The following information is from articles and references on water, air, and thermal envelope:
The building envelope is all of the elements of the outer shell that maintain a dry, heated, or cooled indoor environment and facilitate its climate control. Building envelope design is a specialized architectural and engineering practice area that draws from all areas of building science and indoor climate control.
The many functions of the building envelope can be separated into three categories:
- Support (to resist and transfer structural loads)
- Control (the flow of matter and energy of all types)
- Finish (to meet human desires on the inside and outside)
The control function is at the core of good performance and, in practice, focuses, in order of importance, on rain control, air control, heat control, and vapor control.
Water Vapor Control
Control of rain is fundamental, and numerous strategies exist, namely, perfect barriers, drained screens, and mass/storage systems.
One of the primary purposes of a roof is to resist water. Two broad categories of roofs are flat and pitched. Flat roofs slope up to 10° or 15° but are built to resist standing water. Pitched roofs are designed to shed water but not resist standing water, which can occur during wind-driven rain or ice damming. Typically, residential, pitched roofs are covered with an underlayment material beneath the roof covering material as a second line of defense. Domestic roof construction may also be ventilated to help remove moisture from leakage and condensation.
Walls do not get as severe water exposure as roofs but still leak water. Types of wall systems regarding water penetration are barrier, drainage, and surface-sealed walls. Barrier walls are designed to allow water to be absorbed but not penetrate the wall and include concrete and some masonry walls. Drainage walls allow water that leaks into the wall to drain out, such as cavity walls. Drainage walls such as rain-screen and pressure equalization wall systems may also be ventilated to aid drying. Sealed surface walls do not allow water penetration at the exterior surface of the siding material. Most materials will not remain sealed over the long term, and this system is minimal. Still, ordinary residential construction often treats walls as sealed-surface systems relying on the siding and an underlayment layer, sometimes called house wrap.
Moisture can enter basements through the walls or floor. Basement waterproofing and drainage keep the walls dry, and a moisture barrier is needed under the floor.
Main article: Air barrier
Control of airflow is important to ensure indoor air quality, control energy consumption, avoid condensation (and thus help ensure durability), and provide comfort. Control of air movement includes flow through the enclosure (the assembly of materials that perform this function is termed the air barrier system) or through components of the building envelope (interstitial) itself, as well as into and out of the interior space, (which can affect building insulation performance significantly). Hence, air control includes the control of wind washing (cold air passing through insulation) and convective loops, which are air movements within a wall or ceiling that may result in 10% to 20% of heat loss alone.
The physical components of the envelope include the foundation, roof, walls, doors, windows, ceiling, and their related barriers and insulation. The main factors determining the effectiveness and durability of the building enclosure system are the dimensions, performance, compatibility of materials, fabrication process, details, connections, and interactions.
Standard 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. 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 all climates, but cold and hot-humid climates are incredibly demanding.
The thermal envelope, or heat flow control layer, is part of a building envelope but may be in a different location, such as a ceiling. The difference can be illustrated by understanding that an insulated attic floor is the primary thermal control layer between the inside of the house and the exterior. At the same time, the entire roof (from the surface of the shingles to the interior paint finish on the ceiling) comprises the building envelope.
Building envelope thermography involves using an infrared camera to view temperature anomalies on the interior and exterior surfaces of the structure. Analysis of infrared images can help identify moisture issues from water intrusion or interstitial condensation.
The main physical problem is water infiltrating into buildings’ exterior building envelope (walls and roofs), usually through a weather barrier (e.g., building paper or air barrier membrane) designed to prevent water drops from passing through but allow water vapor through. However, problems in design, installation, and damage during construction can allow water to penetrate the walls. This causes rot and delamination of exterior wall cladding and sheathing, rusting in metal wall studs, rot in the wood structure, saturation of batt insulation, and development of mold and spores inside the walls and building interior. The construction failures ranged from minor to significant failures of the structural integrity of the building. Some buildings became unhealthy for occupants. Most of these buildings are low-rise, 3-4 story buildings constructed of wood-frame construction, some with steel, concrete, and metal stud construction types, including high rises.
The individual owners of the condominium units own most buildings that have experienced these problems in B.C. However, commercial properties and public schools have also been impacted. Many of the properties are homeowners who have been faced with correcting a problem they did not create nor by a contractor they had hired; they purchased the units either from a previous owner, a developer, or a developer/contractor. Typical repair costs are in the tens or even hundreds of thousands of dollars, resulting in significant hardship, bankruptcies, and lawsuits against the developers, contractors, architects, and others involved in the original construction and maintenance of the buildings.
Approximately 45% of the 159,979 condominium strata units and 57% of the 700 school buildings constructed in B.C. between 1985 and 2000 were found to have envelope leak problems. It was reported in 2002 that 90% of 3-4 story units built have severe problems and that some have undergone envelope repairs twice and thrice. In 2008, it was estimated the cost to repair the damage to schools alone would be nearly $400 million.
Pacific Exteriors NW has performed thousands of commercial and residential inspections. We have assisted thousands of property owners here in the Pacific Northwest in discovering and repairing hidden issues from new build issues to construction defect repair, older distressed multi-family or commercial properties where capital investment and return is key, or even due diligence to understand the property before selling or purchasing an apartment or commercial or mixed-use building.
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