Draught Rate

What is draught rate and why does it matter?

Draught rate (DR) is a quantitative measure of local thermal discomfort caused by air movement. Rather than assessing overall room temperature, DR specifically predicts the percentage of occupants who will feel bothered by air velocity at their location, typically at the neck or ankle level. The metric originates from the Fanger model, developed in the 1980s and now embedded in international standards ISO 7730 and EN 16798-1. Understanding draught rate is essential for architects and engineers because ventilation systems, window infiltration, and passive cooling strategies all create air movement that can cause localized complaints if not carefully controlled.

Unlike overall thermal comfort, which depends on metabolic rate, clothing, and mean radiant temperature, draught rate is hyper-local. A person working at a desk two meters from a poorly designed ventilation outlet may feel cold and uncomfortable while the rest of the room maintains pleasant conditions. This mismatch between general comfort and local dissatisfaction is one of the most frequent complaints in mechanically ventilated buildings, and it is entirely preventable with proper design.

How is draught rate calculated?

Draught rate is predicted using a mathematical model that depends on three physical variables: mean air velocity (v, in m/s), air temperature (T, in degrees Celsius), and turbulence intensity (Tu, expressed as a percentage). The relationship is not linear; small increases in velocity or reductions in temperature create disproportionate increases in dissatisfaction. The Fanger model requires that air velocity be at least 0.05 m/s to trigger the calculation; below that threshold, air movement is assumed imperceptible.

The turbulence intensity term captures how much the air speed fluctuates. Steady, laminar flow at a given velocity causes less discomfort than pulsating or chaotic flow at the same mean velocity. If turbulence intensity is unknown, standards typically assume 40% as a conservative estimate. This is important in practice because a ventilation system designed for smooth, low-turbulence flow can operate at slightly higher mean velocities while maintaining the same DR as a system with more turbulent jets.

Standards provide reference tables and diagrams showing acceptable draught rate limits by activity level. For sedentary work (office tasks, reading, leisure activities), a draught rate of 15% is generally considered acceptable. More stringent limits apply in specialized environments such as hospitals or precision work; conversely, light physical activity (light assembly work, standing in a workshop) tolerates slightly higher dissatisfaction rates because metabolic heat production makes air movement feel refreshing rather than cooling.

Where does draught risk arise in residential buildings?

In single-family houses and apartments, draught complaints stem from a few predictable sources. Ventilation diffusers placed too low or aimed directly at seating areas create jet-like streams with high local velocity and turbulence. Heat recovery ventilation (HRVH) units with poorly designed outlets, or ducts that terminate with sharp edges rather than smooth transitions, accelerate air and increase dissatisfaction. Supply diffusers in living rooms should be placed high on walls or in ceilings, angled to break the jet and promote mixing before air reaches the occupant zone.

Window infiltration, particularly around single-pane windows or loose frames, causes drafts that are often blamed on poor ventilation. However, the underlying cause is poor air-tightness, not ventilation design. The air seeps in around seals and edges, creating cold, turbulent flow. Modern windows with low u-values and proper sealing eliminate most infiltration-induced draught, though passive house standards require attention to sill details and sealing of all service penetrations.

Cold internal surfaces near occupants, especially around windows or exterior walls with inadequate insulation, create radiant temperature asymmetry that mimics draught sensation. Someone sitting near a cold window or thin-walled exterior corner may feel chilled even if air velocity is acceptable. This is not draught rate in the technical sense, but it produces the same complaint: 'there's a draft coming from that window.' The solution is robust insulation and high-performance window frames, not ventilation redesign.

Trickle vents and manual window opening, used for fresh air in homes without mechanical systems, naturally create variable air movement. In adaptive thermal comfort models used for naturally ventilated buildings, occupants in such spaces show greater tolerance for localized air movement because they have direct control. However, draught rate still provides a useful baseline: uncontrolled infiltration is worse than planned, designed ventilation.

How does draught rate differ from other thermal comfort metrics?

