Vapour Barriers and Air Tight Envelopes

What is a building envelope?

A building envelope separates the interior of a home from the exterior elements. It includes the exterior walls, windows, doors, mechanical openings (exhaust vents, hose bibs and so on), the roof, and the floor or foundation that connects the home to the ground. Within it sits the thermal envelope: the insulation, vapour barrier and air barrier around the interior of the home.

What is a vapour barrier?

A vapour barrier is most commonly a polyethylene plastic sheet that resists the passage of moisture through a wall assembly. It is installed on the warm side of the insulation (the inside of the wall), behind the interior finish such as drywall. It is fastened to the structure with staples at the framing members, and sealed with acoustical sealant around windows and other openings. Red Tuck Tape joins sheets together and seals around junction boxes and exhaust ducts. At a minimum, building codes across Canada require a 6 mil (0.152 mm) polyethylene vapour barrier.

Why does a vapour barrier matter?

In cold climates we heat the air to keep occupants warm. Cooking, dishwashing, laundry, bathing and even breathing add water vapour to that air. The vapour is constantly trying to escape the home, driven by two forces: vapour pressure (diffusion) and, more importantly, air movement (exfiltration).

As the vapour escapes through cracks and holes in the envelope, it meets cold air and condenses into liquid water or frost. Too often that water ends up inside the wall cavity, where it damages the wood structure, insulation, sheathing and cladding. Wet insulation also loses much of its thermal resistance (R-value), which means more heating and more energy use.

Why are polyethylene vapour barriers only somewhat effective?

Polyethylene was a real improvement for cold-climate construction, but it has flaws, and most of them come down to human error: inadequate detailing by designers, poor installation, and too few inspections.

Designers. On a typical spec house built from a plan-book design, the vapour barrier detailing amounts to one note and, with luck, one typical wall section. The note reads “6 mil poly vapour barrier”. That single line hands the responsibility to the installer.

Installers. Installing a vapour barrier takes a staple gun (which makes holes), Tuck Tape and acoustical sealant. The result is often a patchwork of taped sheets, tape wrapped around wires and outlet boxes, and sealant that is smeared and not properly adhered.

Once the drywall goes on, it helps seal the barrier to the framing, but it still leaves a lot to be desired. A drywall screw that misses a stud punctures the barrier. A 4′ × 8′ sheet needs a screw about every 12 inches, so a whole house takes a very large number of screws, and the installer has to be accurate to puncture none.

Inspections. Some building inspectors review vapour barriers very well, but their main concern is fire, life and safety. On many homes, nobody with building science training reviews the barrier before the walls close, and the inspection falls to the builder. Most Canadian homes are built on spec from purchased plans and are never reviewed by a designer.

Even a well-installed barrier that passed inspection and a blower door test is vulnerable once the owners move in. The nail used to hang a picture on an exterior wall punctures it.

This is widely acknowledged in the industry, yet we keep building this way. We think we have become complacent. North American energy costs have not risen as sharply as they have in Europe, but we expect them to keep rising, and every leaky envelope locks in higher energy bills year after year. Better homes are possible now.

An example of poor installation

We have often arrived on site to find unsealed openings in vapour barriers, unsealed barriers around mechanical penetrations such as exhaust ducts, and in some cases no vapour barrier at all.

In one case in 2010, a builder had to pull down two full layers of drywall because an inspection was missed before the drywall went up. The barrier had been installed, but it was poorly sealed with numerous holes, and the wrong insulation had been installed as well. We could not take their word for it. On a construction job the builder's main concern is the schedule, because finishing faster means more profit.

We are not pointing blame. Not every vapour barrier is installed poorly. But when better and simpler practices exist, it is hard to see why we keep building to the minimum requirements, which cost homeowners more each year.

Why does an airtight building envelope matter?

An airtight envelope matters for two reasons: it reduces overall energy loss, and it makes the envelope durable. No envelope can be 100% airtight, because windows, doors and mechanical openings are needed. But we can take steps to keep air leakage very low.

Polyethylene vapour barriers have been in use since the late 1970s. Nearly every other technology we use daily has changed since then. Would you go back to a first-generation mobile phone, the kind with a carrying case, a large antenna and a cigarette lighter plug? Our homes deserve the same progress. For more on how airtightness is measured, see What Airtightness and Energy Numbers Actually Mean.

What do we recommend instead?

Building science has improved enormously. The system we recommend has not become more complicated; it has become simpler, with the human error designed out.

It starts with a service cavity for plumbing and electrical conduit. The vapour barrier sits behind that cavity, away from the penetrations that would otherwise puncture it. In a conventional house, the 2×6 structural wall sits behind the protected barrier and is insulated. We do not recommend insulating the service cavity, because the dew point can end up inside the house, unless a vapour-permeable smart vapour retarder is used.

In a Passive House, the structural wall can serve as the insulated cavity, with the vapour barrier on the outside of the stud wall because of the large amount of additional insulation outside it. 5/8″ plywood or OSB with taped seams, using a high-performance tape such as Pro Clima Tescon Vana, gives very good airtightness when it is properly detailed at windows, corners and floors. The extra exterior insulation keeps the dew point away from the barrier.

To see how this fits our approach, read the FAQ, or see Passive House and the BC Energy Step Code.