A building envelope refers to the exterior elements of a building, inclusive of walls, windows and exterior doors, subfloors and roofing. It protects the house and its inhabitants from the elements and also contributes to keeping the building structurally sound. The structures also maintain a level of climate control inside and provides a finish for the building, thus material choice is important for presentation stability and durability of the architecture.
To achieve a building that adequately meets these three needs: finish, support and control, the environment in which the building in located must be assessed. Climates that experience extreme heat and humidity, rainfall, cold weather, or see seasonal changes, must place increased consideration into creating an effective building envelope than those in more temperate climates. Ideally, a building envelope must include, a solid structure, drainage planes, a thermal barrier, a vapour barrier and an air barrier.
An awareness of environmental issues and the rising cost of energy has placed increased focus on developing and implementing innovative, sustainable designs and materials to establish ‘passive’ ways of keeping a building thermally and structurally protected with minimal effect on the environment.
Tight and Loose
There are two categories of building envelope: tight and loose.
A loose building envelope allows air to naturally flow through space, resulting in a well-ventilated building. Loose envelopes are suited to continually temperate climates, otherwise, drafts can occur and moisture can gather, which creates damp, mould and mildew. Unless properly executed, a loose building skin can leak heat and cool air, which if controlled by mechanical heating systems, can make for very high energy bills and an unsustainable structure that is subject to damage.
A tight building envelope is the most common choice in environments where the climate can vary dramatically, such as the United Kingdom. These structures are insulated to a level where they significantly restrict air leakage and are considered more sustainable, even when fitted with a mechanical heating system. Tight building envelopes aim to restrict as much air leakage as possible and as a result require consideration to be put into insulation such as wall, roof, and floor placement, thickness, and materials.
A thermal envelope differs from a building envelope in that it is a dedicated layer of insulation that specifically controls the flow and retention of heat within a building. For example, a thermal envelope may not be the roof of the building, which makes up part of the building envelope and is primarily tasked with water control, but rather, the floor of the attic, or the ceiling of the top floor of the house.
Thermal imaging or ‘thermology’ can identify issues related to the control of temperature within a building. Infrared cameras identify thermal irregularities on both the interior and exterior of the structure, such as, structural and water damage, air leakage, and thermal bridging - also known as a cold bridge, which is an area within a building’s structure that has a higher heat transfer than its surrounding elements, such as where there is a break in insulation or close proximity to an element with higher thermal conductivity.
The University of Sheffield, Credits: Graham Hogg
Air and water control
Building envelopes also protect inhabitants from the elements as controlling the airflow throughout a building is key to creating a comfortable, healthy interior environment. By avoiding a buildup of condensation between layers, and controlling energy loss through wind washing, or the passing of cold air though insulation layers, which can result in the loss of up to 20% of heat.
The function of a roof is to resist and drain water and although they can be designed to either be flat, although most flat roofs are angled at 10 – 15 degrees so as to encourage drainage, or pitched (sloping), and are insulated with a waterproof underlayment material such as plastic based underlayment or tar, or felt paper. Roofing can also be natural, such as slate, or thatch, but as these require specialist maintenance. An effective roofing material is usually a composite inclusive of tiles, glass, polycarbonate, cement, and waterproof polymers.
Walls receive less direct water than a roof, but still need to be constructed using low, or non-porous materials so as not to absorb moisture from the air or from the ground. There are three types of wall in a traditional build: drainage, barrier and surface.
Drainage walls are constructed using cavity walls, or two walls, known as building skins. In domestic builds, the exterior is skin is usually made from brick, glass, or wood, and the interior, from breeze block. The cavity is packed with insulation material, or ventilated to encourage drying. The purpose of a cavity wall is to allow any water that may have seeped through the exterior wall to drain or dry out before hitting the second skin.
Barrier walls are made from materials which absorb moisture, such as concrete or stone, but that do not allow moisture to fully penetrate and soak the wall. The surface walls are completely waterproof and usually coated with a sealant or waterproof cladding and fitted with a waterproof underlayment, known as a housewrap. A subfloor is important for moisture control as it creates a barrier between the ground, floor and walls, ensuring that water does not leak through. Ideal materials for a subfloor include wood, plywood, concrete, plastic and composite materials.