Draught rate is one component of local thermal comfort, alongside radiant temperature asymmetry, vertical temperature difference between head and feet, and cold floor surfaces. Thermal comfort more broadly is assessed using the predicted mean vote (PMV) model and predicted percentage dissatisfied (PPD), which capture overall body sensation. A space can have excellent PMV/PPD (everyone feels thermally balanced) while certain locations have high draught rate (some occupants in the supply air jet feel uncomfortable). Conversely, a room with poor overall temperature balance may have low draught risk if air movement is minimal.

Adaptive thermal comfort models, used in naturally ventilated buildings, acknowledge that occupants adjust clothing and behavior in response to outdoor temperature. This expanded comfort range means slightly higher air velocities are tolerated, but draught rate calculations still apply as a local-discomfort check, especially in zones with high turbulence or direct jet impingement.

PMV/PPD and draught rate are complementary. PMV predicts overall satisfaction with temperature; DR predicts dissatisfaction with air movement. A well-designed ventilation system achieves both: neutral operative temperature and low draught rate everywhere. Poor integration of these metrics is why some efficiently insulated, well-heated buildings still generate complaints: the heating system maintains neutral PMV, but the ventilation system creates annoying air jets.

How can designers minimize draught rate in practical terms?

The most effective draught-reduction strategies start at the diffuser. Supply air should exit through low-velocity outlets designed to promote rapid mixing with room air. Linear slot diffusers (long, thin supply grilles) create lower-velocity, more uniform flow than compact round outlets. Ceiling-mounted diffusers angled away from occupied zones are preferable to wall-mounted diffusers that risk direct impingement on seating areas. For heat recovery ventilation units in residential settings, supply velocity should not exceed 2–3 m/s in the duct; beyond this, jets at the room terminal become unavoidable without special diffuser design.

Exhaust outlets are generally non-critical for draught because they remove air rather than introduce it, but they should not draw air directly across occupied zones. Bedroom exhaust should be located in corners or high on walls, not directly above sleeping positions. In kitchens and bathrooms, exhaust-only ventilation or imbalanced mechanical exhaust can create cold drafts if makeup air enters through untreated infiltration; balanced mechanical ventilation with controlled supply diffusers eliminates this problem.

Insulation quality affects draught perception indirectly. Cold windows and thin walls trigger radiant asymmetry, which occupants often misattribute to air movement. Installing thermal break window frames, upgrading facade insulation, and ensuring continuous insulation at corners eliminates this psychologically perceived 'draft' even if actual air velocity remains unchanged. This is especially relevant in passive house retrofits of old dwellings with poor envelope performance.

Turbulence control matters in practice. Flexible ductwork with internal ribs creates more turbulent exit flow than rigid sheet-metal ducts with smooth bends. Duct runs should avoid sharp bends, sudden area changes, and excessive lengths that cause pressure drop and flow acceleration. In balancing during commissioning, technicians should verify that supply velocity matches design intent; an oversized fan pushing too much air through a single outlet creates jets regardless of diffuser quality.

What standards govern acceptable draught rate?

ISO 7730 and EN 16798-1 are the primary standards defining draught rate and its limits. ISO 7730 (Ergonomics of the thermal environment) was established in 1994 and revised in 2005; EN 16798-1 (Indoor environmental quality) adopted the same Fanger model for European buildings and aligns with the newer parametric method. Both standards define three categories of local thermal comfort: Category A (stringent, for high-performance buildings or sensitive spaces), Category B (moderate, for general office and residential use), and Category C (relaxed, for transitional spaces or temporary occupancy).

The acceptable DR thresholds in these categories reflect a balance between cost (lower DR requires more expensive diffuser design and larger ducts) and user satisfaction. For residential buildings in Europe, Category B is typical, allowing DR up to 15% in occupied zones. Buildings targeting passive house certification or premium comfort may aim for Category A (DR less than 10%), requiring more careful diffuser selection and duct design. The standards emphasize that DR must be evaluated at the occupant level (typically 0.1–1.7 m above the floor, depending on activity) and in multiple locations; a single test at one point does not certify the entire room.