Building components · Openings & Frames
Opening Jamb
Explains the jamb as the vertical edge where a wall's build-up is cut through, exposing every layer within it.
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 jamb is
The jamb is the vertical side of an opening. Cutting an opening through a wall exposes every layer of that wall at the jamb: the cavity, the insulation, the control layers, and the two leaves.
The jamb's job is to close and continue those layers so the wall's performance is not interrupted. That is why the jamb of a cavity wall is more than a surface — it is where the cavity is closed and the insulation continued.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the reveal. The reveal is the visible face of the jamb inside the opening; the jamb is the whole edge condition through the wall's thickness.
- It is not a cavity closer. The closer is the component that closes the cavity at the jamb; the jamb is the position.
- It is not a head or a sill, which are the horizontal edges with different water and structural requirements.
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.
- jambs
- opening sides
- reveal jamb
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- How the cavity is closed, and whether the closure allows the leaves to remain separate for moisture purposes.
- How insulation is continued around the edge so there is no gap where the wall is cut.
- How the air and water control layers are returned into the opening and connected to the frame.
- The frame fixing, since jamb fixings carry much of the wind load on the window.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Where insulation stops at a jamb the opening is surrounded by a continuous cold path, and this shows as condensation on the reveal before anywhere else.
Closing a cavity at a jamb creates a potential bridge between the leaves, which is why closers are designed rather than improvised.
The jamb is where several trades meet — masonry, insulation, membrane, window — and each depends on the previous one having done its part correctly.
The gap between frame and jamb absorbs the difference between the opening as built and the frame as made.
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.
Frame to Jamb
Where a frame meets the vertical edge of an opening, and where the wall's every layer must be closed onto it.
Raises: Thermal continuity · Air leakage · Moisture · Movement
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.
- Opening
Clarifications
Commonly misunderstood
- That closing the cavity at a jamb is simply filling the gap. Filling it with masonry bridges the leaves, which is the condition the cavity exists to prevent.
- That condensation on a reveal is a ventilation problem. It very commonly indicates a thermal bridge around the opening.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How is the cavity closed at the jamb, and does the closure avoid bridging the leaves?
- How is insulation continued around the opening?
- How are the air and water control layers returned onto the frame?
- How is the frame fixed at the jambs, and can the fixing take the wind load?
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 dimension, closer type, insulation arrangement or fixing is stated here.
- Opening jamb arrangements are part of wall design and are subject to regulation for thermal and fire performance.
Openings & Frame Elements
The frames, heads, jambs, sills, thresholds and closers that form and finish an opening through a wall.
Browse all openings & frames entries →