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Can we just use the night air?
Five kinds of building crossed with four situations. Where overnight cooling works, where it does not, what it costs to do properly, and the one condition under which everything here reverses.
Three things that apply to every answer below
- This valley genuinely does cool off, and that is the whole opportunity
Inland Solano County has hot dry afternoons and evenings that drop away properly as cooler air moves in off the water. Most of California does not get that, and most contractors here quote as though it did not exist. A building that can take in a large volume of cool air for six hours has done a real share of the next day’s work for nothing. Whether yours can is a question about the building, not about the weather.
- One number decides it: what the inside temperature is at seven in the morning
If the house has shed its heat overnight, it starts the day level and the system only has to handle the afternoon. If it is still warm at seven, it is carrying yesterday forward and every day compounds. That number is invisible without a logger and it is cheap to get. We will leave one for a week before we recommend anything, because it changes the recommendation often enough to matter.
- None of this replaces a system on a genuinely hot week
We are not going to sell you a fan and call it air conditioning. When the overnight low stays high for several days — which happens here — night cooling has nothing cool to bring in and the building needs mechanical cooling. The honest answer for most houses is both, used in the right order, and the reason to get the order right is that it is the cheaper half doing most of the work.
Work out whether your building can do this
Pick the building and pick the situation. Every answer below is on this page whether you pick anything or not — the picker only narrows it down.
This is the one condition that inverts the whole strategy, and it is worth being blunt about. During a wildfire smoke event you keep the building shut, you do not run a whole-house fan, and you do not open up overnight. A whole-house fan moves an enormous volume of outside air through the building in a short time — which is exactly what makes it useful in July and exactly what makes it the wrong thing to switch on when the outside air is full of smoke. Close the windows, run the air conditioning on recirculate if you have it, and use the best filter the system can take without losing the airflow it needs, which is a figure we will give you from the static pressure measurement. What we will not do is tell you when it is safe to open up again. The official air quality reading is the answer, it is free, it is updated constantly, and it is a great deal more reliable than any sensor in a house or any opinion from a contractor. We measure air; we do not assess it, and we make no health claims from anything we measure. Check the official number, and if somebody in the building is unwell, that is a question for their doctor and not for us.
The best case. Log a week of temperatures and then decide.
A single-storey house with an attic above it is the ideal candidate, because the whole ceiling is available as an exhaust path and there is no upstairs to push the heat into. If the logger shows the house shedding its heat overnight already, a modest improvement will do a great deal. If it does not, the attic is usually the reason.
What actually makes it work in a single-storey house
- A big enough exhaust path. A whole-house fan moves air out through the attic, and the attic has to be able to let that air escape. Undersized attic venting is the commonest reason a fan underperforms, and it is cheaper to fix than the fan is to buy.
- Enough open window area, in the right rooms. The air has to come in somewhere. Opening two windows at the far end of the house and closing the rest is how you cool the rooms that matter rather than the hallway.
- Attic insulation that is actually there. A hot attic radiates down all night and undoes the work. This is worth checking before anything is bought.
- A schedule. Shut the house at nine in the morning, open it when the outside temperature drops below the inside one. That crossover is a real time of day and a logger tells you when it is.
Done properly, in this climate, a single-storey house is the case where somebody most often ends up with a smaller system than they expected to buy.
Two storeys makes the physics work for you. The question is the exhaust path at the top.
Warm air rises, so a two-storey house has a natural stack — cool air in at the bottom, warm air out at the top. What it needs is somewhere for that warm air to go, and in most tract houses the only route is the attic, reached through a landing ceiling.
The upstairs problem, and how night air changes it
The reason upstairs is four or five degrees warmer every afternoon is that it gains heat through the roof all day while the downstairs does not, and one system is trying to serve both. Overnight ventilation attacks that from the other end: if the upstairs and the attic above it are properly flushed at night, the following afternoon starts from a lower base and the system spends less of the day losing.
What it needs: a fan or ventilator sited on the upper landing rather than downstairs; open windows downstairs to supply it; enough attic venting to take what the fan moves; and, genuinely important on a two-storey house, somebody willing to shut it all at the right time in the morning, because an open upstairs window at noon undoes the entire night.
What it does not fix: a duct system that was never balanced between the floors, or attic ductwork losing its cooling before it arrives. Those are measured separately and they are frequently the larger item.
A tight house holds yesterday. Night air only works here if you can move a lot of it, quickly.
Newer well-sealed houses are better at almost everything and worse at this. Sealing is what stops heat getting in, and it is also what stops cool air getting in at night unless something is deliberately moving it. Open windows alone frequently will not do it, because there is no leakage path to drive a flow.
Why the tight house is the awkward one
Three things are different. Thermal mass with no leakage means the structure holds its temperature, which is wonderful in the afternoon and unhelpful at eleven at night. The house may already have mechanical ventilation, which is designed to move small volumes continuously and is not a cooling device. And the attic may be conditioned or the ducts inside the envelope, which changes what a whole-house fan would even be exhausting into.
Where a tight house is a good candidate, the answer is a powered ventilator sized against the volume, with a controlled intake, rather than a general invitation to open the windows. Where it is not, we will say so, and the honest recommendation is usually shading, attic and roof treatment, and a correctly sized system.
One more thing worth knowing: on a tight house the logger usually shows a very flat line. That flatness is not a fault. It means the building is doing its job, and it also means the overnight opportunity is smaller than the neighbours’ is.
