Retrofitting internal walls and ceiling

We are in the process of finishing our retrofitting of hempcrete in the back of the old house and wanted to share some tips and tricks we developed along the way.

The wall shown here is an internal wall that we are filling for thermal mass rather than insulation so we are tamping it  very firmly. The wall is only a standard 90mm thick wall, so on the hallway side, we used MgO boards as permanent formwork so that we have only a single surface to render or paint. Otherwise, we would have had a mix of hempcrete and timber substrates to render. The side shown here will be behind floor to ceiling laundry cupboards so it will have no finish or board over it.

Filling to the top of a wall that is only as wide as the top plate or any lintels is challenging as you can't fill from the top. We found that cutting a corner of the formwork to give enough access for a hand allows 2/3 of the space to be horizontally tamped. Filling the final 1/3 is tricky but as the opening is small, the hemp can be pressed in by hand and will stay in place.

We have relined the ceiling in the back of the old part of the house with MgO board. It has been finished and is insulated with over 200mm of hempcrete. The mix in the ceiling space does not include any sand so it is a much lighter and mix and is much quicker to place.  Just tip out the buckets of hempcrete and rake over the MgO boards. No tamping required.