Vented or sealed: what actually works in Florida

The idea the house was built on

Open the foundation vents, let air move under the floor, and the wood stays dry. That was the logic, and in the climate where it was written down it works — provided the air arriving is drier than the wood it touches.

That proviso is the whole argument. In Ohio in October, outside air is drier than a crawlspace and ventilation dries it. In St. Petersburg in July, outside air is wetter than almost anything it touches, and the vents deliver the problem rather than removing it.

Nobody who framed a bungalow in Kenwood in 1926 was wrong about physics. They applied a national convention to a subtropical coast, before air conditioning existed to make the underside of the floor colder than the air around it.

What actually happens under a St. Pete house in August

Warm air holds more water vapor than cool air, and Gulf coast summer air is warm and close to saturated. That air moves through the foundation vents into a space measurably cooler than outside, because the conditioned rooms above hold the subfloor and joists down and the supply ducts running through the space are colder still. When humid air contacts a surface below its dew point, the vapor becomes liquid water on that surface.

The moisture is deposited exactly where you least want it: the underside of the subfloor, the joists, the duct exteriors, the insulation facing. Then more air arrives and does it again. Under summer conditions the ventilation rate is not a drying rate. It is a delivery rate.

The seasonal signature is the giveaway. A ventilation-driven problem worsens from roughly June through September, eases in the dry season, and returns. A problem driven by ground water or a plumbing leak tracks rainfall or runs year-round instead.

What sealing a crawlspace actually involves

“Sealed” is not a single product. It is a sequence, and skipping steps in it is how sealed crawlspaces fail.

• Clear the space. Debris, failed insulation, old sheeting, anything organic and loose.
• Address water first. If water enters the space, sealing it in makes things worse. Grading, downspout discharge, perimeter drainage and any plumbing leak come before the liner. This is the step most often skipped by whoever quoted the lowest number.
• Line the ground. Reinforced polyethylene across the soil, seams lapped and sealed, cut and fitted around each pier.
• Carry the liner up the pier faces and foundation walls, terminated and sealed at the top. This is what separates encapsulation from a vapor barrier.
• Close the vents with sealed, insulated covers, air-sealed rather than capped.
• Seal the penetrations and the access. Plumbing, wiring, duct chases, the hatch.
• Insulate the rim joist where the scope calls for it.
• Manage humidity mechanically. See below, because this is the part left out of bids.

Sealing alone does not dry a Florida crawlspace

This is the single most important sentence on this page, and it is missing from most of what is published about encapsulation.

A sealed space has had its moisture sources reduced — ground evaporation is under a liner, outside air is no longer invited in. It has not had moisture removed. Water remains in the wood, in the soil below the liner and in the air, the enclosure is never perfect, and this coast sits under high dew points ten months a year. A sealed crawlspace with no humidity management drifts, and a sealed space holding elevated humidity with no air movement is worse than a vented one, because now nothing carries it away either.

So a sealed crawlspace in this climate needs one of two things, deliberately specified and sized:

Approach	How it works	Where it fits
Dedicated dehumidifier	A unit sized to the space, with a condensate path to a drain or pump, and a humidistat	The common answer. Runs unattended, drops the space to a target and holds it
Conditioned supply air	A small metered supply from the home’s HVAC with a return path, bringing the space inside the conditioned envelope	Only where the system has capacity and the duct layout allows it. It changes the load calculation for the house

If a sealing quote does not name one of these with a model or a capacity, it is incomplete. You are being sold a box with nothing controlling the inside of it.

When a vented crawlspace is genuinely fine

Plenty of them are, and we are not going to pretend otherwise to sell a larger job.

Leave it alone if the space reads dry: ground covered or naturally dry, no standing water and no tide line on the piers, subfloor moisture content measured in the normal range on a meter rather than guessed at, vents open and unobstructed with real cross-flow rather than one vent on one side, ducts not sweating, no odor.

Leave it alone if the house is elevated with good clearance and good drainage. A crawlspace three feet above grade on a lot that sheds water behaves very differently from one eighteen inches up on a flat lot that ponds.

Leave it alone if water gets in and has not been solved yet. Sealing a space that floods traps the problem. Drainage first, and that may not be us.

And leave it alone if nothing is wrong. A dry vented crawlspace under a well-drained house does not need our money spent on it. We tell you when you do not need us.

The real arguments for leaving a vented space vented

Being even-handed here costs us work and is still correct.

Cost and payback. Encapsulation is several thousand dollars. If the space is dry there is no moisture problem to solve, and the marginal comfort and efficiency gain may not repay that for a long time.

Trapping what you did not fix. Water intrusion, a slow supply leak, a failing pier — all are slower to notice behind a liner. A vented space with an easy hatch is easier to keep an eye on.

Trades work down there. Cast-iron drain replacement is happening across Old Northeast and Kenwood right now. A sealed system installed the year before a plumber trenches your crawlspace is money partly spent twice.

Mechanical dependency. Sealing shifts the space from passive to actively managed. A dehumidifier that fails in July leaves you worse off than a vented space would. That is an acceptable trade against a real problem and a poor one without.

Code and permitting

The Florida Building Code sets requirements for under-floor ventilation, including minimum net free ventilation area, and it contains provisions for unvented under-floor spaces where specific conditions are met — ground cover, mechanical drying or conditioned supply air among them. Those conditions are not optional and not something a contractor gets to read loosely because a sales page calls sealing standard practice.

Closing permanent foundation vents changes the ventilation strategy the building was permitted under. We confirm the scope with City of St. Petersburg Construction Services and Permitting before work starts, every time, and we do not tell homeowners that any part of this work is automatically exempt from permitting, because we have not seen anything published saying it is.

If your house was elevated or substantially rebuilt after the 2024 storms, flood provisions also dictate what may be enclosed and what openings must remain, and they take precedence over anything general on this page.

What a crawlspace evaluation settles

The question is answerable with measurements rather than opinion: relative humidity in the space, moisture content read on the subfloor and joists with a meter, whether the ground is bare, whether there is standing water or evidence of past water, whether the ducts are condensing, vent condition and placement, and clearance at the tightest point.

We crawl the space, take the readings, photograph every bay and write it down. You keep the report and the photographs whether or not anything comes of it, including when the finding is that the space is dry and should be left alone.

We do not perform structural repair, foundation repair or mold remediation — each requires a Florida license we do not hold. If the evaluation finds damaged framing or failing piers, you get photographs of exactly that and a referral to an appropriately licensed contractor. On microbial growth the only thing worth stating is the rule: in Florida anything beyond roughly ten square feet requires a licensed mold remediator, and we are not one.