In a flat you probably cannot do this properly, and it is better to know before anybody sells you a fan.
Overnight cooling needs air in at one point and out at another, and most flats have openings on only one side. Without a second opening there is no path, and a fan in a single-aspect flat simply makes noise. This is a building limitation rather than anything you have done wrong.
What can be done instead
If your flat has openings on two sides, you can do a version of this, and a couple of well-placed fans moving air from the cool side to the warm one at night is worth more than a single fan pointed at a person. Establish which way the air wants to go — it is usually obvious with a door propped and a candle — and help it rather than fight it.
If it does not, the honest levers are: solar control on the glass, which in a west-facing flat is the single largest one; ceiling or pedestal fans, which cool people rather than rooms and are worth several degrees of comfort for very little; and making sure the cooling you do have is working, correctly charged and not fighting a blocked filter.
And the thing we will not do: propose alterations to the building. Windows, vents and anything through an external wall belong to the owner or the association, and a resident cannot authorise them. We will tell you what would work and you can take it to them.
In a working building the question is not comfort. It is what the temperature is at seven in the morning.
A warehouse or workshop that starts the day at eighty is not going to be comfortable at any point, whatever is on the roof. Purging that building mechanically overnight is one of the cheapest interventions available, and almost nobody does it because nobody measures the morning number.
Night purge, in practice
What it is: running the building’s ventilation, or dedicated fans, through the cool hours to flush the accumulated heat out of the structure and the contents, so the day starts from a lower base. On a tall building with a lot of mass this is a genuinely large effect, and it costs the electricity to run fans rather than compressors.
What it needs: an intake and an exhaust that are actually sized for it; controls that run on the difference between inside and outside temperature rather than on a clock; and somebody to have checked that security, weather and dust are all dealt with. A roof ventilator that opens at two in the morning has to close at six.
Where it does not work: buildings with a process that keeps running overnight, buildings whose contents must not be exposed to outside humidity, and buildings on a site where the night air is dusty. All three are common and all three are reasons we would tell you not to.
And the honest caveat: in a heat wave where the overnight low stays high, a night purge has nothing cool to bring in. It is a large saving most of the summer and it is not a substitute for capacity in the worst week.
Nothing in particular
Then it is a straightforward building question and the logger answers it. Get a week of indoor and outdoor temperatures, look at the morning number, and decide from there.
Two things worth doing whatever the answer turns out to be, because they help in both directions. Attic insulation and duct sealing, because a hot attic undoes overnight cooling and wastes mechanical cooling equally. And solar control on west-facing glass, which is the most under-used lever in this valley and is a one-off cost with no running cost at all.
Then, if the building is a good candidate, ventilation. And if it is not, a correctly sized system rather than an oversized one — which, given what we usually measure here, may well be smaller than what is in there now.
Somebody in the building reacts to pollen
Then the honest position is that open-window ventilation brings in whatever is outside, including pollen, and no amount of arrangement changes that. A whole-house fan pulls outside air through the building unfiltered by design.
What can be done mechanically: a powered ventilator with a filter on the intake rather than open windows, which gives you a controlled and filtered flow instead of an uncontrolled one; timing the ventilation to the hours when counts are typically lower, which in most places means late night rather than early morning; and improving the filtration on the cooling system itself — within the airflow the system can afford, which we measure rather than guess.
What we will not do is tell you whether any of it will help the person. We can tell you what we measured and what a filter will and will not capture. Whether it makes a difference to somebody’s symptoms is a medical question and it belongs to their doctor.
Open windows at night are not an option
A completely reasonable position and a common one, and it rules out the free version of this rather than the whole idea.
What still works: ventilation through openings that are not accessible — high windows, skylights, roof ventilators and dedicated ducted intakes. A powered ventilator with a fixed, secured intake gives you the same flow without anything left open at ground level. Security grilles or restrictors on ground-floor openings are another route and are a locksmith’s or a glazier’s job rather than ours.
What we would suggest looking at first: whether the upper floor alone can be ventilated safely. In a two-storey house, an upstairs opening that cannot be reached plus a powered exhaust does most of the work, because the upstairs is where the heat is anyway.
And if none of that suits the building, that is a real answer. The recommendation becomes a correctly sized system, shading, and attic work, which is a perfectly good outcome.
It is smoke season
Read the note above, because it is the substance: the strategy reverses and the building stays shut.
What is worth doing before the season rather than during it: find out what filter your system can take without losing the airflow it needs, which comes from a static pressure measurement and not from the box; check that the return side of the duct system is not leaking, because a leaking return pulls in unfiltered air from an attic or crawl space and defeats the filter entirely; and make sure the system will actually run on recirculation with the outside air path closed, which on some commercial equipment it will not without a change to the controls.
Those three are the useful preparation, they are cheap, and they are far more effective than anything bought in a hurry on a bad air day.
Where this page stops
We will log the building for a week, tell you the morning number, and give you two costed options: what the equipment needs and what the building could do instead. Where ventilation is the better answer we will size and install it properly. Where it is not, we will say so and quote a correctly sized system.
We will not put a percentage saving on a quote, sell a fan as a substitute for cooling, assess air quality, make any health claim, or tell you when it is safe to open the house on a smoke day. That last one is the official reading’s job and it is free.