What is a reveal and why does every window need one?

A reveal is the splay surface created by the thickness of the wall as it meets the window frame. It exists because walls are thick and windows sit within them. Without a properly detailed reveal, you cannot manage thermal bridges, install insulation, or control water and air infiltration. The reveal is therefore not optional; it is the connection detail between the frame and the building envelope.

How does reveal position affect thermal performance?

Windows positioned within the insulation layer create reveals both internally and externally, allowing continuous insulation around the frame. Reveals at the outer face create large uninsulated cavities; reveals at the inner face concentrate thermal bridges at the frame. The best practice is centered positioning with ETICS or ventilated wrap that bridges the reveal space.

Why are reveals especially important in ETICS retrofits?

In a typical retrofit, when you add 200 mm of external insulation, you change the reveal depth by 200 mm. The window frame, which was originally near the outer surface, is now 200 mm recessed. A poor detail creates a cold cavity and a thermal bridge at the frame shoulder. Proper ETICS reveals use insulated reveal boards that extend the insulation around the frame shoulder, eliminating that bridge.

What is the difference between an internal and external reveal?

An internal reveal is the splay you see when standing inside the building looking at the window frame edge. An external reveal is what you see from outside. In passive-house design, both must be insulated to avoid thermal bridges. Many older buildings have only internal splays visible because the frame sits near the external face; retrofits often introduce external reveals for the first time.

Can deep reveals cause solar control problems?

Deep external reveals, especially on south and west orientations, reduce solar gain by 30-50% through self-shading. This is an advantage in summer but can also reduce passive solar warming in winter if overdesigned. The depth should match the sun angle for the building's latitude and window orientation; deep reveals benefit most from the architectural detail rather than thermal need.

How do you insulate a reveal in an ETICS system?

Standard method: extend continuous insulation around the window shoulder using specialized ETICS reveal boards (typically EPS or PIR, same thickness as the main wall insulation). The board is mechanically fastened to the structure and wrapped with reinforcement mesh and finish plaster, bridging from the main facade insulation to the window frame. This eliminates cold edges and reduces thermal transmittance at the junction by 0.08-0.12 W/(m·K).

Window reveal

The splay surface where a window frame meets the wall. Controls thermal bridges, moisture drainage, and air tightness at the window junction.

What is a window reveal?

A window reveal (Slovak: špaleta) is the visible splay or recessed surface created where a window or door frame sits within the thickness of a wall. It is not an optional aesthetic gesture; it is the structural and thermal connection between the frame and the building envelope. The term encompasses both the internal splay (visible from inside) and the external splay (visible from outside), as well as the horizontal surfaces at the head and sill.

The reveal exists because building walls have thickness. A typical residential wall in Slovakia measures 300-500 mm thick when built with masonry, concrete blocks, or modern composite systems. The window frame itself is only 60-120 mm deep. The difference is the reveal: the portion of the wall that must accommodate the frame, insulation, sealing, and tolerances.

Why do window reveals matter for building performance?

Reveals serve several critical functions beyond framing the opening. They accommodate construction tolerances that are inevitable in on-site assembly. A well-designed reveal allows 5-20 mm clearance on each side for installation, packing, foam sealing, and adjustments. Without this tolerance zone, misalignment forces and poor sealing compromise both thermal and water performance.

Reveals also manage thermal bridges. A thermal bridge occurs where a conductive material or geometry bypasses insulation. The junction where a window frame meets the wall is inherently prone to thermal bridging because the frame is often a poor insulator and the connection disrupts the insulation continuity. The reveal detail determines whether this junction is insulated, bridged across, or left exposed to the cold exterior. In passive-house design, this detail controls 5-10% of the building's heating demand.

Moisture management is equally critical. The reveal surface must slope and drain to prevent water from pooling and infiltrating behind the frame. External horizontal reveals (sills) must slope outward and feature a drip edge; internal horizontal surfaces must slope to the exterior to prevent condensation accumulation in cold climates.

How does window position within the wall affect the reveal detail?

The depth and character of the reveal depend on where the window sits relative to the insulation layer. There are three common configurations:

Position	Reveals Created	Thermal Bridge Risk	Common Use
Centered in insulation	Both internal and external	Low (if properly wrapped)	New passive-house construction, quality retrofits
Flush with external face	Large internal splay only	Very high (frame bypass)	Older buildings, minimalist contemporary
Flush with internal face	Large external splay only	High (frame cold side)	Interior-insulated retrofits (not recommended)

Centered positioning is preferred in modern construction because it allows the structural wall and insulation to wrap around the frame, providing thermal protection from both sides. The external reveal can be shallower, accommodating the insulation thickness, while the internal reveal provides adjustment space and thermal buffer.

What changes when retrofitting windows with external insulation (ETICS)?

External thermal insulation composite systems (ETICS) fundamentally alter reveal geometry. Before retrofit, a window frame in a solid masonry wall sits relatively near the exterior face, with a modest external reveal. After adding 150-200 mm of continuous insulation, the frame becomes deeply recessed, creating a new challenge: the cavity behind the insulation and in front of the window frame.

A naive retrofit leaves this cavity uninsulated. The result is a cold path around the frame shoulder, reducing the frame's internal surface temperature below the dew point in winter, causing condensation and risk of mould. Modern ETICS retrofits solve this with insulated reveal boards: rigid EPS, XPS, or PIR boards (typically 80-120 mm thick) mechanically fastened to bridge from the main facade insulation to the window frame. These boards are wrapped in reinforcement mesh and finish plaster, creating a thermally continuous envelope.

