Design studyNatural ventilation

The house that only cools when the wind blows.

A heavily glazed house was designed on the assumption that natural wind would ventilate it. It has two openable windows and they are at the same height. In a 3 m/s breeze that gives 13 air changes an hour; in still air it gives 0.73, and the mezzanine reaches 35.7 °C with the outdoor air at 24 °C. Roof vents remove the dependence on the weather, and shading the glass reaches a similar place on a fraction of the air flow.

Section through the house showing the mezzanine, the sunlit facade, roof and floor, and both openable windows at 1.5 m — The house in section. Both openable windows sit at 1.5 m, so there is no height difference between them for warm air to exploit.

0.73 ACH

STILL AIR, AS BUILT

35.7 °C

MEZZANINE · 24 °C OUTDOORS

−5.8 K

MEZZANINE, FOUR ROOF VENTS

CONFIGURATIONS · 18 SOLUTIONS

THE SYMPTOM

The design intent was natural ventilation. It contains no statement about what happens when the wind stops.

A glazed, open-plan house with a mezzanine over the back half, of the kind that is comfortable on some days and unbearable on others, with the mezzanine always worse than the ground floor. The argument for it is not wrong: when the wind blows it is right, and the building performs.

The house is extensively glazed, but as is usual almost all of that glazing is fixed. Only two windows open, and they are at the same height. That single fact is the whole fault. It leaves the building with one driver, and it is the one nobody controls.

Volume	389 m³	9 × 7.2 × 6 m, mezzanine at 3 m
Openable area	1.44 m²	two windows, both at 1.5 m
Outdoor air	24 °C	a warm day
Solar load	glazing 50 °C	roof 35 °C, floor 32 °C
Design wind	3 m/s	Cp +0.70 windward, −0.30 leeward, −0.60 roof

WHAT THE WIND WAS DOING FOR THE DESIGN

Halving a light breeze does not halve the cooling. It removes most of it.

Ventilation is not imposed here. The openings are held at the static pressure the wind puts on them, and how much air actually moves is an outcome. Between 1.5 and 0.75 m/s the ventilation rate falls from 5.7 to 1.2 ACH and the mezzanine gains 2.9 K. Below about 1 m/s the wind pressures fall to the same order as the buoyancy this house cannot exploit, and neither driver has much left.

Wind · m/s	As built · ACH	Mezz · °C	Ground · °C	ΔT floors · K	Vented · ACH	Vented mezz · °C
0.00	0.73	35.7	31.3	4.39	3.59	29.9
0.75	1.18	34.9	30.5	4.46	4.75	29.0
1.50	5.67	32.0	26.8	5.24	8.72	27.2
3.00	12.79	29.7	26.1	3.59	19.49	26.6

Ventilation rate against wind speed for the as-built and vented house — Ventilation rate against wind speed. The as-built house is almost entirely wind-driven: take the wind away and the ventilation goes with it. The vented house keeps working, because it has a second driver that does not depend on the weather.

The second thing in that table is easier to miss and harder to fix. Stratification is 4.4 K in still air and 5.2 K at 1.5 m/s: cross-ventilation makes the split between the floors worse before it makes it better, because it flushes the storey its windows are on and leaves the one above untouched. At 13 air changes an hour the ground floor is comfortable at 26.1 °C while the mezzanine is still at 29.7. More wind does not settle the argument.

STILL AIR

The height matters more than the hole.

With no wind, the only thing left to move air is buoyancy. Warm air rises and will leave through any opening above the level it entered, and the as-built house has no such opening. Both windows are at 1.5 m, so what is left is each window exchanging air with itself: warm air out of the top, cooler air in at the bottom. That carries 0.73 air changes an hour against the 13 the same house achieves in a breeze.

Roof vents do not fix this by area alone. Adding the same 1.44 m² at window height would do almost nothing. They fix it by putting an opening 4.5 m above the windows, which gives buoyancy something to work against. In still air the vented house reaches a mezzanine at 29.9 °C, very nearly what the as-built house achieves on its best day, in a full breeze. The vents do not make the house better than it has ever been. They make its worst day as good as its best one.

