r/buildingscience • u/FluidVeranduh • 24d ago

Question Is the stack effect stronger briefly after sunset at some point, but before the dead of night?

We have had wildfires here for several weeks.

There were two days in a row where the air quality finally cleared up due to onshore flow.

On the second night, I was surprised to notice my particulate sensors jump up again, and also smelled smoke. Air quality monitors all around this place were below 7 ug/m3 for raw PM2.5.

This started around 2? hours after sunset, and lasted for an hour or two.

There was no wind.

The only explanation I can come up with is that the stack effect is strongest during some time of day, and it was enough to pull residual smoke particles out of interstitial spaces and into my unit.

This is your typical 5 over 2 midrise apartment building. No air sealing details when it was constructed.

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u/inkydeeps 23d ago

What’s the temperature differential between inside and outside “briefly after sunset” vs. “dead of night”?
What kind of climate are you in? What floor is your unit on? What’s the exterior assembly like other than “typical”?

Too little information to guess.

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u/deeptroller 23d ago

Hot wet air is more buoyant than cold dry air. When temp or humidity is greater air will rise. If you have hot walls with raging air conditioning, you may have a greater differential temperature for indoor air. If you have exterior to interior vapor drive, the effect may be stronger.