Retrofit Detail	U-value at Junction	Interior Surface Temperature (Winter)	Condensation Risk
Uninsulated reveal cavity	0.40-0.60 W/(m²·K)	8-12 °C	Very high
Mineral wool reveal board, 100 mm	0.15-0.20 W/(m²·K)	16-18 °C	Moderate
PIR reveal board, 100 mm	0.08-0.12 W/(m²·K)	18-20 °C	Low

The choice of reveal insulation material affects both cost and thermal performance. Mineral wool and EPS are economical but require careful detailing to avoid compression and water damage. PIR (polyisocyanurate) boards offer superior insulation value in a thinner depth, reducing the thickness of the reveal detail and the visual impact of the recess.

How deep can an external reveal be, and what are the trade-offs?

External reveals range from flush (0 mm) to deeply recessed (200-300 mm in historic buildings with thick masonry). The depth affects several design considerations:

Deep reveals create stronger shadow lines and visual emphasis of the opening, which can enhance architectural character. On south and west elevations, a 150+ mm reveal provides significant passive shading, reducing cooling load by 30-50% depending on latitude and sun angle. However, deep reveals also increase the wall thickness perception, which may read as heavy or massive depending on the architectural intent. In narrow alleyways or dense urban settings, deep reveals reduce daylight penetration into adjacent buildings.

From a practical standpoint, reveal depth is constrained by wall thickness, window frame position, and insulation thickness. Retrofits typically impose the greatest constraints: a 200 mm ETICS system paired with a 90 mm deep frame leaves only 110 mm for the reveal insulation, which is borderline for passive-house thermal targets using mineral wool (better with PIR). New construction offers more freedom and should be planned from the beginning to balance thermal, solar, and architectural goals.

How do reveals relate to airtightness?

The reveal space is where the building's air barrier must transition from the structural wall to the window frame. If the reveal is poorly sealed, air leaks around the frame perimeter, bypassing insulation and driving infiltration heat loss. Airtightness requires continuity: the air barrier must seal from the inside surface of the insulation, around the frame perimeter, and down to the sill packing or sealant.

In practice, this means the foam sealant (typically polyurethane or PU foam) must completely fill the gap between frame and wall. This gap is the reveal space. If the reveal is oversized or poorly packed, large air voids remain even after foam application. Sealing tape (membrane) should follow the foam line to ensure continuous coverage. In passive houses, this detail is tested with blower-door testing to verify air tightness; reveal sealing failures account for a significant portion of retrofit leakage problems.

What is a window sill and how does it relate to the reveal?

The window sill is the horizontal surface at the base of the opening, forming the bottom of the external reveal. It must slope outward (typically 5-15 degrees) to shed water and must include a drip edge or ovolo profile to direct water away from the wall face. An internal sill (the horizontal shelf visible from inside) may slope inward or be level, depending on use and maintenance philosophy. Modern practice favors sloped internal sills that direct water outward if any penetrates the frame seal.

Sill details interact with thermal bridges because the junction where sill meets wall is another conductive path. In cold climates, an uninsulated sill can create a local cold zone leading to mould if condensation occurs. Insulated sills or thermal breaks between the external and internal sill surfaces are necessary in passive-house retrofits.

Frequently asked questions

What is a reveal and why does every window need one?: A reveal is the splay surface created by the thickness of the wall as it meets the window frame. It exists because walls are thick and windows sit within them. Without a properly detailed reveal, you cannot manage thermal bridges, install insulation, or control water and air infiltration. The reveal is therefore not optional; it is the connection detail between the frame and the building envelope.
How does reveal position affect thermal performance?: Windows positioned within the insulation layer create reveals both internally and externally, allowing continuous insulation around the frame. Reveals at the outer face create large uninsulated cavities; reveals at the inner face concentrate thermal bridges at the frame. The best practice is centered positioning with ETICS or ventilated wrap that bridges the reveal space.
Why are reveals especially important in ETICS retrofits?: In a typical retrofit, when you add 200 mm of external insulation, you change the reveal depth by 200 mm. The window frame, which was originally near the outer surface, is now 200 mm recessed. A poor detail creates a cold cavity and a thermal bridge at the frame shoulder. Proper ETICS reveals use insulated reveal boards that extend the insulation around the frame shoulder, eliminating that bridge.
What is the difference between an internal and external reveal?: An internal reveal is the splay you see when standing inside the building looking at the window frame edge. An external reveal is what you see from outside. In passive-house design, both must be insulated to avoid thermal bridges. Many older buildings have only internal splays visible because the frame sits near the external face; retrofits often introduce external reveals for the first time.
Can deep reveals cause solar control problems?: Deep external reveals, especially on south and west orientations, reduce solar gain by 30-50% through self-shading. This is an advantage in summer but can also reduce passive solar warming in winter if overdesigned. The depth should match the sun angle for the building's latitude and window orientation; deep reveals benefit most from the architectural detail rather than thermal need.
How do you insulate a reveal in an ETICS system?: Standard method: extend continuous insulation around the window shoulder using specialized ETICS reveal boards (typically EPS or PIR, same thickness as the main wall insulation). The board is mechanically fastened to the structure and wrapped with reinforcement mesh and finish plaster, bridging from the main facade insulation to the window frame. This eliminates cold edges and reduces thermal transmittance at the junction by 0.08-0.12 W/(m·K).