Residential Additions FAQs

Q: How does the code define an addition?

Although not specifically defined in the code, building codes typically define an addition as any increase in a building's habitable floor area, which can be interpreted as any increase in the conditioned floor area.

Additions include new construction, such as a conditioned bedroom or sunspace. Additions also include existing spaces converted from unconditioned to conditioned spaces (converting an existing porch to a conditioned sunroom).

Q: How does the addition comply with code?

The addition (the newly conditioned floor space) complies with the code if it complies with all of the applicable code requirements. For example, requirements applicable to the addition of a new room would most likely include insulating the exterior walls, ceiling, and floor to the levels specified in the code; sealing all joints and penetrations; installing a vapor retarder in unventilated framed walls, floors, and ceilings; identifying installed insulation R-values and window U-factors; and insulating and sealing any ducts in unconditioned portions or exterior components (walls, ceilings, or floors) of the new space.

Q: What are my compliance options for additions?

There are three approaches by which an addition can comply with the code:

The addition as defined above meets all code requirements. This approach does not require that the original portion of the building meet code requirements.
If the building combined with the addition complies with the code, the addition will also comply, regardless of whether the addition complies alone. For example, a sunroom that does not comply with the code is added to a house. If the entire house (with the sunroom) complies, the addition also complies.
Additions less than 500 ft² (46.5 m²) of conditioned floor area may meet the prescriptive envelope requirements in Table1. To use this table, the total area of windows, doors, and skylights cannot exceed 40% of the gross wall and roof area of the addition.

Table 1. Prescriptive Requirements for Additions and Replacement Window

-	Maximum	Minimum	Minimum	Minimum	Minimum	Minimum	Minimum
Climate Zone	Fenestration U-factor (a)	Ceiling R-value (b)	Wall R-value	Floor R-value	Basement wall R-value	Slab perimeter R-value and depth (c)	Crawl space wall R-value (d)
1-4	0.75	R-26	R-13	R-11	R-5	R-0	R-5
5-8	0.5	R-30	R-13	R-19	R-8	R-5, 2 ft	R-10
9-12	0.4	R-38	R-18	R-21	R-10	R-9, 2 ft	R-19
13-15	0.35	R-49	R-21	R-21	R-11	R-13, 4 ft	R-20
16-19	0.35	R-49	R-21	R-21	R-19	R-18, 4 ft	R-20

The area-weighted average U-factor for all windows, doors, and skylights in the addition must not exceed the fenestration U-factor requirement.
Floors over outside air must meet ceiling R-value requirements.
The slab R-value requirements are for unheated slabs. Add an additional R-2 for heated slabs.
The crawl space wall R-value requirements are for walls of unventilated crawl spaces only.
The maximum U-factor for replacement skylights is 0.5 in Zones 5-19.
The area-weighted average solar heat gain coefficient (SHGC) of all windows, glazed doors, and skylights cannot exceed 0.4 in Zones 1-7.

The 2000 IECC Prescriptive Package Worksheet can be used for this approach. (Requires Acrobat Reader)

Q: How do I enter an addition in REScheck?

You will need to include the following inputs for the addition (newly conditioned floor space) only:

Ceiling - gross area (ft²) and insulation R-value
Exterior walls - gross area (ft²) of new exterior walls and insulation R-value (the existing exterior wall(s) that will become interior wall(s) once the addition is built are to be considered interior walls and should not be entered as part of the addition wall area).
Windows/Doors - gross area (ft²) of windows and/or doors with U-factor from NFRC label or default table in the help section of REScheck.
Floor - gross area (ft²) of addition and insulation R-value. If the floor is a slab, the length of the exterior slab edge should be entered in linear feet.

Q: What options do I have if my addition does not comply?

Increase the insulation R-values in the ceiling, walls, and/or floor if possible.
Upgrade the windows (the lower the u-value of the window, the better the window).
Decrease the window area.
If feasible, upgrade insulation levels in the existing part of the house (i.e. the attic). If you try this option, you will need to demonstrate compliance for the entire house not just the addition.