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.
Generally speaking, vapor barriers are considered an essential moisture-blocking tool for climates that experience significant levels of humidity. For that reason, building codes in more humid locations will often require them, in which case you might not have much say about whether to install one or not.
“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.
Most climates require a vapor barrier to cover ceiling insulation, but not all insulation requires a vapor barrier. It's always important to refer to your state's building codes.
A vapour barrier is a damp-proof membrane used to create a perfect seal between the surface you want to insulate and the rest of the loft. This should prevent condensation from the home rising into the roof which would end up degrading the insulating materials.
In Most Cases, Attic Insulation Does Not Need To Provide A Vapor Barrier.
It's important to leave a ventilated air gap between the insulation and the boards to prevent condensation on the underside of the boards. Make sure you don't squash the mineral wool when you fit the boards on top, as this will reduce its insulation value.
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.
Even if the attic space is unheated, it's typically warmer than the outside air in winter. If you have chosen unfaced batting insulation for your rafters, you should staple sheets of plastic over the batting to accomplish a moisture barrier.
Attic Ventilation
area shall be no less than 1/150 of the attic floor area if no vapor retarders are used, or 1/300 of the attic floor area when a vapor retarder having a permeance of one perm or less is used.
Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor retarders are usually mechanically fastened and sealed at the joints.
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.
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.
In order for insulation to do its job, sufficient airflow to the outside surfaces is required. Good ventilation in your attic will allow the moisture to get out. An attic that is well ventilated will reduce the roof surface temperature and eliminate moisture.
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.
Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.
One popular option for attic spaces is unfaced insulation. This type of insulation features no external vapor barrier, allowing it to breathe freely and avoid trapping condensation buildup within the attic. It is also typically less expensive than faced insulation, making it a good choice for homeowners on a budget.
Boards for loft flooring can be installed on top of insulation using plywood, wood, or plastics. Installing loft flooring and boarding over insulation helps save space, prevent heat loss, and increase home value.
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.
The IRC requires either a Class I or II vapor retarder on the interior side of frame walls in climate zones 5, 6, 7, 8 and Marine 4 (see climate zone map).
Installing a vapour barrier is optional when utilising ROCKWOOL solutions, as stone wool insulation is water repellent. However, adding a vapour barrier can help to prevent additional moisture coming through the ceiling space.
A plastic vapor barrier is best installed in a vented attic within climates with over 8,000 heating degree days. If your climate does not meet these conditions, a vapor retarder like latex ceiling paint will work well for your house if you are in all other climates except the hot-dry and hot-humid.
The current government recommendation is 270mm which provides a U value (watts / meter2 / degree C) of 0.15. Insulation of 200mm provides a U value of 0.18 and 100mm provides 0.28. That means that putting 100mm of insulation in a house with no insulation will make a 72% increase in efficiency.
If you consider to insulate your attic or crawlspace it will be hard to beat the quality of spray foam because it outperforms other insulation materials in terms of R-value, resistance to mold and moisture damage, durability, air sealing, and versatility.