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.
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.
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.
It is not recommended to use a polyethylene vapour barrier on the interior of assemblies where the spray foam/board foam combination meets the minimum level of vapour control as calculated by Figure A-9.25.
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.
If a vapour barrier is required, it can be attached over the foam insulation. If the vapour barrier is placed over the insulation, it will need to be sealed carefully to prevent any moisture from entering.
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.
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.
The best paint to use over spray foam insulation is a water-based solution like latex or acrylic paint.
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.
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.
Installation Tips
Here are a few tips to get you started: 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.
Crawl Space Foam Board and Vapor Barrier
Typically what we use is a fastener with our Hilti guns. You can also use a Christmas tree fastener for vapor barrier, or you can even get a ram set fastener if you want to mechanically attach.
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.
Begin at one side of the crawl space and lay down 6-mil or thicker polyethylene plastic over the entire crawl space. Cut the sheets to size, allowing 6 inches or more along walls. Overlap the seams by no less than 12 inches and secure them together with the poly PVC tape.
A vapour barrier is not necessary with closed-cell foam but with open-cell spray foam such as Icynene®, it is sometimes required. Any air that migrates though a building envelope will carry water vapour. As Icynene® spray foam creates a seamless air-seal, it controls air leakage and the moisture in the air.
“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.
For crawl space projecs that require a vapor barrier installed within walls, builders commonly use a thinner plastic sheeting option such as 6 mil, but for the best durability in the crawl space, our vapor barrier experts instead recommend choosing 8 mil plastic sheeting.
Directly against electrical wiring or fixtures: Spray foam insulation should not be applied directly against electrical wiring, fixtures, or recessed lighting unless they are rated for contact with insulation. Contact with insulation can cause overheating and pose a fire hazard.
Therefore providing mechanical fresh air ventilation is especially important in spray foam houses. Mechanical ventilation systems allow a constant flow of outside air into the home and can also provide filtration, dehumidification, and conditioning of the incoming outside air.
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.
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.