Air temperature on a section through the room in still air, as built beside the same room with roof vents — Air temperature on a section through the room, in still air. Left: as built. Heat from the glazing and the floor collects under the roof and floods the mezzanine. Right: with roof vents, the same heat leaves through the top of the room instead of accumulating in it.

Horizontally averaged air temperature against height for each vent area — Horizontally averaged air temperature against height, in still air. The as-built profile is the one that climbs; adding vent area both lowers it and flattens it.

HOW MUCH VENT IS ENOUGH

The first pair does most of it.

All in still air, which is the condition the design has to survive. Returns flatten once the vent area approaches the window area: openings in series combine as 1/A² = 1/A²in + 1/A²out, so beyond that point the fixed 1.44 m² of window limits the flow, not the roof.

Vents	Area · m²	ACH	Mezz · °C	Ground · °C	ΔT floors · K	% out through roof
0	0.00	0.73 ±0.07	35.7	31.3	4.39	0
2	0.72	2.51 ±0.66	31.3	27.9	3.45	99
4	1.44	3.59 ±0.54	29.9	26.9	3.06	100
6	2.16	5.04 ±0.45	29.2	26.5	2.64	100

Ventilation and occupied temperature against roof vent area in still air — Ventilation and occupied temperature against roof vent area, in still air. The ground floor was never the problem; the mezzanine is where the vents do their work.

THE OTHER LEVER

Stop the heat arriving, and you need a sixth of the air.

External shading takes the sunlit glazing from 50 to 32 °C and the sunlit slab from 32 to 26 °C, and changes nothing else about the building. Vents alone give a mezzanine at 29.94 °C; shading alone gives 30.62. Ventilation is ahead by 0.68 K, and the grid convergence index on occupied temperature is 0.58 K, so the two levers are not separable at this level of numerical accuracy. What can be said is the striking part: shading reaches the same place on six times less air.

Still air	ACH	Ground · °C	Mezzanine · °C	ΔT floors · K	kW removed by the air
as glazed	0.73	31.3	35.7	4.39	0.83
as glazed + vents	3.59	26.9	29.9	3.06	1.78
shaded	0.57	26.7	30.6	3.91	0.29
shaded + vents	2.13	25.1	26.9	1.73	0.46

The four still-air arrangements plotted against the as-built house on a windy day — The four still-air arrangements, and the as-built house on a windy day for scale. The best result is not the one with the most ventilation; it is down and to the left, at less than a fifth of the windy-day air change rate.

How much of the mezzanine sits above a given temperature. The threshold is a judgement rather than a measurement, so the whole curve is shown instead of one number.

Does it matter where the vents go? Barely.

Moving all four vents from over the double-height void to over the mezzanine changes ventilation from 3.59 to 4.13 ACH and the mezzanine from 29.9 to 29.88 °C, differences sitting right at the edge of the settled band on either case. That is worth knowing precisely because it is a negative result. What makes a roof vent work is the height difference between it and the inlet, and both positions have essentially the same one. Plan position is a detail; section position is the design.

LIMITATIONS

Stated rather than buried.

Solar gain is imposed as surface temperatures, not computed. No radiation model.

Steady solver on an unsteady flow. Ventilation is over-predicted by 42 to 54%.

Read an absolute air change rate as good to about a factor of two.

One weather condition and one occupancy. No internal gains, no night purge.

BRING US THE DECISION

Before the glazing is ordered.

If a building is being asked to cool itself, the question is what it does on the day the wind stops. Send us the section and the climate, and we will tell you which lever actually moves the answer.

TALK TO AN ENGINEER

OpenFOAM v2512, buoyantBoussinesqSimpleFoam, steady, Boussinesq buoyancy, k–ω SST with wall functions. Hexahedral mesh from blockMesh; mezzanine as a zero-thickness baffle; openings cut on existing face boundaries, held at wind static pressure via totalPressure on p_rgh. Solar gain imposed as raised surface temperatures, no radiation model. 4,000 iterations, settled band taken over the last 1,000. Basis: a representative house, not a named client.