glass foam, note the cork perimeter insulation.

Foamed glass aggregate in and compacted, the foamed glass is insulating and does not allow moisture to wick through so acts as a damp proof membrane.

Cork boards down on top of the glass foam. The cork has a better thermal conductivity than the glass foam, I have used it here to bump up the insulation and I only installed enough glass underneath to be sure that it would act as a moisture barrier.

The larder floor was not insulated. The larder walls and ceiling were insulated to isolate the room away from the heat of the rest of the hose. The floor was left un-insulated so that it could act as a heat sink and keep the room cool. The floor had

The frame studs in the larder were from planned square timber, I put them internally then plastered the hempcrete and put shelves between the studs for all the jars of food to be stored on. The wall is blocks here as its a party wall with the neighbo

Rob finishing off the hempcrete above the larder room.

I lots of little tricky areas like this cork boards were fixed to the frame internally and externally and then cork granules were poured in between to fill the gap. Cork can be rendered or plastered in lime very easily.

There is a flat roof linking old house and the extension which creates a nice large landing area on the first floor. The dark board is cork board which was used for lots of tricky areas.

Wood woolboard ceilings.

Timber I-joists.

The first floor landing area.

Woodfibre insulating sarking board.

Gable ladder.

Where the woodfibre sarking boards are fixed over the hempcrete sheeps wool or hemp fibre insulation is sandwiched between the hempcrete and board to fill any gaps and act as an expansion gasket.

Note the oak board supporting the Breather membrane, this will be seen from beneath.

Cherry Tree Cottage .jpg

Extension and boiler house.

Boiler house.

Rear of boiler house.

Boiler house.

Cedar shingle roof.

cherry tree.jpg

timber frame

hempcrete renovation