How the two differ in kind
A crawl space with exposed earth is a continuous moisture source. Ground moisture evaporates into it indefinitely, and that vapour goes where the air goes — up into the floor above, into the insulation, and into the house. Nothing else done to a crawl space matters much while that continues.
A GROUND VAPOUR BARRIER cuts it off: a sheet laid over the earth, lapped and sealed, so evaporation stops at the source. It is inexpensive, it is straightforward, and in terms of moisture removed per pound spent it is the single highest-value intervention available in most crawl spaces. Where the crawl space is otherwise reasonable and the climate is kind, it may be all that is needed.
What it does not change is the AIR. A vented crawl space exchanges with outdoors through its vents, and that exchange is helpful in some conditions and harmful in others. In winter, cold outdoor air is dry in absolute terms and ventilating dries the space. In a humid summer, warm outdoor air carrying a great deal of moisture enters a crawl space that is cool — cooled by the ground and by the conditioned floor above — and as that air cools, its relative humidity rises toward saturation and it condenses on the coolest surfaces available: ducts, cold water pipes, and the underside of the floor. That is the mechanism behind the mould, the rusting duct straps and the cupping floorboards, and no ground barrier prevents it.
ENCAPSULATION addresses it by changing which side of the envelope the crawl space is on. The liner is carried up the walls and sealed, the vents are closed, the perimeter walls are insulated rather than the floor above, and the space is conditioned — a small dehumidifier, or a supply of conditioned air, depending on the approach and the code. The crawl space becomes part of the building rather than a vented cavity beneath it.
Before either, one check is non-negotiable: any atmospherically vented combustion appliance in the space, which sealing would affect.
The factors that actually differ
Show
| Full encapsulation | Ground vapour barrier alone | |
|---|---|---|
| What it addresses | The source, the air, and the thermal boundary — the space becomes part of the building. | The source only. Ground evaporation stops; the air still exchanges with outdoors. |
| Summer condensation | Prevented, because humid outdoor air is no longer being admitted to a cool space. | Unchanged. This is the failure a ground barrier alone does not touch. |
| Cost | Substantially more — wall liner, sealing, perimeter insulation, conditioning and the electricity it uses. | Low, and it is the best value per unit of moisture removed. |
| Where the insulation goes | At the perimeter walls, since the crawl space is now inside the envelope. | In the floor above, since the crawl space is outside it. |
| Ductwork and pipes in the space | Now inside the envelope — duct losses stay in the building and pipes are much less at risk of freezing. | Still outside it. Duct leaks are lost, and pipes need their own protection. |
| Ongoing energy | A dehumidifier or conditioning supply runs continuously, which is a real running cost. | None. |
| Combustion safety | A sealing operation that changes the air supply. Any atmospherically vented appliance must be addressed first. | Unchanged. |
| Radon and soil gas | A sealed liner is the basis of a soil gas control layer, and can be extended to a sub-slab depressurisation system. | Helps, and a ground barrier alone is not a radon system. |
| Code position | Unvented conditioned crawl spaces are permitted in most modern codes with specified conditioning and details — check locally. | Vented crawl spaces with a ground cover are the traditional arrangement and remain permitted. |
| Inspection | Easier — a clean, dry, lined space that can actually be entered and looked at. | Improved, but the space is still exposed to outdoor conditions. |
Which one, and when
Choose full encapsulation when…
- The climate has humid summers, where venting admits more moisture than it removes.
- There is ductwork or plumbing in the crawl space, which then sits inside the envelope.
- There is a history of condensation, mould, musty smells reaching the house, or cupping floors.
- Radon or soil gas is a concern, where a sealed liner is the basis of the control layer.
Choose ground vapour barrier alone when…
- Budget is limited — this is the highest-value first step and it is worth doing immediately.
- The crawl space is dry, well drained and in a climate where venting genuinely dries it.
- There is an atmospherically vented appliance and no plan yet to address it.
- As the first stage of a phased approach, since a ground barrier is part of encapsulation anyway.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does venting a crawl space make it damp in summer?
- Because warm air carries far more water than cool air, and a crawl space is cool. Outdoor air on a humid summer day holds a great deal of moisture in absolute terms; drawn into a crawl space cooled by the ground and by the air-conditioned floor above, it cools down, its relative humidity climbs, and on any surface below its dew point it condenses. The surfaces that qualify are exactly the ones that matter: uninsulated cold water pipes, air conditioning ducts, and the underside of the floor. The result is standing moisture in a space with poor air movement, which is ideal for mould and for rusting anything metal. The same vents in winter do the opposite and dry the space, because cold air is dry in absolute terms — which is why the traditional advice to vent was not wrong so much as seasonal.
