Building a home begins with the load-bearing structure. It’s not the bit that anyone will see when the house is finished, but deciding how to build the structure is probably the most significant choice you will make when designing your new home.
Since the 1950s, the most popular building method has been a cavity wall construction. The exterior walls consist of two “leaves” – the load-bearing inner leaf, a cavity (usually filled with insulation) and the exterior “leaf”, likely to be brick, but potentially also render or another cladding material. The exterior leaf plays no part in the structural stability of the home, and the choice of material is determined by the aesthetic you prefer.
The use of a cavity wall structure fulfils two purposes – the cavity provides a great barrier to weather (the solid brick walls of Edwardian properties can get very damp during prolonged periods of rain) and, when filled with insulation, gives the energy efficiency that is demanded by modern Building Regulations.
However, the component that actually supports the house is the inner wall, and the choice here is whether to use a timber structure or to build solid masonry walls using dense or lightweight blocks.
Building for now
Your focus when making the decision should be a combination of the immediate performance of the structure; its flexibility to allow changes over time and the durability to ensure your home keeps its resale value after many decades.
In terms of the immediate performance, you will be considering the need to meet Building Regulations and top of the list is thermal performance. Current Building Regulations demand high levels of thermal insulation from the external walls and are currently being revised to become even more demanding.
It is possible to meet the current energy efficiency requirements for new-build homes by “building in” energy efficiency into the structure of the house. To do this, you will need excellent levels of insulation and airtightness. It’s a challenging target, and every component you use in the build should be considered for its contribution towards energy efficiency.
Aircrete blocks are a good choice here for two reasons. Their unique structure traps tiny air bubbles throughout the block, providing built-in insulation, while airtightness is best achieved using a Thin-Joint building method.
Thin-Joint construction, as its name implies, replaces traditional sand and cement mortar (typically giving a 10mm joint between blocks) with a special glue-like mortar that cures to give a much smaller joint, typically around 2mm. The result is an extremely airtight, thermally-efficient wall and is particularly appropriate if you are looking to build a Passivhaus-certified building.
On site, Thin-Joint has advantages for the building process too as it is a very quick method of wall building. The special Thin-Joint mortar cures almost immediately, allowing a full-height wall to be completed in a day. Contractors also like the flexibility of aircrete blocks as they can be cut on site with a handsaw to accommodate the individual design details that so often characterise a self-build project.
Building for the future
Of course, you want a home that is comfortable and energy-efficient immediately, but you will also want to create a building that will last for generations. You should consider not only the declared design life of any building system but also the evidence available to provide reassurance that your home will last for over 100 years.
You are looking not only for reassurance that the structure will not warp, rot or burn but also to focus on how flexible it will be for the long term. Your home will be perfect for you now, but it’s worth considering whether it could be altered in the future to meet the changing needs of your family or the next generation to live there.
As you won’t know what form that alteration might take, you need to understand how the structure could cope with an extension, changing the configuration of internal walls or adding more living space with a room in the roof.
With traditional building methods and recognised masonry materials, this is relatively straightforward to calculate, and a trip down any street of Victorian terraced houses will reveal just how much customisation happens over generations.
It’s much more difficult to calculate the impact of such re-working on a frame-based structure. Modular systems that are unique to a single provider may, in future decades, be difficult for builders to understand or interpret as they are unlikely to have the original drawings and design calculations. This might seem a distant issue but protecting the value of your home relies on ensuring that potential buyers have the freedom to put their own stamp on the building too.
And finally, there is a practical aspect: you will know already that financing a self-build project is no mean feat. During the build phase, when money (and particularly cash flow) will be tight, it makes sense to stagger the purchase of materials and time the expense for the moment when an individual product is needed on site. Using readily-available standard materials, such as aircrete blocks that can be purchased and delivered at short notice from your local builders’ merchant, will help you accommodate unforeseen delays and adapt your build plan to cope with the unexpected.