Wind and infiltration: how the outdoors gets in

No building is air-tight. Even a well-sealed home has a continuous low-rate exchange of indoor and outdoor air through the building envelope, door seals, electrical penetrations, window frames, HVAC duct boots, chimney. This passive air exchange is called infiltration. The rate is driven primarily by wind and by indoor-outdoor temperature differences (the stack effect).

Infiltration varies a lot from house to house and from day to day. Nazaroff's review of residential air-change rates, which pools about 40 studies of roughly 10,000 homes, puts the typical rate at about 0.5 air changes per hour (ACH), with most study averages between 0.2 and 1 ACH, and names wind speed and the indoor-outdoor temperature difference among the main drivers. For comparison, the ASHRAE 62.2 whole-house ventilation rate works out to roughly 0.35 ACH for a typical house, so on a windy day infiltration alone can exceed it, while on a calm, mild day it can fall well short.

For interpretation: wind affects all the outdoor-cross-reference parameters Terrestream tracks. On a windy day, outdoor PM_2.5, NO_x, SO₂, ozone, and pollen all infiltrate your home at higher rates. Terrestream reads outdoor wind from Open-Meteo: strong wind or gusts switch its window guidance to keeping windows closed, windblown dust outside triggers a keep-windows-closed notice, and wind speed and direction help it estimate when a smoke plume will arrive. It does not estimate how much extra outdoor air the wind pushes into your home. A wildfire-smoke day with 30 km/h winds is meaningfully worse for indoor air than the same outdoor PM_2.5 level on a still day.

Practical implication: during outdoor air events (smoke, high pollen, ozone), tightening the building envelope helps, closing windows, sealing obvious gaps, switching HVAC to recirculate. During favorable outdoor air conditions, mild infiltration provides "free" ventilation that lowers indoor CO₂ and VOCs without any energy cost.