Crawl space
Crawl Space Vapor Barrier
A vapor barrier is the simplest thing done to a crawl space: plastic over the bare ground. It is often confused with encapsulation, which is a much bigger job. Knowing the difference lets you tell whether an offer is too little, too much or about right.
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What a vapor barrier does, and what it leaves alone
Soil gives off moisture all the time, even when it looks dry. A sheet of plastic laid over it keeps most of that moisture in the ground. That is the whole job. The vents stay open, the walls stay bare, and the air under the house still follows the weather outside.
- Vapor barrierPlastic over the soil only
- EncapsulationLiner over the soil, up the walls and around the piers
- Vapor barrierVents left open
- EncapsulationVents closed and sealed
- Vapor barrierNo machine to run
- EncapsulationDehumidifier running, with a drain
- Vapor barrierSlows moisture from the ground
- EncapsulationBlocks ground moisture and humid outdoor air
- Vapor barrierSuits a crawl space that is mostly dry
- EncapsulationSuits a crawl space that stays damp in summer
Signs a barrier alone will do
- No water stands under the house, even after heavy rain.
- The joists look clean and the insulation is still in place.
- Ducts and pipes do not drip in July and August.
- The only complaint is bare or patchy soil cover.
Signs it will not be enough
If the wood is stained, the insulation is sagging or the ducts sweat, the moisture is arriving in the air as well as from the ground. Plastic on the soil cannot stop that. Paying for a barrier in that case usually means paying again later for encapsulation.
What a sound barrier looks like
The sheets overlap generously and the overlaps are taped. The plastic reaches the walls on every side and is cut neatly around piers, with no soil showing. It lies flat without pools of water on top. If there are pools, water is getting in from somewhere, and that is the thing to fix.
Questions to ask
- How thick is the plastic, and will it stand up to someone crawling across it to reach a pipe?
- Will the old plastic and any debris be taken out first?
- How are the seams and the edges fixed down?
- If you are recommending more than a barrier, what did you see that a barrier would not solve?