- Is a ground barrier alone worth doing?
- Emphatically yes, and it should be the first thing done in any crawl space with exposed earth regardless of what follows. Ground moisture evaporating continuously is usually the dominant moisture input, and a sealed sheet over the earth stops it at source for very little money. The work that makes it effective is in the detailing rather than the material: laps sealed rather than merely overlapped, the sheet carried up the walls at least a little and fixed, and penetrations for piers and pipes sealed around. A liner laid loose in strips with open joints is doing a fraction of the job. It is also not wasted if encapsulation follows, since a ground barrier is the first layer of an encapsulated space — the later work extends and seals it rather than replacing it.
- What does full encapsulation actually involve?
- Five things beyond the ground sheet, and skipping any of them undermines the rest. The liner is carried up the foundation walls and sealed to them, terminating below the level of any damp-proof course or as the local practice requires, so the walls are covered as well as the floor. The vents are sealed. The perimeter walls are insulated rather than the floor above, since the thermal boundary has moved. The space is conditioned — usually a dedicated dehumidifier draining to somewhere sensible, or a small supply of conditioned air with a return path, depending on the approach and the code. And bulk water is dealt with first: grading, gutters and downpipes discharging away from the building, and a sump where groundwater enters. A sealed crawl space with water coming in is a sealed pond.
- What has to be checked before sealing the vents?
- Combustion appliances, and this one is a safety matter rather than a performance one. An atmospherically vented appliance — a water heater or furnace drawing its combustion air from the space and venting through a flue that relies on natural draught — depends on that space having air available and being at roughly neutral pressure. Sealing the crawl space changes both, and the consequence of getting it wrong is combustion products entering the building. The resolutions are to replace the appliance with a sealed-combustion or direct-vent unit, to relocate it, or to provide a dedicated outdoor combustion air supply designed for it. In many jurisdictions this is explicitly regulated for unvented crawl spaces. It is a question for a qualified installer before the work starts, not a detail to resolve afterwards.
- Where does the insulation go?
- Wherever the thermal boundary is, and encapsulation moves it. In a vented crawl space, the boundary is the floor above, so insulation goes between the floor joists and the crawl space stays at outdoor conditions — which means ducts and pipes down there are outside the envelope and need their own insulation and freeze protection. In an encapsulated space, the boundary moves to the perimeter walls, so insulation goes against the foundation walls and the rim joist, and the floor above is left uninsulated. Insulating both is not a belt-and-braces improvement; it puts insulation on the wrong side of the assembly and can trap moisture, so the change of location is part of the conversion rather than an addition to it.
- Does an encapsulated crawl space need a dehumidifier?
- It needs conditioning of some kind, and a dehumidifier is the usual and simplest form. Sealing the space removes the outdoor air exchange, which was the previous moisture removal mechanism such as it was — so without an alternative, whatever moisture does get in has nowhere to go. The options are a dedicated dehumidifier sized for the space and drained to a condensate pump or a gravity drain, or supplying a small quantity of conditioned air from the house's system with a path back, which some codes specify in terms of a flow rate per unit area. Local codes differ in what they require, and the requirement is generally not optional. The dehumidifier route uses electricity continuously, which is a real running cost and is worth putting in the comparison honestly.
- What about bulk water?
- It has to be fixed first, because neither a ground barrier nor encapsulation is a drainage system. Water entering a crawl space through the walls or rising through the ground is a drainage problem and it will defeat a liner, which simply becomes a sheet with water on both sides. The order of work is grading the ground away from the building, making sure gutters and downpipes discharge well clear rather than beside the wall, dealing with any perimeter drainage that has failed or was never installed, and providing a sump and pump where groundwater genuinely enters. Only then does a liner make sense. A sealed crawl space over standing water is worse than a vented one, since the moisture is now inside an unventilated space with the building above it.
- Will this fix a musty smell in the house?
- Usually a large part of it, because the smell has a specific mechanism. Air moves upward through a building by the stack effect, drawing replacement air from the lowest available space — which in a house with a crawl space is the crawl space. A substantial share of the air in the living space has therefore passed through it, carrying whatever is down there in the way of moisture, mould spores, soil gas and odour. Stopping the moisture source and sealing the space cuts that at both ends: less mould to smell, and less air coming from there. The work is not complete without air sealing the floor above — the penetrations for pipes, wiring and ducts through the crawl space ceiling — which is what actually breaks the path, and which is cheap to do while the space is accessible.