Where is the vapor barrier on a roof?

Author: Holden Bosco  |  Last update: Sunday, September 7, 2025

In the context of flat roofs, these membranes are usually installed directly over the roof structure and under the insulation layer (at its warm side). Properly installed Vapour barriers also contribute to the overall energy efficiency of a building.

Where does the vapour barrier go on a roof?

It's the first line of defence against high vapour pressure air entering the roof structure and causing interstitial condensation. Because of this, the VCL always sits at the warm side of the insulation.

Where is a vapor barrier normally located?

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.

Where do you place a vapor barrier on a ceiling?

Install the vapor barrier on the warm side of the insulation.

Where should a vapour barrier be placed?

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.

You're Using Vapor Barriers Wrong | What You NEED To Know

What is the most ideal location for a vapour barrier?

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.

How to install a vapor barrier on a roof?

And here's how it's done:
  1. Measure the rafters. ...
  2. Cut the insulation material to size. ...
  3. Wedge the insulation material between the rafters. ...
  4. Attach the sealing tape to the rafters. ...
  5. A NOVUS electric tacker makes light work of stapling the vapour barrier in place. ...
  6. Seal the cut edges with adhesive tape to make them airtight.

Where to install a vapor barrier?

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.

Where does the vapor barrier go in an attic?

Tip 3: Determine the most vulnerable side – When installing insulation with a vapor barrier, the vapor barrier should usually be applied on the side of the insulation that's closest to where it will be exposed to the most warmth and moisture.

Do you need a vapor barrier between a ceiling and an attic?

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.

What does a vapor barrier look like?

Your home's vapor barrier, unlike an air barrier which prevents air from permeating through a structure, is generally a thick plastic sheeting that is placed in your home's crawlspace. In Oregon and Washington, in order for this plastic sheeting to be code, it should be black, and no less than 6mm thick.

Should you put plastic over insulation in a ceiling?

Exposed insulation is at risk of catching fire if not covered properly because it is extremely flammable. Installing a fire retardant plastic sheeting over insulation slows the fire damage rate in the case of an adverse event.

Does the vapor barrier go in front or behind insulation?

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.

Where does the radiant barrier go on a roof?

In a new house, an installer typically drapes a rolled-foil radiant barrier foil-face down between the roof rafters to minimize dust accumulation on the reflective faces (double-faced radiant barriers are available).

When should you not use a vapor barrier?

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.

Can you use roofing felt as a vapour barrier?

Roofing felt is a versatile material that can be used for a variety of applications in the construction industry. From providing a protective layer over flat or sloping roofs to acting as a moisture barrier, roofing felt has become an essential component in any roofing system.

Where does the vapour barrier go on a warm roof?

It can be made from a wide array of materials, but the most common options are polythene membrane (at least 125 microns thick) and foil backed plasterboard. This barrier is always applied on the warm (internal) side of the insulation on walls and in lofts.

Where should vapor barriers always be installed?

In cold climates, the barrier goes on the warm (interior) side of the insulation. In hot, humid climates, it's placed on the cool (exterior) side. Under flooring: When installing wood flooring over concrete, a moisture barrier protects the wood from moisture damage.

Does roof insulation need a vapor barrier?

(Source: NRCA.) In the event you are erecting a “cold building” (such as a refrigeration building) that stays at 32 F (0 C) degrees or less inside, you will need a vapour barrier on the outside of the insultation to prevent warm exterior air from entering and potentially damaging the roof's insulation. (Source: NRCA.)

Where is the vapour barrier placed?

In humid regions where warm-weather cooling predominates within buildings, the vapor barrier should be located toward the exterior side of insulation. In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapor barrier may not be necessary at all.

Where should attic vapor barrier be placed?

Attics and Ceilings: In attics, vapor barriers can be placed on the floor to prevent moisture from rising into the attic space from the living areas below. In certain designs, especially in colder climates, a vapor barrier may also be used in ceilings to prevent moisture from passing into the attic.

How do you install a vapor barrier on a roof?

If you use permeable insulation material such as sheep's wool, rockwool or fibreglass, the VCL is installed to the warm side of the insulation. The VCL is stapled to the timber studwork with the insulation wedged between the timber studs. Any joints will need to be sealed with Single Sided Foil Tape.

Is roofing underlayment a vapor barrier?

Although most synthetic underlayments are vapor barriers, there are exceptions.

Is Tyvek 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.

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