In new construction, using a vapor barrier can be the best or worst decision you ever make. If it functions properly, it can serve you well; but on the other hand—an improperly placed vapor barrier can become your worst nightmare that will haunt you as long as you own the building.
The placement of a vapor barrier in relation to the slab insulation is one of those recurring questions in construction that nobody seems to want to give a straight answer to. And there’s a reason for that. It’s because the answer depends on multiple factors and may be different from one project to the next
The Big Question
We’re going to address the question, “What side does the vapor barrier go with insulation?”
It’s an important question to ask, because an improperly placed vapor barrier can ruin your insulation and cause mold and moisture problems inside your building.
The purpose of the vapor barrier is to inhibit the travel of water vapor. Since the building envelope is the barrier that separates the outside climate from the environment inside the building, it’s inside this cavity where there is the biggest risk for vapor condensation. Since we use insulation inside the building envelope to reduce heat loss, it’s critical to keep this space dry and functional.
The sequence of construction steps is up for interpretation and may vary depending on your contractor or construction manager. Some builders swear by installing the vapor barrier first, while others claim that laying the vapor barrier over the insulation is better. Both may be correct. Keep reading to find out why.
Ask These Questions
To figure out correct placement of your insulation in relation to your vapor barrier, evaluate these questions.
What are the Code Requirements for a Vapor Barrier on Slab on Grade?
Building codes help contractors make buildings that are safe and functional. The 2021 International Building Code (IBC), is an example of a model building code. Chapter 19 covers concrete requirements—and specifically, Section 1907 calls for a 6-mil polyethylene vapor barrier between the subgrade and the concrete floor slab.
Local building codes may have additional requirements or amendments that conflict with the model code, so always verify requirements with your local code officials.
What is the Construction Type and Use?
Not all buildings require a vapor barrier with the slab on grade. Even Section 1907 of the IBC lists some exceptions where a vapor barrier is not required with a concrete slab, including:
Detached unoccupied structures
Unheated storage rooms
Buildings where moisture intrusion would not be detrimental to the occupants
Other exceptions as approved locally due to site conditions
When looking through the building codes, make sure you’re looking at the sections that apply to your specific building type (residential vs commercial) and building use. Some construction types, such as cold storage facilities, will have different vapor barrier requirements than car washes or residential homes.
Where is Your Building?
Geographical location is another factor that may change the requirement of your vapor barrier under the slab. Where climates are very mild and dry year-round, a vapor barrier may not be required at all. In climates that are hot and humid, a vapor barrier should be installed on the warm side (for walls, this would be just beneath the exterior cladding, over the insulation). For very cold climates, the warm side is the interior, so the insulation should have a vapor barrier separating it from the interior wall.
Is Under Slab Insulation Required?
Under slab insulation isn’t always required. In fact, for unheated slabs in climate zones 1, 2, and 3, there is no insulation requirement per the International Energy Conservation Code for commercial or residential buildings. Check with your local code officials to get the most up-to-date information for your location.
Above or Below?
So, what side does the vapor barrier go with insulation? Should you place the vapor barrier over or under foam insulation? You may have noticed that the building code referenced above didn’t exactly answer that question. It just states that the vapor barrier is required between the ground and the concrete. Since it left insulation out of the equation, you’ll need to use the answers to the other questions to determine the best course of action for your project. You can also consult the insulation manufacturer, vapor barrier manufacturer, or local construction professionals for additional insight and advice.
Vapor Barriers and Insulation: A Team Effort
Vapor barriers and insulation work together to provide thermal comfort to your slab on grade. The vapor barrier keeps the insulation dry and at its optimal performance. The dry insulation insulates and protects the slab and interior structure. Some insulation products, such as Rmax ECOMAXci FR, even come with a built-in vapor barrier option. The joints of these foil faced insulation boards can be taped in the field to create a complete vapor barrier seal.
Protect Your Slab with Rmax Polyiso
Rmax is dedicated to helping customers find the right products for their project. Reach out to our experts today to get help picking the right polyiso for the job and get our take on whether we recommend placing your vapor barrier over or under foam insulation for your project.