That vapor barrier would go between the subfloor and the underlayment. “Combination underlayments,” as described on learn.
Location: Floor underlayment is positioned between the subfloor and finished flooring, whereas floor vapor barriers are typically installed beneath the underlayment or directly on top of the subfloor, depending on the specific requirements of the flooring system.
On most projects where insulation meets the under-slab vapor barrier, we have found that the project teams elect to install their insulation first and to lay the vapor barrier over top. Various industry leaders have commented on the advantage of placing the vapor barrier directly beneath the slab.
Lastly, the geographic location of the construction needs to be taken into careful consideration when deciding the proper placement of a vapor barrier in relation to the wall assembly. Properly placed vapor barriers should be installed on the warm, humid side of a wall.
Vapour control layer building regulations
12 require that all new builds have an approved VCL fixed to the warm side of all insulation and cover the external framed wall area. It must be fitted at 250mm centres to the top and bottom of frames, around openings, at laps and wherever there has been any damage.
Add a vapour barrier on the warm side of the wall within the first third of the thermal resistance value of the overall wall insulation.
General rule of thumb is that the WATER barrier goes on the exterior wall, just under the outer cladding, and the VAPOR barrier goes on the warmest side of the wall, up against the sheathing. You can have only one water barrier, and only one vapor barrier, so you would need to remove the existing one.
The combination of a vapor barrier and gravel covering your crawl space sounds like a good idea. But it's very important in which order you install them. You need to put the gravel down first and then put the vapor barrier on top to keep your crawl space dry and free from crawl space mold.
The air barrier may be installed on the interior side of the insulation, the exterior side of the insulation, or both, depending on the building component and the climate.
If only adding a quality vapor barrier and not sealing the crawlspace, the experts recommend extending the vapor barrier up the wall, six to twelve inches above outside grade. In either case, the vapor barrier needs to be permanently attached and sealed to the foundation wall.
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.
This is because an air gap provides an additional layer of insulation that helps to prevent heat from passing through the barrier and into the living space. If you are installing a radiant barrier, it's recommended to leave an air gap of at least 25mm (1 inch) between the barrier and the surface it's installed on.
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.
When using this approach, it is important to ensure that the vapour barrier is installed correctly. The vapour barrier should be installed on the warm side of the insulation to prevent moisture from penetrating the insulation and potentially causing damage or reducing the insulation's effectiveness.
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).
You have the top layer that is your floor covering (Hardwood, Laminate, Vinyl, Carpet, etc.), next you have the underlayment (usually a thin layer of fiber, foam, rubber), below that you have your subfloor (plywood, OSB, concrete), and lastly you have the joists which are the foundation to your floors.
Vapor barriers are installed along, in, or around walls, ceilings, and floors. Of course this is done to prevent moisture from spreading and potentially causing water damage.
A vapour barrier is placed on the inner side of the insulation and is designed to stop moist air from passing through it. A breathable membrane, by contrast, is installed on the outer side of the insulation. Breathable membranes must repel water from the outside while still allowing moisture from the inside to escape.
Quite simply an air barrier reduces the flow of air, a vapor barrier reduces the flow of water vapor and a water resistive barrier reduces the flow of liquid water. Each function is completely different.
After the insulation is in place you will want to add a vapor retarder, sometimes called a vapor barrier, if you need one. Not every wall does. A vapor retarder is a material used to prevent water vapor from diffusing into the wall, ceiling or floor during the cold winter.
A continuous underslab vapor barrier is the most effective against moisture control, but in most cases the material has to be cut to size, so using the appropriate tape to overlap seams and seal holes is very important.
They perform best when installed closest to the warm side of a structural assembly -- toward the interior of the building in cold climates and toward the exterior in hot/wet climates. Vapor retarder installation should be continuous and as close to perfect as possible.
Extend vapor retarder at least 6 inches up walls, columns, and footings. Use physical attachments where practical and necessary for long-term adhesion of vapor barrier to vertical surfaces. Overlap all seams at least 12 inches using a reverse or upslope lapping technique.
Okay, so we get asked all the time, “Does installing the vapor barrier control humidity?” Yes, it does… but it doesn't control humidity enough to where you still don't need a dehumidifier. You should also install a crawl space dehumidifier to properly control humidity.