Natural ventilation assessment

Plan "Apartment (CADCraft example)" (12.0 × 8.0 m) · 25/10/2026, 21:15:23 · prepared by amin

MEETS TARGET

Every habitable room reaches 4.0 ACH for all 8 wind directions tested.

Target: 4.0 air changes per hour in every habitable room · wind 4.1 m/s at the building · outside windows fully open (purge) · 8 wind directions · uncertainty ±10%–39%

Rooms are judged on the lower of the simulation and the textbook orifice-flow calculation; the two agree for 4 of 8 wind directions (section 5).

What was assumed: Wind: 4.10 m/s, given as the speed at the building (no terrain correction applied). Outside windows: Fully open (purge).

1. Summary

Best direction135.1 ACHwind from S

Worst direction35.0 ACHwind from SW

Median fresh air4371 L/swhole house

Rooms meeting target (all directions)3 / 3habitable 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

Orientation matters a lot here: 135.1 ACH with wind from S versus 35.0 ACH from SW. If the site has a prevailing wind, set Wind From to it for the key study.
For wind from NE, E, SW 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.
Cross-check: for wind from SE, S, W, NW the simulated fresh air is above the textbook orifice-flow estimate (SE 1.60×, S 1.46×, W 1.32×, NW 1.34×), usually an inlet jet crossing the house straight to an outlet. The verdict already uses the lower, textbook figure for those directions.
Possible draughts: mean air speed above 0.8 m/s in R1, R3 for some directions. Partly closing the inlet windows in windy weather would reduce it.

3. Rooms: air changes per hour by wind direction

Room	N	NE	E	SE	S	SW	W	NW	Worst, dm³/s per m²	Result
R1 main room · 51.9 m²	75.7	10.5	20.4	115.5	151.4	25.4	153.6	115.5	7.7 ≥ 6	Pass
R2 room · 22.5 m²	87.2	135.6	180.3	68.9	174.3	135.6	91.7	68.9	50.8 ≥ 6	Pass
R3 room · 12.6 m²	154.8	242.5	322.4	39.6	309.6	242.5	54.3	40.8	29.2 ≥ 6	Pass

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 × 2.65 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°)	67.6 ±12%	74.0	4738	5388	12%	0%
NE (45°)	35.0 ±26%	35.8	2293	3092	26%	1%	in/out mismatch over 25% (steady): wider band
E (90°)	46.6 ±36%	48.2	3087	4808	36%	1%	in/out mismatch over 25% (steady): wider band
SE (135°)	69.0 ±24%	110.6	7082	5411	24%	0%
S (180°)	135.1 ±18%	197.7	12662	10367	18%	0%
SW (225°)	35.0 ±39%	52.2	3342	2049	39%	0%	in/out mismatch over 25% (steady): wider band
W (270°)	91.7 ±10%	121.3	7766	8616	10%	0%
NW (315°)	69.0 ±10%	92.5	5922	6375	7%	0%

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	4738	4327 (3606–5048)	4783	1.09 (0.73–1.31)	Agrees
NE	2293	2244 (1870–2618)	3248	1.02 (0.62–1.47)	Agrees
E	3087	2984 (2487–3481)	4318	1.03 (0.54–1.58)	Agrees
SE	7082	4416 (3680–5152)	4547	1.60 (0.64–1.44)	Simulation higher
S	12662	8654 (7211–10096)	9566	1.46 (0.68–1.38)	Simulation higher
SW	3342	2244 (1870–2618)	3248	1.49 (0.51–1.62)	Agrees
W	7766	5871 (4893–6850)	6045	1.32 (0.75–1.28)	Simulation higher
NW	5922	4416 (3680–5152)	4547	1.34 (0.75–1.28)	Simulation higher

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

Room ACH: simulated / network	N	NE	E	SE	S	SW	W	NW
R1	97.9 / 75.7	10.5 / 25.4	20.4 / 33.7	163.4 / 115.5	220.7 / 151.4	34.4 / 25.4	203.1 / 153.6	154.9 / 115.5
R2	118.8 / 87.2	138.5 / 135.6	186.6 / 180.3	85.8 / 68.9	273.8 / 174.3	190.8 / 135.6	155.1 / 91.7	111.9 / 68.9
R3	210.7 / 154.8	247.7 / 242.5	333.5 / 322.4	39.6 / 40.8	417.6 / 309.6	301.1 / 242.5	130.9 / 54.3	89.0 / 40.8

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

Id	Type	Position	Setting	N	NE	E	SE	S	SW	W	NW
W1	window 2.00 m	S facade	Auto	1874 in	165 out	439 out	3210 in	4833 in	491 in	336 out	324 out
W2	window 2.00 m	S facade	Auto	1528 in	171 out	454 out	2391 in	4088 in	705 in	6092 out	4471 out
W3	window 2.00 m	S facade	Auto	1335 in	2483 out	3458 out	1052 in	3741 in	1752 in	1134 out	885 out
W4	window 2.00 m	W facade	Auto	3778 out	274 out	457 out	5411 out	7541 out	394 in	7766 in	5922 in
W5	window 1.40 m	E facade	Auto	1609 out	2293 in	3087 in	429 in	2826 out	2049 out	1053 out	694 out
D1	door 0.90 m	interior	Open	303	192	209	1483	416	1039	2630	1865
D2	door 0.80 m	interior	Open	2292	3175	4220	304	4905	3525	1370	954

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: 67.6 ACH

Air speed, wind from NE — Wind from NE: 35.0 ACH

Air speed, wind from E — Wind from E: 46.6 ACH

Air speed, wind from SE — Wind from SE: 69.0 ACH

Air speed, wind from S — Wind from S: 135.1 ACH

Air speed, wind from SW — Wind from SW: 35.0 ACH

Air speed, wind from W — Wind from W: 91.7 ACH

Air speed, wind from NW — Wind from NW: 69.0 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, 17.8 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 three flow-through times (Study Length "Long") and then sampled 8 times. The flow through each opening was integrated on the faces between the indoor air and the opening.

Wind: 4.10 m/s, given as the speed at the building (no terrain correction applied).
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 2.65 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 ±39%). 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.