Vapor barriers play an important role in keeping buildings dry, safe, and inhabitable. A building that is prone to moisture problems, like mold and mildew, likely has a design flaw related to a missing or inadequate vapor barrier.
Designing a functional vapor barrier starts with choosing the right building materials. Each material requires an individual assessment of its vapor permeability. Once individual materials are selected, the designer must verify material compatibility and collective vapor barrier properties.
In this article, we explore this relationship and answer the question, “Does mineral wool insulation need a vapor barrier?”
Mineral Wool: What is it?
Mineral wool is an insulating material often used in commercial construction to insulate walls. It’s used both inside wall cavities and on the exterior face of walls. Its raw materials include minerals found in volcanic rock or byproducts of molten metal production. It’s naturally fireproof, carries a respectable insulating value of R-5 per inch, and even has sound-absorbing properties.
Downsides to Mineral Wool
So far, mineral wool sounds like an ideal building material. However, no building material is perfect and mineral wool is no exception. Here are some of the shortcomings of mineral wool insulation.
Limited Applications
Mineral wool is not an all-in-one insulation. It can not be used for every part of the building envelope. Typically, it’s only used in walls and ceilings (and rarely in roofs). It can’t be used below grade where moisture is present.
Heavy Weight
Mineral wool is very heavy. It may require multiple installers to lift a single batt, which increases both installation time and cost. The weight must also be factored into the building’s design since it adds significant dead weight to the structure.
Vapor Permeable
Mineral wool is unfaced and vapor permeable. It will not keep moisture from traveling through the envelope and into the wall cavity. If a vapor barrier is desired, it must be installed separately from the mineral wool insulation.
What is a Vapor Barrier?
A vapor barrier is an impermeable building material that blocks water vapor from traveling through. Examples of vapor barriers include the following:
Polyethylene plastic sheeting
Foil-faced insulation
Vapor retardant coatings
These can be applied by loose laying, fastening, and taping, and can be brush or roller-applied.
The role of vapor barriers is to keep moisture out of your walls. Without a vapor barrier in environments where there is a big difference in inside and outside humidity and temperature, condensation and moisture can build up inside walls and cause moisture problems like mold and mildew. Vapor barriers should be located on the warm side of the wall.
Does Mineral Wool Insulation Need a Vapor Barrier?
Depending on your geographic location, yes. If a vapor barrier is required in your location, then a separate vapor barrier is required for mineral wool insulation. If no vapor barrier is required by code in your area, then you do not need to add a vapor barrier to your mineral wool insulation in most cases. Exceptions may occur if your building houses a high-moisture interior environment, like a pool or carwash.
Polyiso: A Superior Alternative
Polyiso iso an alternative to mineral wool if you are looking for a more versatile insulation that can be used on all parts of the building envelope, including foundations. Polyiso can also be foil-faced to combine air and vapor barrier properties with the insulation in a single product.
Even more advantages to choosing polyiso over mineral wool insulation include:
Easier installation with a single laborer
Lightweight material reduces dead load impact
Higher R-value at R6.0 per inch
Here are Rmax air and vapor barrier insulation products designed for use throughout the building envelope:
Rmax ECOMAXci® FR Air Barrier
The Rmax ECOMAXci ® FR Air Barrier uses foil facers on both sides of the polyiso foam core to provide a built-in vapor barrier for steel, masonry, and treated wood exterior walls. Water vapor permeance is less than .1 perm. The air and vapor barrier is completed by detailing joints with either liquid sealant or joint sealing tape.
Rmax DuraSheath® Building Envelope Insulation
When a vapor barrier is not required, your insulation options open up to include various facers, including inorganic polymer-coated glass fiber mat facers found in Rmax Durasheath® insulation. This product can be used in many wall applications, ceilings, attics, crawl spaces, roofing, and more. DuraSheath® still carries a respectable vapor permeance of less than 1.5 perms.
Rmax ThermaSheath® Insulation
Upgrade your facer with Rmax Thermasheath® insulation, which features foil-facers bonded to a polyiso foam core. This expands the application to include even more applications where vapor permeability needs to be reduced, but a full vapor barrier system is not required.
Get Help Choosing Compatible Envelope Materials
Building codes and product selection can get complicated in a hurry. Get help from experts in the insulation industry to decide which products are needed for your project. Reach out to your local Rmax representative today!