This crawlspace floor grate/vent is located in the furnace/water heater closet of my rambler house outside of Seattle, WA, built ~1959. The closet is covered by two bifold doors, so not airtight, there is also a vent in the ceiling of the closet that leads to the roof. My neighbor has the same setup, so I know this is intentional and original to the house design. My neighbor claims it is for ventilation, drawing 'fresh' air in from the outside. I can confirm I occasionally feel airflow coming into the house. However all my google-fu can't find anything similar in description or purpose.
Tried doing some research here, two related posts I found here on DIY:
- This post also has an indoor crawlspace grate. Like my neighbor, the landlord in that post claims it is for ventilation. Unlike that post, I do have a central furnace and my grate is original to the house.
- This post has a duct that is directly heating the crawlspace, perhaps this is a passive way of doing the same? Seem like a huge heatsink though (not seen in practice).
I'm fairly certain this is also how mice found their way into my house. I am considering putting in 1/4" hardware mesh in from the crawlspace side to keep them out however not knowing the purpose I'm a little hesitant to slow down airflow. What is this for?