What Is? Guides

Building a home comes with a language of its own — cavity closers, trench-fill foundations, SIPs, U-values, air tests. The What Is? series exists to demystify that language for homeowners and self-builders, one term at a time.

Each guide follows the same simple structure: a short, plain-English answer to the question in the title, a clear explanation of how it works, and a labelled diagram showing the main parts in context. Every guide then goes a little deeper, covering why the element or system is used, what can go wrong, and the key things to check or ask your builder, designer or assessor about.

Crucially, every guide is deliberately concise — just three pages long. There’s no wading through lengthy technical documents or manufacturer literature to find the answer you need. Each one is designed to be read in a few minutes, giving you enough understanding to feel confident in conversations with architects, builders and Building Control, without replacing their professional advice.