In many colder North American climates, vapour barriers are a required part of building construction. You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good.
Absolutely, interstitial condensation can cause mould growth which can affect the health of building occupants. Over time, condensation will also damage the structure of the building by rotting timbers or causing corrosion.
Yes, you should leave an air gap between the insulation and the roof to allow proper ventilation and prevent moisture buildup. This is especially important for materials like fibreglass and mineral wool but not necessarily for spray foam insulation.
Don't: Put Vapor Barriers on the Bottom of the Floor Joist in the Crawl Space. Because vapor barriers are good at retaining moisture, putting plastic sheeting on the joists can cause moisture to build up under the floor.
A warm roof is designed to prevent any moisture from forming on cold surfaces in the roof construction. This means that warm roofs do not generally require ventilation, but there should be an Air & Vapour Control Layer (AVCL) or vapour barrier on the warm side of the insulation depending upon the detailing.
One main issue to think about with a warm roof construction is the ceiling height. If it is limited, the insulation will reduce the height of a room. It could also restrict access to doors or windows. This is why warm flat roof constructions are not as common as a pitched alternative.
A hot roof is simply a roof without ventilation. The important distinction between these two roof types is in their application.
If you live in a mixed climate – hot and humid with several heating months in the winter, you probably need a vapor retarder. Specifically, if you live in climate zones 4C (marine), 5, 6, 7 and 8. Not sure your climate zone? You can check here.
The Cons of Vapor Barrier Installation
Although vapor barriers aim to prevent moisture from entering the structure, they can also trap moisture inside. If not properly installed or upheld, this could result in the proliferation of mold and other issues related to 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.
In many colder North American climates, vapour barriers are a required part of building construction. You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good.
In the context of a pitched roof, the prevailing Building Regulations stipulate a minimum U-value of 0.18 W/m2K for a warm roof, where insulation is positioned above and between the rafters, and 0.13 W/m2K for a cold roof, where insulation is placed between and beneath the rafters.
Cold flat roof insulation requires more work, taking off old boards and replacing them and the roof surface. The thermal efficiency isn't as good as with warm roof insulation. A warm deck roof is much easier to install and offers great thermal performance but adds height to the flat roof.
Can you use a DPM as Vapour barrier? You can use a DPM sheet as a VCL and it will do the same job as long as it is sealed correctly and placed in the correct position – on the warm side of the insulation.
In colder climates, people often install vapor barriers on the exterior side of the insulation or between insulation layers. This is because cold air can enter the building and come into contact with warm, moist air, leading to condensation and potential moisture infiltration.
Vapor barrier materials are installed on the warm side of the insulation in a building assembly, as determined by climatic conditions. In warm climates, it will be on the exterior and in cold climates, it will be on the interior.
“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.
Depending on where you live, the vapour barrier will either go on the inner surface of the insulation (if you live in a cold climate) or the outer surface of the insulation (if you live in a hot climate). It's safe to say that a vapour barrier will be needed for almost all projects.
Carton board air barrier paper is an excellent alternative to vapor barrier plastic.
You can use plastic if you put polyethylene between your drywall and the wall studs. This way, you should get a tighter wall without trapping moisture inside the exterior walls. If you are framing out a below-grade basement, however, do not use plastic as a vapor barrier.
Is DuPont™ Tyvek® a vapor barrier? No, DuPont™ Tyvek® is not a vapor barrier. It is made with unique material science to keep air and bulk water out while allowing moisture vapor inside walls to escape.
Install a New Vapor Barrier
If your vapor barrier was damaged or not installed correctly, it's important to replace it with a new one. This will help prevent moisture from seeping into your crawl space and creating the perfect environment for mold growth.
During the summer, a hot roof does not allow for air exchange between exterior and interior air. Depending on the quality of the insulation, hot air from outside can permeate the insulation and warm the conditioned space, raising energy bills and making HVAC units work overtime.
Poor ventilation can damage shingles by causing extreme roof temperatures, water damage to roof decking, and more. That's why it's important to be alert to the state of your roof's exterior.
ROCKWOOL warm roof insulation is made from stone wool and designed to achieve maximum thermal performance. It's also non-combustible, with exceptional acoustic properties.