Rijtjeshuis (voorbeeld) / Terraced house (example)

Natural ventilation assessment · Plan "Terraced house" (5.6 × 11.0 m) · 25-10-2026, 19:34:57

PARTLY MEETS TARGET

0 of 2 habitable rooms reach 4.3 ACH in every direction tested; 2 fall short in at least one direction.

Target: 4.3 air changes per hour in every habitable room (Bbl spui, verblijfsruimte (3 dm³/s·m²), at a 2.50 m ceiling) · wind 4.0 m/s at 10 m = 2.4 m/s at the building · outside windows fully open (purge) · 4 wind directions · uncertainty ±31%–50%

Rooms are judged on the lower of the simulation and the textbook orifice-flow calculation; the two agree for 4 of 4 wind directions (section 5).

What was assumed: Wind: 4.0 m/s at a weather station (10 m, open country), converted to 2.42 m/s at the building for terrain "Suburban / urban" and building height 9.0 m (BS 5925 / CIBSE: U = U₁₀·K·H^a). The facade Cp values refer to this wind at the building. Outside windows: Fully open (purge).

1. Summary

Best direction46.7 ACHwind from N

Worst direction0.0 ACHwind from E

Median fresh air849 L/swhole house

Rooms meeting target (all directions)0 / 2habitable rooms

Whole-house air changes per hour for each wind direction (blue), the lower of simulation and textbook calculation. The dashed red ring is the target. Whole-house ACH = fresh air entering the house / total indoor volume.

2. Recommendations

R1 (main room), worst 0.0 ACH with wind from E: fine for some winds but below target for wind from E, W. A window on a different facade from its existing W2, D4 would give it a cross-flow path for those directions.
R2, worst 0.0 ACH with wind from E: fine for some winds but below target for wind from E, W. A window on a different facade from its existing W1, D3 would give it a cross-flow path for those directions.
For wind from E, W all openings sat at the same facade pressure, so no wind-driven cross-flow is possible. On those days only stack (temperature-driven) ventilation or fans will move air. Openings on more facades help.
Orientation matters a lot here: 46.7 ACH with wind from N versus 0.0 ACH from E. If the site has a prevailing wind, set Wind From to it for the key study.
For wind from N, E, S, W the measured outflow differs from the inflow by more than 25% even though the flow is steady. ACH is taken from the inflow, which is the more stable figure, but those directions are judged with the wider uncertainty band shown in the tables.

3. Rooms: air changes per hour by wind direction

Room	N	S	Worst, dm³/s per m²	Result	Notes
R1 main room · 23.9 m²	100.3	83.0	0.0 < 3	Below target
R2 room · 21.3 m²	101.8	114.8	0.0 < 3	Below target
R3 hall / corridor · 7.1 m²	292.4	328.1	—	Not assessed	treated as hall / corridor (a guess from its size and shape); no window or door to the outside

Green = at least the target plus that direction's uncertainty band; amber = within the band of the target; red = below. Values are the lower of simulation and textbook calculation, capped at the fresh air entering the house. Room volume = floor area × 2.50 m. dm³/s per m² = fresh air per m² of floor, shown against the Bbl purge-ventilation figures (3 for a verblijfsruimte, 6 for a verblijfsgebied) for orientation only; the formal check is NEN 1087.

4. Whole house by wind direction

Wind from	Judged ACH	Simulated ACH	Fresh air in (L/s)	Air out (L/s)	In/out mismatch	Fluctuation	Note
N (0°)	46.7 ±37%	50.5	1835	1625	37%	0%	in/out mismatch over 25% (steady): wider band
E (90°)	0.0 ±50%	0.0	0	847	100%	0%	All open windows/doors face the same wind pressure, so wind alone drives no cross-flow (single-sided ventilation needs turbulence / temperature effects, not modelled). Change the wind direction or set roles by hand.; in/out mismatch over 25% (steady): wider band
S (180°)	46.7 ±31%	54.8	1990	1097	31%	2%	in/out mismatch over 25% (steady): wider band
W (270°)	0.0 ±50%	0.0	0	783	100%	0%	All open windows/doors face the same wind pressure, so wind alone drives no cross-flow (single-sided ventilation needs turbulence / temperature effects, not modelled). Change the wind direction or set roles by hand.; in/out mismatch over 25% (steady): wider band

5. Cross-check: textbook orifice-flow calculation

Each direction recalculated with hand-calculation methods that do not use the simulation, with the same opening sizes and facade pressures. Orifice network: every room is a zone at its own pressure and every open window and door (interior doors included) is an orifice, Q = C_d·A·√(2Δp/ρ); the room pressures are solved so each room's flows balance (the multizone method of CIBSE AM10 and CONTAM). Single zone: BS 5925 cross-ventilation, inlets and outlets in series and interior walls ignored. Discharge coefficient C_d = 0.6; the range in brackets is C_d 0.5–0.7.

