Vented vs sealed crawl space
Decades of building code said vent it. Current practice in most US climates says seal it. Both are defensible; the half-measure is not.
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For most of the twentieth century, code required crawl space vents on the theory that outdoor air would carry moisture away. In a dry climate that works. In a humid one it does the opposite, and the mechanism is simple enough to check for yourself.
Why vents can make it wetter
Warm summer air holds a lot of water. Push it into a crawl space where the ground and the surfaces sit at 60 degrees or below and it cools, which raises its relative humidity. Cool it far enough and it reaches dew point and condenses — on the joists, the ductwork and the underside of the subfloor.
That is why vented crawl spaces in humid regions often have the worst condensation in July, and why opening the vents further makes it worse rather than better.
The two defensible positions
| Vented | Sealed | |
|---|---|---|
| How moisture leaves | Air exchange with outside | A dehumidifier or conditioned air |
| Summer performance in a humid climate | Poor — incoming air condenses | Good |
| Summer performance in a dry climate | Fine | Fine |
| Winter heat loss | Higher — the floor above is the thermal boundary | Lower — the walls become the boundary |
| Running cost | None | A dehumidifier, continuously in summer |
| Up-front cost | None | Barrier, sealing, insulation, dehumidifier |
| Combustion appliances | Usually straightforward | Requires professional assessment |
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  AlorAir Sentinel HDi90Sealing the vents means owning the humidity, and this is the unit built for the conditions down there. | The sealed route | 8.7 | Check price on Amazon #ad we earn a commission · Amazon shows the current price |
| 2 |  10-mil reinforced crawl-space vapor barrierGround cover is the right first move whichever side of this argument you land on. | Either route | 9.2 | Check price on Amazon #ad we earn a commission · Amazon shows the current price |
Scores are ours and editorial, not lab measurements — see how we pick. Tap a row for the full write-up.
AlorAir Sentinel HDi90
Sealing the vents means owning the humidity, and this is the unit built for the conditions down there.
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| Type | Basement and crawl-space dehumidifier with a built-in condensate pump |
|---|---|
| Capacity | 90 pints/day at AHAM (80 F, 60% RH); 198 pints/day at saturation |
| DOE rating | 47.71 pints/day at DOE (60 F, 60% RH) |
| Coverage | Up to about 2,600 sq ft |
| Airflow | 210 CFM |
| Noise | Under 58 dB(A) |
| Pump lift | Pumps condensate over 16 ft of hose |
| Operating range | 33.8 to 104 F |
| Filter | MERV-8 |
A sealed crawl space is a conditioned space whether you planned for that or not. Something has to manage its humidity, and in a crawl space that means a dehumidifier with a pump.
What it gets right
- Built-in condensate pump
- Rated to operate in a cold crawl space
- Large coverage area
Where it falls short
- Expensive
- Physically large
Specifications from AlorAir Sentinel HDi90 specification listing, AlorAir Sentinel HDi90 retail listing.
10-mil reinforced crawl-space vapor barrier
Ground cover is the right first move whichever side of this argument you land on.
#ad we earn a commission · Amazon shows the current price
| Type | Reinforced polyethylene ground vapor barrier |
|---|---|
| Thickness | 10 mil, the usual floor for a crawl space you will ever crawl on |
| Use | Full ground cover, lapped and taped, run up the walls and sealed |
This is the one recommendation that holds regardless. Covering bare ground reduces the moisture load whether the vents are open or closed.
What it gets right
- Helps in a vented crawl space too
- Cheap
- Nothing about it forecloses sealing later
Where it falls short
- Does not address vented air
- Tedious to install
Specifications from Amazon listing (search).
The middle ground that is actually unsafe
If you seal, do all of it
- Fix any liquid water first — drainage, grading, a pump if needed.
- Lay and tape a 10-mil or heavier ground barrier, run up the walls and sealed.
- Seal the vents with purpose-made covers, not plastic and tape.
- Air-seal and insulate the rim joist, which is where most of the remaining leakage happens.
- Add a dehumidifier with a condensate pump and set it to 50-55% relative humidity.
- Put a hygrometer at the far end and actually look at it occasionally.
Local code still varies and some jurisdictions have specific requirements for conditioned crawl spaces, including insulation and fire-rating rules for exposed foam. Check before you buy materials.
Questions people ask
Should crawl space vents be open or closed?
In most US climates, current practice is to seal them and condition the space. In a genuinely dry climate, vented works fine. The one thing not to do is seal them without adding moisture control.
Will sealing the crawl space make my floors warmer?
Usually yes. Sealing moves the thermal boundary from the floor above to the crawl space walls, which generally makes the rooms above noticeably less cold in winter.
Is a sealed crawl space a code violation?
No, conditioned crawl spaces are recognized in modern codes, but the requirements vary by jurisdiction and often cover insulation and fire protection. Check locally before starting.
What about a gas furnace in the crawl space?
That changes the analysis entirely. An appliance drawing combustion air from the space needs an assessment before any vents are closed. Treat this as a safety question, not a preference.
Sources
Read next
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