The two questions most often raised regarding the thermal performance of changes to existing walls, roofs and windows are:
We’d like to add insulation to the building, but if we do, does it have to meet code?
If a window or storefront is being replaced, can we just match the existing so the look remains the same?
Every discussion regarding an upgrade to an existing building begins with this or similar concept: “Additions or alterations to any building or structure shall conform with the requirements of the code for new construction.” Of course every code has a chapter that describes the compliance path of an existing building renovation. This chapter is nearly silent on the thermal performance of the building, but is it entirely silent? One section on glass begins with this statement: “The installation or replacement of glass shall be as required for new installations.” One might conclude that all alterations to walls, roofs and windows are to meet the same requirements as new installations. Not so fast—just as in the creation of construction documents, all of the codes have to be considered. One book, chapter or statement rarely stands alone.
The energy code or referenced standard like ASHRAE 90.1 carry the provisions for thermal performance. These codes have very carefully worded passages that allow the alterations to maintain or improve the building’s thermal performance without mandating meeting the prescriptive requirements for new construction.
Examples (not excerpts, but simplified explanations):
Walls: Insulation may be added to a wall without bringing its thermal performance up to current code unless the wall alterations accommodate the full upgrade. So if your alteration includes the furring, the full void is to be filled with insulation, but you don’t need to increase the depth of the furring to full code compliant thermal performance.
Roofs: Insulation may be added to a roof without bringing its thermal performance up to current code, but if the insulation is being replaced, it is to meet code, unless it is “impossible,” such as the structure is unable to support the weight of the new insulation. Glazing: Individual panes of glass, entire windows, and even storefronts may be replaced with glass that meets the existing thermal performance as long as the area being replaced or upgraded falls below 25% in one version of code. Another code uses a more complicated calculation.
The key is to realize that all codes are meant to work as a full unit and you need to consult all applicable sections before concluding the path to compliance. One must also be careful to keep in mind that there are also still many different codes, referenced standards, jurisdictions, and periodic updates of all the aforementioned. Each instance needs to be individually considered.