What is a Building Envelope


A building envelope is the separation between the interior and the exterior environments of a building . It serves as the outer shell to protect the indoor environment as well as to facilitate its climate control . Building envelope design is a specialized area of architectural and engineering practice that draws from all areas of building science and indoor climate control.

Building envelope design includes four major performance objectives:

  • Structural integrity
  • Moisture control
  • Temperature control
  • Control of air pressure boundaries of sorts

Control of air includes air movement through the components of the building envelope (interstitial) itself, as well as into and out of the interior space, which affects building insulation greatly.

The physical components of the envelope include the foundation , roof , walls , doors and windows . The dimensions, performance and compatibility of materials, fabrication process and details, their connections and interactions are the main factors that determine the effectiveness and durability of the building enclosure system.

Common measures of the effectiveness of a building envelope include physical protection from weather and climate (comfort), indoor air quality (hygiene and public health), durability and energy efficiency. In order to achieve these objectives, all building enclosure systems must include a solid structure, a drainage plane, an air barrier, a thermal barrier, and may include a vapor barrier. Moisture control is essential in cold climates.

What is a Vapour Barrier?

A vapour barrier is an impermeable membrane that blocks the flow of air through the building envelope. A vapor barrier is an essential part of the building envelope. Because the purpose of a vapor barrier is not obvious, this important component is often omitted or installed incorrectly. The main purpose of a vapor barrier is preventing the passage of the water vapor that is contained in air. Vapor barriers and the insulation affect each other. They must both be installed so that they interact beneficially rather than harmfully.

In Orillia, the climate is cold, so the vapour barrier is installed on the interior of the house. This prevents moisture from entering the wall cavity, passing through the insulation where it meets the cold wall. Moist warm air, when coming into contact with cold, will create moisture, this is the Dew Point. The vapour barrier, 6 mm in Ontario, is required to prevent moisture in your walls where mould and moisture damage will take place. In tropical climates the reverse would be true. You would want to prevent moist warm air coming into your wall cavity from the exterior and contacting air conditioned cool air, creating moisture in your insulation.

House Wrap – What it Does

House wrap is a breathable membrane installed directly over your home’s plywood or OSB (oriented strand board) sheathing, underneath the exterior finish.

House wrap has been around for over 30 years — since the 1970s energy crisis. It became a hot commodity back when R-2000 homes were introduced to the market in Canada in the 1980s and is now required by code in many cities.

The most commonly used house wraps are made of threads of vinyl on the inside surface of a synthetic sheet. There are also newer types, in a spray-on elastomeric version, that aren’t as common in use.

House wrap works in several important ways. The membrane stops direct moisture, like rainwater from penetrating the plywood or OSB sheathing on the outside of your home, yet allows water vapour from within to migrate outside. Don’t forget, we create a lot of moisture inside by living in the house — breathing, showering, doing the laundry, cooking. Moisture vapour that gets into your wall cavities must be allowed to escape back out through the house wrap, or it will rot the studs and wood inside and lead to mould.

House wrap also stops outside air from penetrating the home. But it is not airtight: It is a breathable membrane. House wrap has no insulation value, but it improves energy efficiency by preventing air leakage. So, it helps retain heat in winter and retains cool, air-conditioned air in summer.

The synthetic house-wrap system replaces the old-fashioned tarpaper membrane used on some older homes, though many older homes only had sheathing without tar paper. Wrap is much more effective and durable and comes in larger rolls than tar paper, making this step of the building process much more cost-effective for a builder.

House wrap comes in a nine-foot-long roll, which helps to avoid seams and gives you a continuous sheet with minimal seams and overlaps. It takes two men less than a day to wrap a small house, with only a stapler and a roll of Tuck Tape. But it isn’t a job for just anyone. It takes experienced contractors to do the job properly and efficiently.

There must be a minimum six-inch to 12-inch overlap at all joints. Every — and I mean every — hole and overlap is sealed tight with tape. If the taping isn’t done properly, the system won’t be effective and you will have air movement and the possibility of water damage.

Details like wrapping the membrane around window and door openings are important, too. Taking the membrane into the inside edge of the frame, which will be covered by window and door trim, guarantees an airtight envelope — providing your contractor does a good job of spraying foam insulation around all windows and doors. The same applies for any accidental tears or holes.

Let’s face it, on a busy construction site, accidents happen.

Don’t let your contractor start covering the house wrap with the external sheathing until you are satisfied every opening is taped over.

Once the house is properly wrapped up in its new windbreaker, it’s all right to apply the finishing layer, the outside coat.

Without this breathable membrane, moisture would collect on the underside of your wood, brick, vinyl or metal siding, as well as on the outside surface of the plywood. In time, this can lead to mould and wood rot, and very expensive repairs.

Foamed Insulation

Buildings using foamed insulation are not required to have vapour barriers installed as the foam is considered to meet the requirements of the Ontario Building Code.

Problems with Building Envelope

The majority of water intrusion through the envelope occurs at material transitions and changes-in-plane. Doors, windows, skylights, electrical boxes, plumbing penetrations, HVAC penetrations and sloppy workmanship are all common causes of breakdowns in your building envelope.

Written by Roger Frost

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