Joe Lstiburek, an engineer and principal with Building Science Corp. in Westford, Mass., responds: Installing a vapor barrier on both sides of a wall is never a good idea in any climate.
The common taboo against a double vapor barrier arises from a very real concern: If you have a Class I vapor retarder (less than 0.1 perm) on both sides of a wall, that wall has virtually no drying potential in either direction.
In cold climates, vapor barriers go on the warm side of insulation. In hot, humid climates, they go on the exterior side. Even with faced insulation appropriately installed, it's wise to have a separate vapor barrier facing the heated space.
Direction Matters: The vapor barrier should always face the warm side of your insulation. In the UK, that's usually the inside of your building.
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.
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.
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.
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.
With the insulation built into the plasterboard, there is no need for an additional installation process, saving time and money. One of the key benefits of insulated plasterboard is the option for foil-backed varieties, which serve as a built-in vapour barrier.
If doing an encapsulation (including the crawlspace in the building envelope and making it conditioned space) the experts agree that the vapor barrier should extend up the wall, stopping four to six inches below the sill plate to allow for an unobstructed pest control inspection area.
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.
In finished attic rooms with or without dormers, insulate (2A) between the studs of "knee" walls, (2B) between the studs and rafters of the exterior walls and the roof, (2C) and ceilings with unconditioned spaces above. (2D) Extend insulation into joist space to reduce air flows.
Where should vapor retarders be installed? In areas where the climate is cold in the winter, the vapor retarder should be installed on the inner side of the wall near the warm interior space — or on the warm side in winter.
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.
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.
Double Vapor Barriers.
Generally, do not install vapor impermeable products on both the interior and exterior wall surfaces – this would create a double vapor barrier condition that could trap moisture and reduce the ability of the wall assembly to allow drying of any incidental moisture.
In Most Cases, Attic Insulation Does Not Need To Provide A Vapor Barrier. The good news is that blown-in insulation should not need a vapor barrier. Blown-in is primarily used on the attic floor, and vapor barriers are not a concern for attics in our climate zone.
Aluminium foil is considered to be very resistant, tear- resistant and vapour diffusion-tight. A PE-aluminium laminated vapour barrier is particularly suitable for a vapour-tight construction.
If you are wondering how to install the foil backed insulation on your walls here you have few tips to consider. if it is hot outside, the reflective part of your foil insulation must face outside. if it is freezing outside, the best orientation of the reflective part is inside.
Joe Lstiburek, an engineer and principal with Building Science Corp. in Westford, Mass., responds: Installing a vapor barrier on both sides of a wall is never a good idea in any climate.
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.
Kraft-faced insulation includes a paper vapor retarder, which helps prevent mold and mildew. Kraft-faced insulation should be installed in exterior walls, exterior basement walls, and attic ceilings by pressing the product into the wall cavity with the paper side facing outward, towards the installer.
If you are installing insulation to the inside and/or outside of your external walls, then yes, you need vapour control to avoid any serious issues occurring. You should never miss this step.
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.
Thickness: The vapour barrier must possess a minimum thickness of 0.2mm (200 µm) and demonstrate resilience against impacts, punctures, and moisture penetration. Installation Standards: The film should provide a 200mm overlap at all joints, with penetrations meticulously sealed or taped using a snug-fitting sleeve.