You can also use an underlayment with a moisture barrier built-in for water vapor control.
Function: While both underlayment and vapor barriers contribute to moisture management, underlayment focuses on enhancing stability, comfort, and surface preparation, whereas vapor barriers prioritize moisture control and protection against mold and mildew.
Very often, particularly in a below-grade installation on a concrete basement floor, a vapor barrier is recommended in addition to the underlayment. That vapor barrier would go between the subfloor and the underlayment.
The underlay will act as a moisture barrier and it will also make the floor more comfortable.
An underlayment that offers a vapor barrier is essential to protect any flooring installed over a subfloor that emits moisture, like concrete. A thin foam underlayment made of polyethylene or polypropylene usually works best.
Polyisocyanurate (PIR) foam board insulation has a relatively low permeability, meaning it can resist the passage of moisture to a certain extent. However, it is not typically recommended to rely on PIR foam board insulation as a standalone vapour barrier.
Felt underlayment's do an exceptional job at insulating your flooring from the subfloor. This will keep your flooring warmer during the colder months of the year and result in you being more comfortable in your own home.
Both are designed to keep moisture out of your home and can be made from various materials. But there are a few key differences, the most apparent being that moisture barriers are better at keeping out liquids, and vapor barriers are better at keeping out water in its gaseous state.
Underlay protects your laminate against moisture damage
Cold ground floor subfloors are often prone to damp, which can damage your laminate. An underlay with a vapour barrier helps prevent this.
Since these underlayments don't seal around fasteners, there is some risk of water infiltration through fastener holes. That's why they're considered to be water-resistant. To get a truly waterproof barrier, you'll need to use a peel-and-stick underlayment.
Because a vapor barrier is a low permeable barrier, it keeps moisture from passing through. This is beneficial to keep moisture away from a dry surface but will trap moisture behind it if installed on a water-damaged concrete wall, causing the wall to fail further.
“For minor projects, the absence of a vapor barrier leads to water intrusion, leading to an unstable foundation or a cracked slab.” The American Society for Testing and Materials (ASTM) outlines national standards for underslab vapor barriers.
Subfloor Material
Therefore, because concrete is porous, it's a good idea to install a vapor barrier over your concrete subfloor before installing your flooring. This way you can stop the vapor drive of water upward to your floors, protecting them from any damage (source).
The Critical Role of Vapor Barrier Underlayment
Vapor barrier underlayment, also known as moisture barrier underlayment, is designed to prevent moisture from seeping into your floors. In regions where humidity levels fluctuate, or in installations over concrete subfloors, the risk of moisture damage is significant.
Although most synthetic underlayments are vapor barriers, there are exceptions. Several manufacturers produce products with a vapor permeance that is as high as, or even higher than, asphalt felt. The higher the permeance, the faster water vapor can pass through a material.
If you live in a mixed climate – hot and humid with several heating months in the winter, you probably need a vapor retarder. Specifically, if you live in climate zones 4C (marine), 5, 6, 7 and 8. Not sure your climate zone? You can check here.
Because a large percentage of breathable air in a residential building comes from the crawl space, it is crucial to use a vapor barrier in your encapsulation that meets crawl space code requirements.
Depending on where you live, the vapour barrier will either go on the inner surface of the insulation (if you live in a cold climate) or the outer surface of the insulation (if you live in a hot climate). It's safe to say that a vapour barrier will be needed for almost all projects.
Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor retarders are usually mechanically fastened and sealed at the joints.
No, DuPont Tyvek is not a vapor barrier. It is breathable, allowing moisture vapor to pass through it.
Some underlays may also provide thermal protection, although it is important to note that because it is installed over the subfloor, rather than underneath it, it will never provide quite the same level of assistance in this department as purpose built, properly installed insulation.
Underlayment has thermal properties that can increase the warmth of the floor. Select an underlayment with a thermal R rating to help keep your floors warm. Know that underlayment can only do so much to keep your floors warm, especially on concrete floors in your basement.
While most hardwood and laminate underlayments play a key role in keeping your flooring safe, they often cannot provide the right level of protection against moisture or vapor.