Wind from	Simulated fresh air (L/s)	Orifice network (L/s)	Single zone (L/s)	Simulated / network	Result
N	1835	1697 (1414–1980)	4333	1.08 (0.53–1.59)	Agrees
E	0	0 (0–0)	0	— (0.42–1.75)	No flow (both)
S	1990	1697 (1414–1980)	4333	1.17 (0.58–1.53)	Agrees
W	0	0 (0–0)	0	— (0.42–1.75)	No flow (both)

Agreement range: C_d 0.5–0.7, widened by that direction's uncertainty band.

Room ACH: simulated / network	N	E	S	W
R1	100.3 / 102.3	0.0 / 0.0	83.0 / 102.3	0.0 / 0.0
R2	101.8 / 114.8	0.0 / 0.0	120.5 / 114.8	0.0 / 0.0
R3	292.4 / 344.5	0.0 / 0.0	328.1 / 344.5	0.0 / 0.0

6. Openings: airflow (L/s) by wind direction

Id	Type	Position	Setting	N	E	S	W
D1	door 0.90 m	interior	Open	1386	34	1766	140
D2	door 0.90 m	interior	Open	1495	131	1466	18
W1	window 2.40 m	S facade	Auto	47 out	171 out	1053 in	255 out
D3	door 0.90 m	S facade	Auto	1578 out	214 out	744 in	170 out
W2	window 1.90 m	N facade	Auto	1050 in	245 out	1097 out	259 out
D4	door 0.90 m	N facade	Auto	785 in	217 out	192 in	98 out

Settings "Auto" become an inlet or an outlet depending on the wind (green = air in, blue = air out). Interior doors show the air passing through them.

7. Air speed in the plan

Air speed, wind from N — Wind from N: 46.7 ACH

Air speed, wind from E — Wind from E: 0.0 ACH

Air speed, wind from S — Wind from S: 46.7 ACH

Air speed, wind from W — Wind from W: 0.0 ACH

Colour = air speed (blue slow, red fast); white streaks follow the air. Snapshots taken at the end of each case.

8. Method and limits

For each wind direction the plan was simulated as a 2D incompressible flow (grid 320 × 184 cells, 13.7 cells per metre, LES turbulence model). Each window and door to the outside was held at the wind pressure of its facade, 0.5·ρ·U²·Cp. The flow was allowed to settle for two flow-through times (Study Length "Standard") and then sampled 8 times. The flow through each opening was integrated on the faces between the indoor air and the opening.

Wind: 4.0 m/s at a weather station (10 m, open country), converted to 2.42 m/s at the building for terrain "Suburban / urban" and building height 9.0 m (BS 5925 / CIBSE: U = U₁₀·K·H^a). The facade Cp values refer to this wind at the building.
Outside windows: Fully open (purge).
2D plan model: each window and door acts over its height (window 1.20 m, door 2.10 m), and the flow is converted to L/s and ACH with those heights and a 2.50 m ceiling.
Wind acts only through the pressure it puts on each facade (Cp windward 0.60, side -0.60, leeward -0.30, generic values for a low-rise building). Nearby buildings, trees and terrain change these, often by a factor of two.
Room figures: a room is credited with the air passing through it, but never more than the fresh air entering the whole house, and it is judged on the lower of the simulation and the orifice-network calculation (section 5).
Single-sided ventilation (air in and out of the same opening) and temperature-driven stack ventilation are not modelled; both add air the study does not count. On calm summer nights stack ventilation through stairwells is often the main driver.
Wind direction 0° = from the top of the drawing. If the drawing is not north-up, read the directions relative to the drawing.
Uncertainty: each direction is judged with its own band, the larger of its measured in/out mismatch and twice its sample-to-sample fluctuation (at least ±10%; widest here ±50%). Rooms within the band of the target are reported as marginal.
Bbl comparison: the dm³/s per m² figures are compared with the purge-ventilation capacities of Bbl art. 4.131 for orientation only. The formal check is NEN 1087; this study does not replace it.
This is a design-stage comparison tool, not a certified calculation. Where a building regulation applies, confirm the result with an accepted method.

Generated by OTOM CFD (WebGPU), House Ventilation tab. Every number in this report comes from the simulation or the textbook calculation; the recommendation wording follows fixed rules.