Short answer:
In the Pacific Northwest, crawlspace vents should almost always be closed in winter.
Here’s why — and how to know if your home is the exception to have them open.
Why Crawlspace Vents Matter in Winter
Many homeowners still remember the old rule: “Open your vents to let moisture out.”
But in the PNW, where winters are wet, cold, and humid for months at a time, that logic no longer applies.
Cold, damp outdoor air rushing into your crawlspace does more harm than good, leading to:
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Higher moisture levels
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Condensation on framing
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Insulation damage
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Mold growth
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Higher heating bills
Closing vents helps keep the crawlspace drier, warmer, and more energy-efficient.
PNW Logic: Humidity + Temperature Swings = Moisture Problems
Here’s what makes our region unique:
1. Humidity stays high all winter
Even when it’s cold, PNW air can hold a lot of moisture. When that humid air enters a cooler crawlspace, it condenses onto wood and insulation.
2. Temperature swings push moisture into the crawlspace
A warm home + a cold crawlspace = condensation risk.
Outdoor air entering through open vents makes that difference even more extreme.
3. Rain and groundwater rise
Open vents allow damp air and wind-driven moisture to move straight inside.
Bottom line:
Open vents = a wetter crawlspace in winter, not a drier one.
So… Should Crawlspace Vents Be Open or Closed in Winter?
CLOSE THEM.
For the majority of PNW homes, closing vents in winter is the safest, most energy-efficient choice.
This helps:
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Reduce moisture levels
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Protect insulation from sagging
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Prevent mold
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Lower heating costs
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Keep floors warmer
The Only Exception: Unconditioned, Very Dry Crawlspaces
A small number of older homes with exceptionally dry soils and excellent drainage may operate fine with traditional venting.
But these cases are rare—and still benefit from modern sealing and moisture control.
Quick Visual: Open vs. Closed Vents in Winter
| Vents OPEN (Winter) | Vents CLOSED (Winter) |
|---|---|
| Humid air enters crawlspace | Blocks damp outside air |
| Cold floors above | Warmer floors above |
| Higher heating bills | Better energy efficiency |
| Greater risk of mold | Moisture stays controlled |
| Insulation freezes or sags | Insulation stays effective |
Quick 10-Second Quiz: What Type of Crawlspace Do You Have?
Choose the option that sounds like your home:
1. “My crawlspace has insulation, a vapor barrier, and no standing water.”
Close your vents in winter.
2. “I’ve seen moisture, musty smells, or rodents before.”
Closing vents helps, but you likely need a moisture inspection.
3. “My crawlspace is bare dirt and very open.”
You may have an older airflow-dependent design, but moisture control upgrades are recommended.
4. “I’m not sure what type I have.”
Most PNW homes fall into category 1 or 2 — closing your vents in winter is the safer choice.
When to Reopen Crawlspace Vents
In the PNW, vents can reopen in:
Late Spring → Early Fall
When temperatures are warmer and humidity drops, vents can help circulate air — if your crawlspace is already dry and healthy.
Final Takeaway
For homeowners wondering “Should crawlspace vents be open or closed in winter?”
The answer is simple:
In the PNW, keep them closed in winter to prevent moisture intrusion and protect your home.
If you’re seeing drafts, cold floors, or moisture signs, that’s your cue that your crawlspace needs attention.