Crawl space insulation

[mmarcley]

I'm in Wilmington, NC. I am adding a 30 x 30 addition to my home. My current crawl space is sealed and uninsulated. If I add a 2" foam board around the foundation, do I need to insulate the floor?

[Jerry Peck - Codeman]

Hi Mark,

Based on the North Carolina Residential Code, with Wilmington being located in Brunswick County, Wilmington is located in Climate Zone 6.

(underlining and bold are mine)
- SECTION N1101 GENERAL
- - N1101.1 Scope.
- - - This chapter sets forth energy–efficiency–related requirements for the design and construction of buildings regulated by this code.
- - - - Exception:
- - - - - Portions of the building thermal envelope that do not enclose conditioned space.

- N1102.1 Thermal performance criteria.
- - The minimum required insulation R-value or maximum required U-factor for each element in the building thermal envelope (fenestration, roof/ceiling, opaque wall, floor, slab edge, crawl space wall and basement wall) shall be in accordance with the criteria in Table N101.4.2.5 of the International Energy Conservation Code.
- - Residential buildings, Type A-1, with greater than 15–percent glazing area; residential buildings, Type A-2, with greater than 25–percent glazing area; and any residential building in climates with HDD equal to or greater than 13,000; shall determine compliance using the building envelope requirements of the International Energy Conservation Code.
- - TABLE N1102.1 SIMPLIFIED PRESCRIPTIVE BUILDING ENVELOPE THERMAL COMPONENT CRITERIA MINIMUM REQUIRED THERMAL PERFORMANCE (U-FACTOR AND R-VALUE) shows floors in Climate Zone 6 as requiring R-13 minimum insulation and crawl space wall in Climate Zone 6 as requiring R-7 minimum insulation.

For clarity, let us first start with N1102.1, which states: "The minimum required insulation R-value or maximum required U-factor for each element in the building thermal envelope (fenestration, roof/ceiling, opaque wall, floor, slab edge, crawl space wall and basement wall) shall be in accordance with the criteria in Table N101.4.2.5 of the International Energy Conservation Code." Being as that does not specify the year of the IECC, and neither my 2003 nor my 2006 IECC has any section resembling N anything, but that the IRC (International Residential Code) does have N sections in its Chapter 11 Energy Efficiency chapter, I will post the latest closest section from the 2003 and 2006 IRC for comparative and explanatory purposes.

From the 2003 IRC.
- N1102.1.4 Floors. The required R-value in Table N1102.1 shall apply to all floors, except any individual floor assembly with over 25 percent of its conditioned floor area exposed directly to outside air shall meet the R-value requirement in Table N1102.1 for "Ceilings."
- N1102.1.7 Crawl space walls. Where the floor above the crawl space is uninsulated, insulation shall be installed on crawl space walls when the crawl space is not vented to outside air. The required R-value in Table N1102.1 shall be applied inside of the crawl space wall, downward from the sill plate to the exterior finished grade level and then vertically and/or horizontally for at least an additional 24 inches (610 mm). The exposed earth in all crawl space foundations shall be covered with a continuous vapor retarder having a maximum permeance rating of 1.0 perm [(57 mg)/(s · m2 · Pa)], when tested in accordance with ASTM E 96.

N1102.1.7 says that if you do not choose to insulate the floor above the crawl space that you must insulate the crawl space walls to the values shown in Table N1102.1, which shows the crawl space walls as requiring minimum R-7 insulation. N1102.1.7 (above) states that this insulation is to is to be installed from the sill plate down to the exterior finished grade level "and then vertically and/or horizontally for at least an additional 24 inches". The earth in the crawl space also needs to be covered with a continuous vapor barrier (a vapor barrier which meets the requirements stated in N1102.1.7).

The North Carolina Residential Code (NCRC), from what I can find, is not as clearly stated as the IRC, the NCRC also implies that both the floor and the crawl space walls would need to be insulated, except that you could apply this exception and not insulate the crawl space walls as the crawl space would now be outside the thermal envelope:
- N1101.1 Scope.
- - This chapter sets forth energy–efficiency–related requirements for the design and construction of buildings regulated by this code.
- - - Exception:
- - - - Portions of the building thermal envelope that do not enclose conditioned space.

The code makes that a slightly complex issue, however, to put it into simple terms ... there are two choices:
a) Insulate the floor to the required R-value.
b) Insulate the crawl space walls to the required R-value and seal off the floor with a continuous vapor barrier.

I have to wonder about the reason that the crawl space walls, without the floor being insulated, are not required to have the same R-value for other exterior walls ... R-13.