Define the boundary to the environment
The envelope separates heated space from outdoor air, the ground and unheated areas. It includes walls, roof, floor, windows and doors together with their connections. First draw this boundary consistently on plans and sections. A garage or basement does not become part of the conditioned volume merely because it is structurally attached to the house.
Several control functions coexist
Insulation limits heat transfer, the airtight layer limits air leakage, and water protection addresses rain or ground moisture. These are distinct functions. One material may perform more than one, but that capability must be established for its particular use. At roof, façade and foundation transitions, each control layer needs continuity while the assembly retains an appropriate drying route.
Geometry changes the energy question
With other conditions equal, more exposed enclosure area increases potential heat exchange. Glazing introduces solar gains and daylight as well as heat loss and overheating risk, so U-value alone cannot determine its suitability. Compactness, orientation, shading and window area belong in the same energy model. Annual demand still does not replace a separate assessment of peak heating requirements.
Coordinate in a useful sequence
Fix the conditioned volume first, then the assemblies, junctions and service penetrations. Changing a window or moving a ventilation duct may alter an agreed connection. Acceptance needs drawings and inspection of concealed layers, rather than only an insulation thickness in the estimate. Explore the individual wall, roof and foundation solutions while retaining the same envelope boundary throughout. That common reference makes conflicts between separate drawing packages easier to detect before construction.