Moisture barriers are a key part of home construction. A moisture barrier, also called a vapor barrier or vapor retarder, is a material that stops water and moisture from passing through walls, floors, and other parts of a building. These barriers can come in different forms, like sheets, coatings, and membranes, and they’re categorized based on their permeability—how much moisture they allow through. Moisture barriers help protect your home against water damage and mold growth. In this guide, we’ll discuss moisture barrier types, where they go, and how to install them.
Types of Moisture Barriers
Moisture barriers fall into three main categories based on their permeability, as measured in units called “perms.” The lower the perm rating, the less moisture the barrier allows through.
Pro Tip: Gary Schinning, national sales and marketing manager for performance-strength films at Van Leer Flexibles, explains the role of moisture barriers: “It’s a secondary barrier, not a pool liner.” He emphasizes that these products are designed to provide extra protection beyond the siding and sheathing, not to be waterproof membranes.
Class I Barriers
Class I barriers are the most impermeable, with a rating of 0.1 perms or less. They include materials like polyethylene sheets, rubber membranes, sheet metal, and glass, and they’re best for cold or humid climates.
Pro Tip: Tom Silva, TOH general contractor, recommends: “Though some batts come with a paper or foil facing that can act as a vapor barrier. Tom Silva prefers to use 6-mil polyethylene sheeting. cut to fit between the joists and with seams sealed by foil tape, to prevent moisture from seeping into insulation.”
Class II Barriers
Class II barriers are semi-permeable, allowing some moisture to pass through. They have a perm rating between greater than 0.1 and 1.0. Examples include 30-pound asphalt-coated paper, bitumen-coated kraft paper, plywood, and certain vapor retarder paints. They’re suitable for a range of climates and uses.
Class III Barriers
Class III barriers are the most permeable, with a perm rating between greater than 1.0 and 10.0. They include brick, gypsum board, concrete block, and house wrap. They’re typically used in milder climates or as part of a well-thought-out moisture management plan.
The Importance of Moisture Barriers in Home Construction
Moisture barriers protect your home from moisture-related problems. Too much moisture can wreak havoc on a home’s structure, leading to wood rot, metal corrosion, and cracked concrete. A good moisture barrier can help prevent these issues and extend the life of your home.
Mold thrives in damp conditions, and if moisture builds up inside walls or other hidden areas, it can lead to health problems and costly repairs. Moisture barriers help keep these areas dry, reducing the risk of mold growth.
Pro Tip: Tom Silva, TOH general contractor, explains: “The vapor barrier needs to be on the outside of the wall and the same thing in the attic. By stopping that moisture when the air-conditioning’s on, you’re not going to have to worry about mold or mildew collecting on the outside of the wall.” However, in colder climates, Tom advises: “In the part of the country where I live, our air is colder in the winter and the moisture that we have to worry about comes from the inside, so we have to put our vapor barrier on the inside wall.”
Where To Install Moisture Barriers
Here are some key areas where moisture barriers are typically installed:
- Basement floors: A moisture barrier is a must for basement floors in contact with the ground. Place a polyethylene sheet under the concrete slab to stop moisture from seeping up.
- Crawl spaces: Moisture barriers in crawl spaces prevent ground moisture from seeping into the house. Install a heavy-duty polyethylene sheet over the entire crawl space floor, extending it up the foundation walls by about 6 inches.
- Exterior walls: In exterior walls, moisture barriers keep outdoor moisture out and prevent indoor moisture from condensing within the wall. In cold climates, the barrier goes on the warm (interior) side of the insulation. In hot, humid climates, it’s placed on the cool (exterior) side.
- Under flooring: When installing wood flooring over concrete, a moisture barrier protects the wood from moisture damage. Use a suitable underlayment or moisture barrier designed for this specific application.
Pro Tip: Tom Silva, TOH general contractor, emphasizes the importance of proper crawl space moisture control: “The reason we’re covering this floor with the poly, if you look at the dirt below, you can see how wet it is. It’s actually damp.” He recommends taping the polyethylene sheets at the seams and to the side walls, explaining, “We’re going to seal up all that so none of the moisture can escape and come up into this area here.”
How To Choose the Right Moisture Barrier
Choosing the right moisture barrier for your project depends on several factors:
- Climate: Your local climate plays a big role in what kind of barrier you need. In cold climates, go with less permeable barriers (Class I or II) on the warm side of the insulation. In hot, humid climates, opt for more permeable barriers (Class II or III) on the cool side.
- Material compatibility: Make sure the barrier works well with the materials it will be touching. Some barriers might react poorly with certain building materials. For example, certain plastic barriers could break down when in contact with chemically treated concrete.
