The chief challenge is that of retrofits vs. new construction. Much of California's school infrastructure is 50--100 years old.
In general, newer buildings with sealed windows have pretty good air exchange (with heat recovery) and filtration.
> For much of the state, through much of the year, open windows are a reasonably minor consideration.
Yah, tell that to me languishing masked a 95 degree classroom between building heat gain and a temperature in the 90s, and now ending up with a 60 degree room. It's workable, but it sucks, and we're wasting a lot of energy just to maintain a few degree delta T from outside with open windows. We've had a few weeks where the open windows have been great, but a lot more crummy weeks.
I'm a little jealous of those upstairs in the high school building that don't need to open windows.... and have nice temperatures in their classrooms... and have also had 0 secondary COVID cases. It's also really hard to conduct class when there's music, PE, etc, activities outside.
> The chief challenge is that of retrofits vs. new construction.
I thought we were talking in the context of a new school building being built without windows that open.
That said, windows are nice. If you have a class full of kids that are restless, sometimes opening windows and getting some outside noise or a breeze through the room changes the entire mood. It would be sad to lose that.
(Keep in mind that mixing air also mixes infectious particles.)
A slight improvement is to have a single exit point (vented with a fan) and rely on structure permiability to introduce fresh air. That's effectively a negative-pressure system.
From there, exhaust and intake with heat recovery is the next obvious step.
I mention older construction as replacing building stock is a long-term and expensive process. New construction will account for an exceedingly small percentage of all utilised buildings. New construction currently planned or in process should if at all possible incorporate lessons from the COVID-19 pandemic, but that's a drop in the bucket. Retrofits are highly likely.
If you want to read further, I'm finding numerous results for a search on school ventilation in new construction and retrofits:
https://duckduckgo.com/?q=+school+retrofit+construction+vent...
Among the first of those, from the CDC:
Reoccupying a building during the COVID-19 pandemic should not, in most cases, require new building ventilation systems. However, ventilation system upgrades or improvements can increase the delivery of clean air and dilute potential contaminants. Consult experienced heating, ventilation, and air conditioning (HVAC) professionals when considering changes to HVAC systems and equipment. Buildings that provided healthy, code-compliant indoor air quality prior to the pandemic can be improved for pandemic occupancy using less costly interventions. Below is a list of ventilation interventions that can help reduce the concentration of virus particles in the air. They represent a list of “tools in the mitigation toolbox,” each of which can contribute towards a reduction in risk. Implementing multiple tools at the same time is consistent with CDC’s layered approach and will increase overall effectiveness of ventilation interventions. These ventilation interventions can reduce the risk of exposure to the virus and reduce the spread of disease, but they will not eliminate risk completely.
...
Open windows and doors, when weather conditions allow, to increase outdoor air flow. Do not open windows and doors if doing so poses a safety or health risk (e.g., risk of falling, triggering asthma symptoms) to occupants in the building. Even a slightly open window can introduce beneficial outdoor air.
https://www.cdc.gov/coronavirus/2019-ncov/community/ventilat...
That cites an industry study by ASHRAE (professional association):
https://www.ashrae.org/file%20library/technical%20resources/...
It also recommends on exhaust fans rather than interior circulation fans to minimise strong internal air currents, with intake provide as I've indicated above.
During a pandemic, it makes a lot of sense-- at least for the "extremes" of weather faced here.
> A window cracked open for ventilation, and either at the top of the classroom (warm weather w/ cooling active) or bottom (with heating), and a fan within the room for vertical mixing ... might help.
Might, but who knows how much? How does one weight the ordinary risk of sickness and infection (outside of a pandemic) with energy costs? (Especially when there are less energy-intensive ways to get the ventilation).
> A slight improvement is to have a single exit point (vented with a fan) and rely on structure permiability to introduce fresh air. That's effectively a negative-pressure system.
> From there, exhaust and intake with heat recovery is the next obvious step.
Yes, and pretty quickly from here you're ending up designing a structure without windows that open, so you can ensure that your fancy ventilation system does the right thing, filters don't get fouled, and you're not wasting heat.
> I mention older construction as replacing building stock is a long-term and expensive process.
Well, sure. But we were talking about it in the context of someone complaining a new building doesn't have windows. Older, smaller, poorly sealed buildings are a lot less likely to be retrofitted with no windows than new construction.
> It also recommends on exhaust fans rather than interior circulation fans to minimise strong internal air currents, with intake provide as I've indicated above.
Well, duh.
An otherwise well-sealed building, with a porous wall structure held at a small negative prssure, is effectively a very simple heat recovery ventilation system, too.
Good modern buildings are being built with more ventilation and heat recovery built in. Taking this over windows that open can be a reasonable trade. Yes, on the pandemic axis, many of these buildings may be a little worse than a classroom with a whole long wall of opening windows.