Vapor Barriers: Where They Belong in a Colorado Home

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Few building materials cause as much confusion as the vapor barrier. Some homeowners assume every wall needs a sheet of plastic, others have heard that plastic causes mold, and both camps can point to houses that prove their case. The truth depends on climate, on which side of the wall stays warm, and on whether the material is meant to stop water vapor, air or liquid water. On the Colorado Front Range, where the air is often dry, winters are cold and many homes have crawl spaces or basements, getting placement right matters more than the product itself. This guide explains the difference between barriers and retarders, where they belong, the double-barrier trap, how house wrap differs, and what home inspectors look for.

Part of our Crawl Space & Attic Guide guide — start there for the full picture →

Vapor Barrier Versus Vapor Retarder

Water vapor moves through most building materials by diffusion, from areas of higher vapor pressure toward lower, which in winter usually means from the warm, humid inside toward the cold outside. A vapor retarder is any material that slows that movement. A vapor barrier is the informal name for a material that slows it so much that very little gets through. Building scientists and codes generally prefer the term “vapor retarder” and sort materials into classes by their permeance, measured in perms.

The commonly cited classes, which are general industry ranges rather than precise product ratings, look like this:

Product data sheets list a measured perm rating, and the exact class boundaries are set by the building code a jurisdiction adopts. The practical takeaway is simple: a 6-mil polyethylene sheet and a coat of latex paint are both vapor retarders, but they behave very differently, and choosing between them is a design decision.

Climate Decides Which Side Gets the Retarder

The long-standing rule of thumb is that a vapor retarder belongs on the side of the insulation that is warm in winter. In a cold climate, that is the interior side, just behind the drywall. Placed there, it keeps humid indoor air from diffusing into the cold wall or ceiling cavity, where vapor could condense on the back of sheathing or framing.

In hot, humid climates the logic reverses, since the outdoor air is the humid side for much of the year and air conditioning keeps the interior cool. Mixed climates fall somewhere in between, and many designers there prefer assemblies that can dry in both directions.

The Front Range generally falls in cold climate zones, with mountain areas colder still. The air is also relatively dry much of the year, which is an advantage: wall cavities tend to dry out faster than they would in a humid region. That does not make moisture problems impossible. Bathrooms without working exhaust fans, humidifiers set high, large households, crawl spaces over damp soil and finished basements against cold concrete can all create local conditions far wetter than the regional climate suggests. Because codes and recommendations vary by zone and assembly, a local building department or an energy professional is the right source for a specific project.

Where Vapor Retarders Belong in a House

Crawl Space Ground Cover

The most important vapor retarder in many Front Range homes is the one lying on the dirt. Exposed soil in a crawl space releases moisture continuously, and a polyethylene ground cover is the standard way to block it. The sheet is commonly overlapped at seams, taped and run partway up the foundation walls. Thicker, reinforced sheets hold up better to foot traffic from service techs. Our guides to crawl space vapor barrier installation and crawl space vapor barrier insulation cover materials and sequencing in detail, and the crawl space hub collects related topics.

A ground cover can also play a role in radon resistance. EPA’s radon-resistant construction basics describe placing heavy-duty plastic sheeting (6-mil polyethylene) or a vapor retarder on top of a gravel layer beneath the slab to help prevent soil gases from entering the house, and note that the sheeting also keeps concrete from clogging the gravel when the slab is poured. In crawl spaces, sealed ground covers are often part of radon mitigation designs as well, though a ground cover alone is not a substitute for testing.

Basements

Basements are where vapor retarder mistakes are most common. Concrete walls and floors carry moisture from the surrounding soil, and the inner face of a basement wall stays cool for much of the year. Polyethylene stapled to framing directly against a concrete wall can trap moisture between plastic and concrete, or on the back of the plastic where warm basement air meets it. Many current approaches instead use rigid foam board or closed-cell foam directly against the concrete, which acts as both insulation and a vapor retarder, with framing and drywall inside. See basement vapor barrier and basement insulation for options. ENERGY STAR’s recommended insulation levels for existing homes call for adding R15 insulative wall sheathing or an R19 batt to basement or crawlspace walls in zones 4C and 5 through 8.

