Bay window (oriel)

An arkier is a bay window that projects from the face of a wall and adds a small enclosed space to a room. The projection is not one element but a set of parts: a floor, a roof, glazed faces and several junctions with the main wall. Each junction is a place where structure, heat and moisture meet. The thermal sources cited below discuss balconies. A bay floor that projects through the wall has the same junction geometry, so the same findings apply to it.

What is an arkier?

An arkier is a window bay that stands proud of the external wall line. The glazing sits on a projecting base and under a roof or cap. The enclosed space is usually a shallow alcove or part of a living room. This entry treats arkier and bay window as the same element.

How is an arkier supported?

The base of the bay has to carry its own weight and pass the load back to the main structure. The support method decides the structural behaviour and the heat flow through the envelope. STRUCTURE magazine describes three approaches for balconies, and the table applies them to a bay floor.

Support approach	How it works	Thermal behaviour
Continuous cantilever slab or joists through the wall	The floor of the bay extends past the wall line as one continuous element	A linear thermal bridge. The article calls the detail problematic because the penetration through the envelope is continuous
Discrete exterior supports	A limited number of members carry the bay from the outside	The bridging elements are discrete, so the linear bridge becomes point bridges
Structural thermal break assembly	A proprietary manufactured element in the plane of the thermal envelope carries the load across the insulation	Interrupts the conduction path. The load and thermal properties come from the manufacturer

The same article warns that the high thermal conductance of steel can cause significant heat loss unless the detail reduces it. For the structural side of the cantilever, see cantilever.

Why does an arkier create a thermal bridge?

A thermal bridge is a part of the envelope where heat flows more easily than in the surrounding wall. The RICS Building Control article by Jon Denyer and Chris Willett (26 September 2018) says an uninsulated cantilever has a cooling fin effect. It considers cantilevered balconies and exposed slab edges to be the most critical thermal bridges in the envelope. A projecting bay floor has the same geometry.

The risk is not only heat loss. A cold internal surface at the junction can cause condensation, which supports mould growth. The article describes the fRsi factor as the ratio of two temperature differences. The first is the drop from the outside air to the internal surface. The second is the drop from the outside air to the inside air. For residential properties the article says fRsi should be equal to or greater than 0.75. For the thermal side of the question, see linear thermal bridge coefficient and thermal bridge.

How do the sources say to reduce the bridging?

The two thermal sources give four points. Each applies to balconies, and a projecting bay floor has the same junction.

• Avoid a continuous cantilever through the insulation where you can, because it is a linear thermal bridge.
• Carry the projection on discrete exterior supports to turn the linear bridge into point bridges.
• Use a proprietary structural thermal break assembly at the critical connections. The RICS authors describe such a break as typically made of EPS in place of highly conductive materials, with stainless steel reinforcement.
• Do not rely on extra insulation elsewhere. The RICS authors say it is not an effective way to reduce the risk of surface condensation at a cold spot.

The balcony entry covers the open platform, see balcony. The element that carries the load across the insulation has its own entry, structural thermal break element.

Does an arkier need a special permit under Act 25/2025?

Act 25/2025 does not mention an arkier. The text has no rule on projections. It also has no definition of the built-up area (zastavaná plocha), which it uses for size limits. Section 2 (6) a) defines a simple building as a residential building with at most three flats and two above-ground floors. Its built-up area is at most 300 m2. The act does not say how a projecting bay counts toward that area. Ask the building office before design starts.

The act gives these general rules.

Provision	What it says
Section 18 (2)	A decision on the building intent is required for every building, change of a finished building and building alteration, unless the act says otherwise
Section 5 (3)	An extension (prístavba) is a plan expansion of an existing building that keeps its structural and operational unity
Section 2 (6) a)	Simple building: residential building with at most three flats, at most two above-ground floors and a built-up area of at most 300 m2

A bay added to an existing house widens its plan. The office decides whether it counts as an extension. The decree on general technical requirements (532/2002) stays in force to 31 March 2027 under section 84 (23) of the act. This entry did not use its text.

What applies to an arkier on a heritage building?

The heritage act 49/2002 sets two rules. The owner of a cultural monument must ask the regional heritage office for a decision on the restoration intent before work starts (section 32 (2)). The owner of a property in a protection zone must ask the same office for a binding opinion before changing it (section 32 (11)). Act 25/2025 also sets notification rules for work on a cultural monument and in a heritage area (section 18 (3) f) and g)).