Skylight / Roof Window
A fixed or tilted glazed opening in a roof that admits daylight into rooms below, commonly used in attics to reduce reliance on artificial lighting.
What is a skylight and how does it differ from a dormer or roof window?
A skylight is a fixed or tilted glazed opening set directly into the roof plane, allowing daylight to enter rooms below without projecting outward. Unlike a dormer window, which is a protruding roof structure with vertical walls and standing headroom, a skylight is a simple glazed panel that provides daylight but no additional usable floor area or ability to stand upright. Unlike a roof window (an operational glazed frame that opens and closes for ventilation), most skylights are fixed and offer no air exchange. Skylights suit attic conversions seeking daylight without major structural modification, while dormers solve the headroom problem at higher cost and complexity. Roof windows bridge the gap, offering ventilation and some daylight but less headroom than dormers.
How much daylight can a skylight provide?
Daylight delivery depends on skylight area, roof angle, and room layout. A horizontal skylight achieves a daylight factor of 2–5 percent in the directly lit zone, bright enough to eliminate artificial lighting for daytime hours. North-facing skylights deliver steady, glare-free light; south-facing skylights provide more winter sun but risk summer glare. Daylight falls sharply beyond 2–3 m, so skylights suit rooms directly below. Reflective ceilings extend penetration. Professional simulation guides sizing for residential projects.
What are the thermal risks of skylights?
The primary challenge is heat loss in winter and overheating in summer. A double-glazed skylight typically has U-value 1.0–1.5 W/m²K, compared to insulated walls at 0.15–0.25 W/m²K, making each square metre of skylight lose 5–10 times more heat. In Slovakia's climate, large unshaded skylights cause heating demand spikes and summer overheating. Solar heat gains can drive temperatures above 28°C. Passive-house designs typically avoid skylights or use very small, triple-glazed units with exterior shading. Standard new builds should limit skylight area to less than 5 percent of roof and provide operable exterior shading to mitigate both risks.
What causes condensation at skylights and how is it prevented?
Surface condensation forms when warm indoor air meets the cold skylight glazing or frame, worsened by thermal bridging where the frame conducts outside cold inward. Risks are highest in humid spaces (bathrooms, kitchens) with poor ventilation.
Prevention requires: deep insulation (200–300 mm around the skylight in Slovak climate), a frame with a thermal break to reduce conductivity, secondary internal glazing to raise interior surface temperature, humidity control via ventilation below 50 percent relative humidity in winter, a continuous vapour retarder on the warm side of insulation, and careful air-sealing around the frame to prevent air infiltration into the roof cavity.
How does skylight flashing prevent water leaks?
Water entry is a leading cause of roof failures because the skylight breaks the continuous roof membrane. Proper flashing has three parts: base flashing (adhered around the skylight), step flashing (overlapped shingles or tiles), and counter-flashing (at the frame, directing water outward). All joints must be sealed with sealant or membrane. On pitched roofs, the downslope edge must not trap water; flashing slopes steeply past the curb. On flat roofs, the curb must project at least 150 mm above the surface to prevent ponding, sealed with EPDM extending onto the roof membrane. Common failures include sealant cracking, flashing installed below shingles, or curbs too low. Professional installation and pre-fabricated kits reduce failure risk.
What size and placement work best?
A rule of thumb is 5–15 percent of room floor area as skylight area; bedrooms need 5 percent, studios/offices 10–15 percent. North-facing skylights offer steady, glare-free light; south-facing skylights provide winter sun but need expensive exterior shading to prevent overheating. Multiple smaller skylights spread light more evenly than one large one. Avoid direct placement over desks or beds where glare is uncomfortable. Steep roof pitches (greater than 35°) shed water reliably but gather less winter daylight; gentle pitches capture more winter sun but risk water pooling if flashing fails.
| Skylight Type | Frame Material | Typical U-Value (W/m²K) | Operability | Best Use |
|---|---|---|---|---|
| Fixed double-glazed | Aluminium or timber | 1.2–1.5 | None | Daylight only; attics, galleries |
| Tilting roof window | Timber-aluminium | 1.0–1.3 | Tilts for ventilation | Ventilation + daylight in pitched roofs |
| High-performance (triple-glazed with thermal break) | Timber or composite | 0.5–0.8 | Fixed or tilting | Passive-house compatible; premium cost |
| Flat rooflight (minimal curb) | Aluminium frame | 1.3–1.8 | Usually fixed | Flat roofs; high water-leak risk if poorly flashed |
What are the key design and regulatory considerations in Slovakia?
