Building envelope basics

The building envelope is everything that separates the inside of a building from the outside: the roof, walls, windows, doors and foundation. Its job is to control four things: liquid water, air, water vapour and heat. Most envelope problems come from a gap in one of those control layers, often at a joint where two parts of the building meet.

The four control layers

Building scientists describe the envelope as a set of layers, each controlling one thing. A single material can do more than one job, but each function has to be continuous around the whole building.

Water control

Keeps rain and snowmelt out. Cladding sheds most water, but some always gets behind it, so modern walls use a water-resistive barrier behind the cladding and often a drainage gap (rainscreen) to let that water drain and dry. Flashings at windows, doors, roofs and floor lines direct water back outside. In BC's coastal climate in particular, rainscreen wall design became standard practice after widespread envelope failures in the 1980s and 1990s.

Air control

The air barrier stops air leaking through the envelope. Air leakage wastes energy, causes drafts, and, in cold climates, carries moisture from indoors into walls and roofs where it condenses. An air barrier can be a membrane, sealed sheathing, sealed drywall or concrete, as long as it is continuous and joined at every transition. See air leakage around windows and doors.

Vapour control

Water vapour moves by diffusion from warm, moist areas toward cold, dry ones. In Canadian heating climates, a vapour retarder is normally placed on the warm side of the insulation to limit how much moisture reaches cold parts of the wall. Vapour control matters, but air leakage typically carries far more moisture into walls than diffusion does.

Thermal control

Insulation slows heat flow. Its effectiveness depends on being continuous. Studs, steel framing, concrete slab edges and balcony connections can all act as thermal bridges that let heat bypass the insulation, creating cold spots where condensation and mould can form.

Where envelopes fail

Most failures happen at transitions and penetrations, where different trades, materials and layers meet:

• Window and door perimeters, sills and heads.
• Roof-to-wall junctions, parapets and roof edges.
• Balconies and decks tied into the wall.
• Floor-line transitions and changes in cladding.
• Penetrations for pipes, vents, ducts, lights and signage.
• Foundation-to-wall junctions and below-grade waterproofing.
• Sealant joints that have cracked, debonded or reached the end of their life.

Signs of envelope problems

• Water stains, leaks or efflorescence on interior or exterior walls.
• Peeling paint, swollen trim, or rot at window sills and cladding.
• Spalling or cracking masonry and concrete. See freeze-thaw damage.
• Condensation or frost on the inside of windows and walls in winter.
• Drafts, cold rooms and uneven temperatures.
• Icicles and ice dams at roof edges, which can point to heat loss into the attic.
• Musty smells or mould near exterior walls.
• Rising heating costs with no change in use.

Assessing and maintaining the envelope

A building envelope assessment usually combines a review of drawings and maintenance history, a visual inspection inside and out, and targeted testing. Tools can include moisture meters, thermal imaging to find heat loss and wet areas, water testing at suspected leak points, and air leakage testing. Sometimes small exploratory openings are needed to see what is behind the cladding.

Routine maintenance that pays off:

1. Inspect sealant joints and flashings every year or two and replace failed sealant.
2. Keep roof drains, scuppers and eavestroughs clear.
3. Repair damaged cladding promptly so water does not get behind it.
4. Keep landscaping and soil away from cladding and below the start of the wall.
5. Record leaks and repairs so patterns show up over time.

A typical case: tracing a recurring window leak

A four-storey office building has water staining below several windows on its windward side after heavy rain. The tempting fix is to run new sealant around the outside of every window. A better approach works through the control layers:

1. Map the leaks. Record which windows leak, on which floors and elevations, and in what weather. Leaks concentrated on one side during wind-driven rain point at the water control layer rather than at plumbing or condensation.
2. Look at the details. Check sill flashings, end dams, sealant joints and how the window ties into the wall's water-resistive barrier. Compare a leaking window with a dry one of the same type.
3. Test, do not guess. Controlled water testing at one part of a window at a time, starting low and working up, can isolate whether water enters through the glazing, the frame corners or the joint between window and wall.
4. Open up if needed. A small exploratory opening at a typical leaking window shows whether the sill flashing is missing or sloped back toward the wall, and how far damage has spread into the sheathing.
5. Repair the cause. If the problem is a missing sill flashing, sealant alone will only delay the next leak. The repair restores the flashing and its connection to the barrier, then the other windows of that type are checked.

Common mistakes in envelope repairs

• Sealing the bottom of cladding or window frames shut, which traps the water the drainage gap is meant to release.
• Adding interior insulation to an older masonry wall without assessing how it will change wetting and drying.
• Installing a second vapour barrier, which can trap moisture between two impermeable layers.
• Repairing one window or one section of wall without connecting the new work to the existing air and water barriers.
• Treating condensation as a leak, or a leak as condensation, because the cause was not confirmed.

Envelope requirements are set in the National Building Code of Canada and provincial codes, which differ in details, and BC and Ontario have their own building codes. When planning retrofits or repairs, work with a qualified envelope consultant or engineer and check the code that applies to you.

How ehZee can help

ehZee's commercial building inspections include the roof, walls, windows and foundation, looking for the leaks, condensation and failures described here. We document what we find and help you prioritize repairs so small envelope issues are dealt with before they damage the structure.

Common questions