- Permeability: Choose a barrier with the right perm rating for your project. A lower perm rating isn’t always better—allowing some moisture movement is sometimes beneficial for your home’s overall moisture management.
Regional Considerations: As Tom Silva explains about vapor barrier placement: “In the North it goes on the inside to keep moist heated air out of the wall cavity in the winter; in the South, it goes on the outside to keep humid heat from getting into air-conditioned rooms.” He notes he’s seen this done incorrectly many times because builders don’t consider local conditions.
Moisture Barrier Installation Techniques
Proper installation is key for moisture barriers. Follow these general guidelines.
- Preparing the surface: Before installing the barrier, clean and dry the surface and fix any damage to the structure underneath.
- Sealing seams and edges: If you’re using sheet-type barriers, overlap seams by at least 6 inches and seal them with tape or adhesive. Pay extra attention to edges, corners, and where different materials meet.
- Addressing penetrations: Carefully seal around any penetrations through the barrier, such as electrical outlets, plumbing pipes, or windows. Use specialized tapes or sealants designed for this purpose.
The Role of Moisture Barriers in Energy Efficiency
Moisture barriers can help with your home’s energy efficiency by keeping moisture out and maintaining steady indoor temperatures. This reduces the strain on your heating and cooling systems and helps prevent drafts and cold spots.
Insulation Performance
Moisture barriers help keep insulation dry. Wet insulation doesn’t do its job well, leading to higher energy bills and less comfort. By keeping insulation dry, moisture barriers help you save on energy costs and stay comfortable.
Air Quality
A dry indoor environment means better air quality. High humidity can lead to dust mites and other allergens. A well-installed moisture barrier helps maintain healthy humidity levels inside.
Expert Insight: Richard Trethewey, This Old House’s expert on indoor air quality, explains the modern challenge: “When the building was nothing but plaster, you could boil spaghetti all day long, you could take a shower all day long. But the modern home—the home built from 1975 on—is a tight thermos bottle. Whatever humidity we create can’t escape. That’s why the mold problems tend to come.”
Building Codes and Moisture Barriers
Always install moisture barriers according to building codes. Following these codes not only keeps you on the right side of the law but also helps you get the best results for your project
Different areas have specific rules for moisture barrier installation based on climate. Check your local building codes to make sure you’re using the right barrier in the right place.
If you’re unsure about how to install your moisture barrier, ask a professional for advice. They can guide you in the right direction and help you choose the most suitable materials and installation techniques for your needs.
Pro Tip: Tom Silva, TOH general contractor, emphasizes the importance of placement: “Whether using loose fill or batts, put the vapor barrier closest to the warm side of your installation, where hot, moist air would get in—facing the house’s interior in cold climates (beneath floor insulation) and the attic’s interior in hot climates (on top of floor insulation).”
Common Mistakes To Avoid When Installing Moisture Barriers
Be aware of these common pitfalls when installing moisture barriers:
- Improper sealing: Failing to seal seams, edges, and penetrations properly can create weak points in your moisture barrier system, allowing moisture to bypass the barrier entirely.
- Inconsistent overlapping: Make sure to overlap seams uniformly to avoid gaps that could compromise the barrier.
- Incorrect placement: Installing the barrier on the wrong side of the insulation can lead to moisture problems. Consider your climate and follow local building codes for proper placement.
- Using the wrong barrier type: Choosing a barrier that’s too impermeable or too permeable for your application can cause moisture to become trapped or allow too much moisture to pass through.
Pro Tip: Tom Silva, TOH general contractor, emphasizes the critical importance of vapor barrier placement: “If the studs are 16.5 inches on center and you’re using batts made for 16 inches, you’ll have a crack on each side that air can move through.” He also warns about a common mistake he sees frequently: “Laying a second layer of fiberglass batts in the attic is an easy way to boost R-values. But if the new layer has a kraft-paper backing, it can trap moisture and turn the layers underneath into a soggy mess.”
Enhancing the Longevity of Moisture Barriers
Maintenance and regular inspections can extend the life of your moisture barriers. Periodically inspect visible portions of your moisture barrier for signs of damage or deterioration. Pay special attention to areas prone to moisture issues, such as basements and crawl spaces.
If you notice any tears, gaps, or other damage to your moisture barrier, address it right away to prevent moisture problems from developing.
Moisture Barriers vs. Vapor Barriers
While the terms “moisture barrier” and “vapor barrier” are often used interchangeably, there are subtle differences between the two. Moisture barriers block both liquid water and water vapor, while vapor barriers focus on blocking water vapor only.
Many products do both jobs, but you should know what your barrier is designed to do and pick the right one for your project. When deciding between moisture and vapor barriers, think about what your project needs. Consider the climate, the materials you’re using, and the nature of your construction to make the best choice.
