Building envelope can be described as the exterior enclosure of a building structure which seperartes the interior conditioned space.These include the following 

1. Roof: the upper most covering, providing protection against external agents like rains,wind,sunlight etc.
2. Fenestration: comprising of transparent,translucent elements of a building.These are structures which allow sunlight into the interior spaces.
3. Walls: opaque structures which are inclined from the horizontal.
4. Flooring: acts as the separation or bifurcation between the interior and the ground below.

This building envelope is an essential component in determining the flow of heat in a building.Heat enters the building through a combination of conduction,convection,radiation processes,facilitated by the building envelope.Each of these components have a different bearing on the overall thermal effect, felt in the interiors.

Roofs

As mentioned above, is the top most cover. In urban settlements, roof surfaces consist of approximately 25-30% of all surfaces.Roofs,usually made up of concrete, is the first line of defence against the overhead sun.The direct, incident solar rays heating up a building is absorbed primarily by this structure.The solar heat gain and thermal emissivity of the roof surface is dependent upon the thermal characteristics of the building material.Absorptive heat,mainly on account of conduction is experienced more in tropical countries like India.This conductive heat,travels to the other parts of the building envelope,thereby raising the overall temperatures.Convective heat from the roofs can also accumulate on the top floors of a building, increasing cooling loads of air conditioning,fan operated cooling structures. According to India Cooling Action Plan Report, roofs are responsible for 30% of the heat build up during urban heat island phenomenon.Roofs constitute an essential part of the building matrix,especially when it comes to heat management.

Fenestration:

These are the structures controlling  the flow of light in the interior of the building envelope.Unlike the roof, which is an opaque structure, these have transparent or translucent structures, regulating the entry/exit of heat and wind.This is an arrangement of the design of windows,doors,skylights and other openings.The amount of solar heat gain, thermal loss/gain , natural ventilation have a bearing on the heat consumption of a building. These structures require strategic placement, innovative materials, crafted engineering for optimum thermal management.

Walls: 

Another opaque structure preventing the majority of external agents from interacting with the interiors of the building.Just like the walls, it stands against the direct incoming rays of the sun.The placement of the walls is a strategic factor in heat management.Following the apparent movement of the sun, the wall structures facing east and south are exposed to maximum duration of sunlight.Similar to the roofs, the walls are also made up of concrete.The walls,depending on the thermal properties of the material used in construction, absorb heat through a combination of conductive and convective processes.The thermal emissivity of the concrete used,length of sun exposure,use of protective cover( external paint), all have a bearing in the temperatures found in the interior of buildings.

Flooring:

This is done to seperate the interior of the building from the ground on which it stands.The ground is a great absorber of heat as well as a reflector. The choice of the flooring material is therefore highly crucial. Depending on the need, that is, heat gain in colder climates, heat reduction in tropical climates, materials are to be chosen. In cold climates, due to excessive loss of heat, the flooring is made up of materials like wood,which by nature is a poor conductor of heat.The absorption of heat in warmer climates, necessitates the construction of a floor which conducts minimum heat from the floor below.

These components of a building envelope, together determines the heat management in a building.