Bygningsreglementets vejledning om korrektioner til 10 pct.-reglen for dagslys – Danish Building Code Guidance on Daylight Corrections
The Bygningsreglementets vejledning om korrektioner til 10 pct.-reglen for dagslys is a formal guidance document issued by the Trafik-, Bygge- og Boligstyrelsen (Danish Transport, Construction and Housing Authority) in January 2019. It operates within the legal framework of the Bygningsreglementet 2018 (BR18). The document provides technical specifications and methodologies for calculating and correcting the required glass area in buildings to ensure adequate daylighting for a healthy visual indoor climate.
This guidance governs the application of the 10 pct.-reglen (10% rule), which mandates that the glass area of a room must correspond to at least 10% of the floor area. Its scope covers residential, office, and commercial building projects. The document defines a comprehensive set of korrektionsfaktorer (correction factors) to account for variables such as lystransmittans (light transmittance), vægtykkelse (wall thickness), shading from the environment, and structural protrusions. It includes technical chapters on skyggende omgivelser (shading surroundings), fremspring (protrusions), atrier (atriums), and rumdybde (room depth).
The following sections detail the specific mathematical methods and variables required to document compliance with the daylight requirements established in BR18 § 379 and § 380.
The 10% Rule and Basic Methodology
The primary requirement for daylight in the Danish Building Regulations is that the glass area must constitute at least 10% of the net floor area. This is expressed as AG,min = 0.1 · Agulv.
- AG,min: The minimum required glass area in square meters.
- Agulv: The net floor area of the room in square meters.
- Glass Area Calculation: Measured as the free opening area, excluding frames, sills, and glazing bars.
- Skylights: Windows in the building envelope with an inclination of 60° or less relative to the horizontal are considered skylights and can be counted with a factor of 1.4.
System of Correction Factors
To determine the actual effective glass area (AGkor,i), the actual glass area of a window (AGvin,i) must be multiplied by all relevant correction factors. The sum of these corrected areas must meet or exceed the minimum requirement (AG,min).
- FLT (Light Transmittance): Correction for glass types with light transmittance values differing from the reference value of 0.75.
- FVÆG (Wall Thickness): Reduction factor for windows placed in thick external walls, particularly significant for small windows or walls exceeding 40 cm.
- FOMG (Surrounding Shading): Accounts for shading from opposite buildings or terrain based on the profilvinkel (profile angle).
- FOH (Overhangs): Correction for shading caused by balconies or eaves directly above the window.
- FSF (Side Protrusions): Accounts for shading from building wings or external structural elements beside the window.
Special Building Conditions
The guidance provides specific factors for complex architectural scenarios where standard window placement does not adequately describe the light intake.
- FATR (Atriums): Calculation for windows facing an internal atrium, incorporating factors for the transparent roof surface (FLT-tag) and the roof's support structure (FTAG).
- FRUM (Room Depth): Correction for deep rooms where the distance to the farthest workspace exceeds 5 meters, requiring an increased glass area to compensate for the rapid fall in illumination.
- FFL (Multiple Surfaces): A factor of 1.2 is applied when windows are placed in more than one external surface (e.g., side window and skylight) to acknowledge improved light distribution.
- FAFS (Fixed Solar Shading): Permanent external shading devices such as fixed lamellas reduce daylight and must be accounted for in the total calculation.
Frequently Asked Questions
What is the reference light transmittance value for glass in BR18?
The reference light transmittance value is 0.75. If the actual glass has a different value, the factor FLT is calculated as LTakt / 0.75.
How are skylights weighted compared to vertical windows?
Skylights are weighted with a factor of 1.4 because they receive significantly more daylight than vertical windows.
At what wall thickness do correction factors begin to apply?
Corrections for wall thickness (FVÆG) are typically applied for walls 40 cm thick or greater; the factor for a 30 cm wall is 1.00.
What happens if a room is deeper than 6 meters?
For residential rooms deeper than 6 meters from windows, a fixed correction factor for room depth (FRUM) of 0.9 is applied.
How do fixed solar shading lamellas affect the daylight calculation?
Fixed or rotating lamellas placed vertically in front of a window reduce daylight by at least 40% for the covered portion, even when in a horizontal position.
Can multiple correction factors be applied to the same window?
Yes, for each window, the actual glass area is multiplied by the product of all relevant correction factors (FG,kor,i).
