What is the build-up of a building element?
The build-up — in Dutch opbouw — is the layered composition of a building element: which layers sit on top of one another, and in what order, from inside to outside. The same Dutch word is also used for an element placed on top of an existing structure.
On a drawing the build-up usually appears as a list beside the detail. That order is not arbitrary: the position of the insulation, the vapour control layer and the ventilated cavity determines whether a construction stays dry or whether interstitial condensation forms.
Examples of a build-up
Warm roof (inside to outside): roof deck or boarding — vapour control layer — insulation — waterproofing. The insulation sits on the cold side of the structure and the vapour control layer on the warm side.
Underfloor heating floor: structural floor — insulation — pipework — screed — floor finish. Insulation under the pipes is essential, otherwise you are heating the crawl space as well.
Timber frame external wall: internal lining — service void — vapour control layer — studwork with insulation — water-resistive, vapour-permeable membrane — ventilated cavity — cladding.
Rule of thumb: vapour-tight inside, vapour-open outside. The further out you go, the more easily moisture must be able to leave the construction. And always provide a ventilated cavity behind an outer finish that can let water through.
Other meanings
- A build-up as an addition — an extra element on top of existing construction, such as a roof extension on a flat roof. The question is always whether the existing structure and foundation can take the additional load.
- Surface-mounted versus recessed — a surface-mounted luminaire, downlight or socket box is fixed on the surface, while the recessed version disappears into it. In an insulated ceiling or an airtight construction, surface mounting is often the better choice because the vapour control layer stays intact.
Related terms
- Roof extension
- Vapour-permeable membrane
- Timber frame construction
- Airtightness
- Vapour control layer
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