Cooling · Ventilation strategy
Move the stored heat out at night, and the day starts cooler.
Most British houses overheat because the fabric absorbs solar and internal gains all day and has nowhere to release them at night. A deliberate ventilation strategy is the difference between waking up in 26° and waking up in 21°, with no equipment involved.
July afternoon, house closed against direct sun
During the day the house is closed against direct sun. The shutters do the shading; the ventilation strategy waits for dusk.
Night purge is the free cooling most homes never use.
Between roughly 10 p.m. and 6 a.m. from May to September, outside air in most of the UK is 10 to 15° cooler than a west-facing bedroom that has baked all afternoon. Moving that cool air through the house dumps the day's heat back into the sky and resets the fabric before morning.
The trick is airflow, not volume. A single tilted window in a hot bedroom achieves almost nothing; the warm air stratifies and stays put. Open a low window on the cool side of the house and a high window on the warm side, and you get measurable cross-flow within minutes.
Same night, one tilted bedroom window
A single tilted bedroom window. The outside air is now cooler than the bedroom by eight degrees, but nothing is moving; the warm air sits at ceiling height and stays there.
Same night, cross-flow path opened; fabric resetting
Open a low window on the cool elevation and a high window on the warm elevation; the same house resets by dawn.
The three ventilation modes; where each one belongs.
Cross ventilation
- Strength
- Strongest of the three; a well-oriented Victorian terrace can shift its full internal air volume every eight to ten minutes on a mild night.
- Trade-off
- Needs openings on opposite elevations; single-aspect flats cannot do it.
- Best for
- Semis, terraces and detached houses with front-to-back permeability.
Stack effect
- Strength
- Works in still weather and in landlocked flats where cross ventilation is impossible; warm air rises and escapes through a landing window or roof light while cool air draws in low down.
- Trade-off
- Needs a real height difference between the two openings; a single-storey flat has almost no stack to work with.
- Best for
- Terraces, town houses and any home with a stairwell or landing that reaches an openable roof light.
Single-sided
- Strength
- Works for small rooms with tall openings; a Victorian sash on one wall is not nothing.
- Trade-off
- Does almost nothing for a large open-plan space; single-sided kitchen-diners are a well-known failure mode.
- Best for
- Small rooms with a single external elevation; usually the fallback, not the plan.
When mechanical ventilation earns its keep.
MVHR with a summer bypass detects when outside air is cooler than inside and shunts around the heat exchanger, delivering filtered, unheated air straight into bedrooms overnight. The best summer feature in any airtight home; almost universally under-used because the installer never explained it.
Whole-house extract fans on a boost cycle can achieve similar effects in older houses, particularly for pulling hot air out of the top of the stack. Not silent, but far quieter than a portable AC unit and running at a fraction of the power.
In noise-sensitive locations, mechanical night purge is often the only workable option; you cannot leave windows open on a road that carries HGVs at 4 a.m.
Verdict
Ventilation strategy, in order.
Close during the day, purge across the top of the house at night, and use mechanical assist only where noise or pollution rule out open windows; done well, this alone shifts wake-up temperatures by four to six degrees on a summer morning.
As outside temperature drops below inside temperature (usually 8 to 9 p.m. in July), open the ventilation path. Cross-flow first, stack second, single-sided last. Sleep with the path open and internal doors ajar or hooked. Aim to have the fabric back to under 22° by dawn. If it is not, you either need more shading upstream or a mechanical assist.
What it gives you
- Free; the house does the work while you sleep.
- Resets the fabric so the next day starts cooler by three to five degrees.
- Pairs with every other cooling intervention; halves the load on any active kit.
What it costs you
- Cannot beat physics; a truly tropical night has nothing cooler to draw in.
- Requires a real airflow path; some flats and dormer conversions simply do not have one.
- Noise-sensitive locations need mechanical intake and acoustic vents.
Why we think thisOpenClose
Reasoning
The 10 to 15° night-to-day differential comes from Met Office 1991 to 2020 climate averages for London, Bristol and Manchester in July; the differential is smallest during the two or three heatwave nights each year, which is exactly when purge falters. UKCP18 projections push those nights up in count.
Cross-flow, stack and single-sided effectiveness ratings come from CIBSE Applications Manual AM10 (Natural ventilation in non-domestic buildings), which is the industry reference used by mechanical engineers on domestic overheating cases.
MVHR summer bypass under-commissioning is a documented finding of the AECB and Passivhaus Trust post-occupancy surveys; roughly a quarter of installed MVHR units in domestic contexts have the bypass either disabled or misconfigured.
Assumptions
- The house has a workable airflow path; single-aspect flats are the notable exception.
- The occupants are prepared to open windows overnight; secure restrictors are a prerequisite in ground-floor rooms.
- MVHR guidance assumes a modern installation with a functioning summer bypass valve; some older units do not have one.
Sources
If this were our house
If this were our house, we would open a proper cross-flow path across the top of the house every summer night, before we bought any equipment at all.
- 1Fit lockable window restrictors upstairs so sashes can stay open safely overnight; £15 to £40 per window.
- 2Open a low window on the cool elevation and a high window on the warm elevation from dusk to dawn.
- 3If the house has MVHR, book a commissioning visit specifically to confirm the summer bypass is enabled and set to the right threshold.
- 4For a busy road frontage, fit acoustic trickle vents on the noise-facing rooms and treat mechanical assist as the primary strategy, not the backup.
This strategy assumes the house has an airflow path to open; a single-aspect flat or a mid-terrace with sealed party walls needs a different play, usually mechanical.