Skylights are subject to STN 73 0540 (thermal protection), which requires minimum insulation and mould-risk compliance (temperature factor fRsi). The building act 25/2025 Z. z. (effective April 2025) classifies roof alterations as potentially requiring authorization or notification depending on extent and local rules. Energy performance regulations (NZEB) apply to new builds and renovations; passive-house projects require triple glazing, thermal breaks, and exterior shading.
Practical Slovak considerations: roof snow loads (up to 150 kg/m² in central regions) require curved or low-profile skylights. Condensation risk is high, so thermal breaks and secondary glazing are standard. Many municipalities restrict skylight visibility from public streets; pre-approval is wise in conservation areas. Discuss placement early to avoid neighbour glare concerns on snow.
| Design Parameter | Recommended Range / Requirement | Rationale |
|---|---|---|
| Skylight area as % of roof area | 3–8% (typical); up to 15% (daylighting priority) | Balance daylight benefit against thermal loss and overheating risk |
| Interior surface temperature (fRsi) in winter | >0.65 (STN 73 0540 requirement) | Prevent mould growth and surface condensation on interior surfaces |
| Frame U-Value | 1.0 W/m²K or better (passive-house: 0.8 or better) | Reduce thermal bridging at frame-to-roof junction |
| Glazing composition | Double-glazed minimum; triple for passive-house or harsh climates | Higher insulation resists heat loss and condensation |
| External shading control | Operable exterior blind or movable shutter on south-facing skylights | Reject high summer sun; operator can close to prevent overheating and glare |
| Thermal break in frame | Mandatory for new builds and retrofits | Prevent local cooling of interior surface directly above frame |
| Roof pitch (for pitched-roof skylights) | 25–45° (steeper for snow shedding) | Steep slopes drain water and snow; gentle slopes capture winter sun |
Frequently asked questions
- How is a skylight different from a roof window or dormer?
- A skylight is a fixed or tilted glazed panel set directly into the roof plane with minimal projection. A roof window (in pitched roofs) is an operational (openable) window that tilts or slides, offering ventilation. A dormer is a protruding roof structure with vertical walls and windows that provides standing headroom and usable floor area. Skylights offer the most direct daylight but the least thermal performance and no ventilation; dormers solve attic conversion needs but are structurally complex.
- Can skylights cause excessive heat loss in winter?
- Yes. Glazed skylights lose significantly more heat per square metre than typical walls because they expose both surfaces to outdoor air and sky radiation (clear winter nights radiate heat away). A double-glazed skylight typically has a U-value of 1.0–1.5 W/m²K, compared to a well-insulated wall at 0.15–0.25 W/m²K. In passive-house design, skylights are often avoided entirely due to this thermal penalty.
- What causes condensation at skylight edges and how do you prevent it?
- Condensation occurs when warm indoor air meets the cold surface of the skylight frame or the roof assembly around it, causing moisture to condense. This is worsened by thermal bridging (the frame conducts cold from outside inward). Prevention requires insulating the roof assembly thoroughly, sealing air leaks around the frame, using a thermal break in the frame, and ensuring adequate ventilation or humidity control in the room. Secondary glazing or a skylight with a very high-performance thermal break can help.
- How much daylight does a skylight actually provide?
- Daylight contribution depends on skylight size, orientation, and room depth. A 1 m² skylight on a horizontal roof can deliver a daylight factor of 2–5 percent in the room directly below (very bright), but the benefit drops sharply with distance and roof angle. Skylights on south-facing sloped roofs provide less consistent daylight than those on flat or north-facing roofs. Modelling or consulting a daylighting specialist is wise for attic conversions.
- What are the main advantages and disadvantages of skylights in Slovakia?
- Advantages: significant daylight gain, no loss of wall area for windows, psychological benefit of sky views. Disadvantages: high heat loss in winter, risk of summer overheating, condensation risk at edges, glare in summer, structural complexity (requires roof reinforcement), higher cost than wall windows per unit glazing, potential snow/ice dam issues on sloped roofs. Thermal performance is the limiting factor in a climate like Slovakia's.
- Do I need a building permit to install a skylight in Slovakia?
- Generally yes. Adding a skylight involves altering the roof structure and may affect the building envelope (energy performance) or exterior appearance. Under the building act (zákon o výstavbe 25/2025 Z. z., effective since April 2025), roof modifications are typically considered substantial changes requiring notification to the building office (stavebný úrad). Consult your local authority, as thresholds vary by municipality.