Building components · Wall Leaves & Backing
Inner Leaf
Explains the inner leaf as the layer that commonly does the structural work, in contrast with the leaf that faces the weather.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a inner leaf is
The inner leaf of a cavity wall sits on the warm side of the cavity. In most arrangements it is the leaf that carries the floors and roof, and it is the leaf that internal finishes are applied to or fixed against.
Because it is inside the insulation and cavity, it also has a role in the wall's air and moisture behaviour: it is commonly where the air barrier is formed, and where warm internal air is either stopped or allowed into the construction.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the outer leaf. The two occupy opposite sides of the cavity and commonly have opposite duties: one faces weather, the other carries load.
- It is not a backing wall in the facade sense. A backing wall supports a cladding system; the inner leaf is part of a masonry cavity wall.
- It is not a lining. Linings are applied to the inner leaf; the leaf itself is masonry.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- internal leaf
- inner skin
- structural leaf
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What it carries, including floors and roof, which is what makes it structural rather than merely enclosing.
- Whether it is continuous and sealed enough to form part of the wall's air barrier.
- The cavity and insulation arrangement outboard of it, which determines whether it is warm or cold.
- How it is tied to the outer leaf and how those ties cross the insulation.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Where the insulation sits relative to the inner leaf determines whether the leaf is warm — which affects both condensation risk and how the wall stores heat.
Warm moist internal air reaching a cold surface within the construction is the mechanism behind interstitial condensation, which is why continuity of the internal control layers matters.
The mass of the inner leaf contributes to how much external noise reaches the room, though the whole build-up governs.
Services chased into the inner leaf are common and can interrupt an air barrier formed on its face.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Materials
Commonly incorporates
Materials this element commonly contains as a distinct constituent part, rather than being made of.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Joist End to Masonry Bearing
Where a floor joist is supported by a masonry wall, and where structure meets a wetting risk.
Raises: Structural transfer · Moisture · Thermal continuity · Buildability
Outer Leaf to Inner Leaf Restraint
Where the two leaves of a cavity wall are connected so they act together while remaining separate for moisture.
Raises: Structural transfer · Moisture · Thermal continuity · Buildability
Slab Edge to External Wall
Where a floor meets the external wall, forming a line that runs around the whole building at every level.
Raises: Thermal continuity · Moisture · Fire continuity · Structural transfer
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- External Wall
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That both leaves do the same job. In most cavity walls only one carries the building.
- That the inner leaf is finished work. It is a structural and control layer whose continuity matters before anything is applied to it.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What is the inner leaf carrying in this wall?
- Where is the air barrier formed, and does it rely on this leaf being continuous?
- Where does the insulation sit relative to the inner leaf?
- How are services routed so that they do not interrupt the control layers?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No thickness, material, air-permeability or thermal figure is stated here.
- Whether an inner leaf can be altered or opened up is a structural question requiring qualified assessment.
Wall Leaves & Backing
The distinct vertical layers that make up a wall — outer and inner leaves, the cavity between them, and the backing that supports what is fixed to the face.
Browse all wall leaves & backing entries →