Free cold downdraught
Without an effective interruption, the cold boundary layer grows over the facade height and spreads into the room at floor level.
Glass facades · Cold downdraught · Thermal comfort
Assess cold downdraught at glass facades reliably
Topic
Comfort near the facade
Cold glazing can create noticeable room air movement
Air next to a tall cold glass surface cools and descends along the facade. At floor level it spreads into the occupied zone. Whether this causes draught or local discomfort depends on facade height, surface temperature, geometry and the heating concept.
Four typical flow cases
Consider projection depth and boundary layer together
Horizontal facade projections interrupt the downdraught effectively only when their depth is appropriate for the cold boundary-layer thickness. The four diagrams show how the airflow changes with projection depth.
Free cold downdraught
Projection smaller than the boundary layer
Projection similar to the boundary layer
Projection larger than the boundary layer
Projection smaller than the boundary layer
Short projections disturb the flow only locally. A substantial part of the cold airflow continues downwards.
Projection similar to the boundary layer
The flow separates more strongly at each level. Local deflection and recirculation reduce the continuous downdraught.
Projection larger than the boundary layer
Deep projections intercept most of the downdraught and deflect it into the room at each level.
Winter case · Draught risk
The temperature field reveals the cold downdraught
The CFD visualisation shows how room air cools directly at the cold glazing and descends as a coherent flow. Its velocity can increase with falling height. At floor level, the cold air turns into the occupied zone – precisely where winter draught discomfort may occur.
- Cooling of the near-wall air layer at the glazing
- Increasing downward velocity over the facade height
- Deflection of cold air into the occupied zone at floor level
- Assessment together with air velocity, temperature and heating system
Method
From glass surface temperature to comfort assessment
The CFD model represents the glazing, facade projections, parapets, heating surfaces and relevant occupied zone. For a winter case, surface temperatures, outdoor climate, room temperature and heating output are combined.
- 01
Capture facade height, glazing, projections, parapets and heating system
- 02
Define surface temperatures and thermal boundary conditions for the winter case
- 03
Simulate downdraught, mixing and interaction with convectors or underfloor heating
- 04
Evaluate air speed, temperature and draught risk in occupied zones
Results for design and verification
Size mitigation measures with confidence
The simulation shows where the downdraught develops, how far it penetrates into the room and which measure is effective. Design variants can be compared before facade construction and heating concept are fixed.
Velocity and penetration distance of the cold downdraught
Temperature and velocity distribution in the occupied zone
Effectiveness of projections, convectors and heating surfaces
Comparison of facade and building-services variants
FAQ
Frequently asked questions
When should cold downdraught be analysed?
For tall or extensive glazing, low internal surface temperatures and sensitive occupied areas close to the facade.
Can a simplified calculation be sufficient?
Yes for an initial assessment. For complex geometry, multi-storey facades or interacting heating systems, spatial CFD gives a more robust evaluation.
Can convectors and underfloor heating be compared?
Yes. Output, position and supply temperature can be varied and compared for comfort effect and energy demand.
Next step
Request a projectInclude cold downdraught early in facade design
We define the geometry, winter boundary conditions and suitable assessment criteria with you.