Walls

The following set of posts contain details of design elements. This is the plan for the wall.

Specification

A cross-section of the exterior wall structure

The wall is a super-insulated, Larsen truss stud framed wall which wraps around the outside of the timber frame. The wall can be built up on site from partially prefabricated units allowing for rapid assembly without requiring heavy lifting equipment such as cranes. All openings will be fabricated on the ground so that no cutting need take place once the units are installed.

Description

The insulating heart of the wall is 200mm of blown in cellulose insulation contained within two sheets of OSB. The sheets of OSB are separated by a series of Larsen trusses, a sort of ladder truss consisting of two pieces of 38mm² timber separated by regular panels of OSB. A set of 38mm² battens on the inside of the internal sheet of OSB provide a mounting for the plasterboard (or other internal finish), leaving space to run cabling and some plumbing. This space is based on the Open-Build system by Bensonwood (more on that in a future post). It future-proofs the house to a great extent as well as insuring that the insulation and air-tightness is not compromised by cabling or plumbing. The outer sheets of OSB are wrapped in a breathable water-proof membrane. Over this are thinner timber battens which provide the mounting for the wooden siding. These are not shown on the image as they could as easily be replaced by an appropriate render, or a brickwork skin.

Construction

The walls are built from the inside out. First the internal OSB sheet with the 38mm² battens is lifted into place and fastened to the frame. These panels are built up on the ground, to the full height of the wall, but to a width of 1.25m (corresponding to the standard width of an OSB panel). Door and window openings are precut and framed. The panels (weighing less than 50kg) can then be hinged upwards into place. Where an opening cuts completely across the width of a panel, the internal battens remain full length to ease installation and are cut out once the structure is rigid enough. Expansion gaps are left between panels and the joints are taped. This internal layer forms the primary air-barrier for the house.

Once the whole house is skinned, the trusses are attached. These trusses can also be pre-fabricated, even before the frame arrives on site. Geotextile stapled to the uprights prevents the cellulose from migrating and ensures the dense packing necessary to prevent settlement. The tops of the trusses attach to the corresponding roof trusses which overhang the frame (Detailing of the floor-wall and wall-roof joints will follow in later posts). Once the trusses are in place the final OSB layer is installed. This is installed horizontally rather than vertically to improve the shear strength of the wall. The vertical joints can also be staggered. Although the wall is non structural the extra rigidity won’t hurt.

Once the whole house is wrapped, the insulation is blown in. Holes are cut in the outer skin at 60cm intervals and the cellulose is blown in through these holes from the ground up. As a hole is completed, it is taped and once the house is completed it is wrapped in waterproofing membrane.

Finally, when the house is completely wrapped, the internal plasterboard is installed. Installing the plasterboard last like this ensures that it is protected from the weather as it is never exposed to the elements during installation. It also makes it easier to replace in the future should that prove necessary. The disadvantage is that as the frame shrinks, cracks will appear between the frame and the plasterboard, but given the nature of the build, this minor maintenance issue is outweighed by the benefits when dealing with the uncertain Normandy weather.