Walls and Ceilings

In above-grade walls in cold climates, a Class 1 or Class 2 retarder on the warm side has long been common, often as kraft-faced batts or polyethylene behind drywall. Some codes now allow Class 3 retarders, such as latex paint, in certain assemblies that include enough exterior insulation to keep the sheathing warm. Top-floor ceilings below a vented attic often benefit more from thorough air sealing than from plastic, because air leaking through gaps around lights, wiring and hatches can carry far more moisture into an attic than slow diffusion through drywall. Our crawl space insulation guide also covers how floor assemblies above crawl spaces fit into this picture.

The Double Vapor Barrier Trap

A wall or floor assembly that gets wet needs a way to dry. When a low-perm layer is installed on both sides of an assembly, any moisture that gets in, whether from a leak, a plumbing drip, construction moisture or air leakage, can become trapped. This is often called the double vapor barrier problem. Common examples include:

• Polyethylene behind drywall in a wall that also has foil-faced foam or vinyl wallcovering on the other side
• Plastic over a finished basement wall that already has closed-cell foam or a sealed concrete surface behind it
• Kraft-faced batts plus a separate layer of polyethylene in the same cavity
• Vinyl wallpaper on the interior of an exterior wall in a cooling-dominated space
• Crawl space floor insulation with a facing installed toward the cold crawl space, combined with flooring above that limits drying

The fix is design, not more plastic: pick one location for the low-perm layer and let the other side remain permeable enough to dry. When remodeling, it is worth identifying what is already in a wall before adding a new retarder.

Installation Details That Make or Break a Ground Cover

A crawl space ground cover is simple in concept, but small details decide whether it works. The points below reflect common practice; the liner manufacturer’s instructions and any local code requirements take priority.

• Prepare the soil first. Sharp rocks, old debris and construction scraps can puncture plastic over time. Raking the surface reasonably smooth helps the sheet lie flat.
• Fix water problems before covering them. A liner is not a drainage system. If water pools in the crawl space after storms or snowmelt, grading, gutters and downspouts, or interior drainage usually need attention first. Plastic laid over a wet crawl space can hide the problem without solving it.
• Overlap and seal the seams. Seams are commonly lapped generously and taped so soil moisture cannot wick up between sheets.
• Run the liner up the walls and around piers. Many designs carry the sheet partway up the foundation walls and wrap support piers, then fasten it with termination strips or adhesive.
• Leave access to the sill. Covering the wood sill plate can hide insect activity or decay. A strip of exposed foundation below the sill keeps it visible for inspection.
• Plan for traffic. Plumbers, HVAC technicians and pest control workers will crawl across the liner. A thicker or reinforced sheet, or a protective path, holds up better than thin plastic.

Whether the crawl space stays vented or is sealed and conditioned is a separate design decision. A vented crawl space with a ground cover relies on outdoor air for drying, which can work in the dry Front Range air for much of the year. A sealed crawl space removes the vents and treats the space as part of the house, which generally calls for wall insulation, careful air sealing at the rim joists and a plan for controlling humidity. Mixing the two approaches, for example sealing the vents without adding any drying mechanism, can make conditions worse rather than better.

Vapor Retarders in Remodeling Projects

Remodels are where many vapor problems start, because a new layer gets added to an assembly built decades earlier. A few situations come up often in older Front Range homes:

• Finishing a basement. Framing a wall against concrete, adding batts and covering it with polyethylene and drywall is a familiar pattern that can trap moisture. Foam against the concrete first is the more common modern approach.
• Adding exterior foam during re-siding. Rigid foam on the outside changes how the wall dries. If the interior already has polyethylene, the wall may end up with low-perm layers on both sides, so the foam thickness and type deserve a closer look.
• Converting an attic or bonus room. Sloped ceilings with limited space for insulation and ventilation are prone to condensation if air sealing is incomplete.
• Bathroom remodels. Showers on exterior walls see a lot of moisture. A working exhaust fan vented outdoors does more for that wall than an extra layer of plastic.

