Basements and crawlspaces can be damp. That’s why many homeowners assume that a basement or crawlspace needs at least one vapor barrier.
They’re right. But where does the vapor barrier go? Do you need one for your walls? The ceiling? The floor? And if some of these assemblies include insulation, you probably have another question: on which side of the insulation does the vapor barrier belong?
These are all good questions.
Two preliminary points
Before providing advice on vapor barrier placement in basements and crawlspaces, it’s worth answering two questions: Why are basements and crawlspaces damp? And what type of performance can we expect from a polyethylene vapor barrier?
There are three basic mechanisms that explain moisture accumulation in basements:
- Water can enter a basement or crawlspace in liquid form—through cracks in the walls, cracks in the slab, or the crack at the perimeter of the slab.
- Water vapor can enter a basement or crawlspace by evaporating from the interior surface of a damp wall or slab.
- Moisture in the indoor air can condense on cold surfaces in your basement, forming drips and puddles.
For more information on these three mechanisms, see Dealing With Basement Water From the Inside.
The next question is: What performance can you expect from a vapor barrier—for example, 6 mil polyethylene?
- You can expect polyethylene to greatly limit the migration of water vapor (that is, evaporation from a damp surface).
- If warm moist air ever contacts cold polyethylene, you can expect condensation to form on the surface of the polyethylene that faces the warm air.
- You can’t expect 6 mil polyethylene to stop liquid water, since water will almost always find a small hole or seam to seep through.
For example, let’s say you have an old-fashioned damp basement with uninsulated…
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