Asking the contractor where the vapor retarder sits in the finished assembly, and what already exists behind the old finishes, is one of the simplest ways to avoid trapping moisture.

House Wrap Is a Different Product

House wrap is often confused with a vapor barrier, but it does nearly the opposite job. Installed on the exterior of the sheathing behind siding, house wrap is designed to shed liquid water and limit air leakage while letting water vapor pass outward, so walls can dry. Many house wraps are highly vapor permeable. Putting polyethylene where house wrap belongs on a cold-climate wall would create an exterior vapor barrier on the cold side, which is the wrong side. Our guide to house wrap explains the product and its installation details.

Condensation, Mold and Humidity Control

A missing, misplaced or doubled vapor retarder often shows up as condensation or mold. EPA’s brief guide to mold, moisture and the home states that the key to mold control is moisture control, and advises keeping indoor humidity below 60 percent relative humidity, ideally between 30 and 50 percent, if possible. EPA also notes that condensation can be a sign of high humidity and advises acting quickly to dry wet surfaces and reduce the moisture source when condensation or moisture collects on windows, walls or pipes.

Warning signs that may point to a vapor or humidity problem include:

• Repeated condensation on interior window glass in winter, covered in our guide to window condensation
• Dark spotting on sills and frames; see mold on windows from condensation
• Musty odor from a crawl space or basement
• Water droplets on the underside of crawl space plastic or on ductwork
• Peeling paint or bubbling on exterior walls of bathrooms
• Damp or compressed insulation and staining on floor framing above a crawl space

Vapor retarders work best alongside humidity control. In a sealed or encapsulated crawl space, a dehumidifier or a small supply of conditioned air is often part of the design. Our guide to choosing a crawl space dehumidifier covers sizing and drainage. In the house itself, exhaust fans that actually vent outdoors and sensible humidifier settings during cold snaps do a lot of the work.

What Home Inspectors Look For

The ASHI Standard of Practice directs inspectors to inspect and describe insulation and vapor retarders in unfinished spaces, and to inspect ventilation of attics and foundation areas. It also states that the inspector is not required to disturb insulation. That means a standard inspection usually covers what can be seen in an unfinished crawl space, basement or attic, but not what is hidden behind drywall.

Typical observations include:

• Missing ground cover in a crawl space, or gaps and torn sections exposing soil
• Ground cover that is not lapped, sealed or run up the walls where the design calls for it
• Standing water or wet soil on top of or beneath the plastic
• Faced batts installed with the facing toward the cold side
• Polyethylene against a basement concrete wall with visible moisture or staining behind it
• Evidence of condensation, mold-like growth or wood decay on framing near the retarder
• Inadequate foundation area ventilation, or a sealed crawl space without apparent humidity control

When these turn up, the usual recommendation is further evaluation by a qualified contractor, and sometimes a moisture assessment if staining or odor suggests an ongoing issue.

Rough Costs

Costs vary with access, material thickness and region, so these are general, hedged ranges rather than quotes:

• Crawl space ground cover, DIY: material costs are often modest for a standard sheet, with thicker reinforced liners costing more per square foot. Seam tape, fasteners and termination strips add up.
• Professional crawl space ground cover: often several hundred to a few thousand dollars depending on size, prep, wall coverage and liner grade.
• Full crawl space encapsulation: commonly several thousand dollars or more, especially with wall insulation, sealing of vents and a dehumidifier.
• Basement wall insulation with an integrated retarder: varies widely with foam type, thickness and finishing.
• Correcting a double barrier: may require removing drywall, so costs depend on how much wall area is affected.

Getting at least two written quotes, and asking each contractor to explain where the vapor retarder will sit and why, is a reasonable way to compare proposals.

References

• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
• Radon-Resistant Construction Basics and Techniques — U.S. Environmental Protection Agency
• Recommended Home Insulation R-Values — ENERGY STAR
• ASHI Standard of Practice for Home Inspection — American Society of Home Inspectors

Seeing condensation, a torn crawl space liner or musty basement air? Ask our team to connect you with an inspector who knows Front Range homes.