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U0 (Btu/h-sqft-ºF)

Heating Degree Days (HDD)

Maximum U0-Factor

0 - 2,500 2,501 - 3,900 3,901 - 6,000 6,001 - 16,000 16,001 - 16,500 > 16,500

0.05 - (HDD x 0.0000056) 0.036 0.036 - [(HDD - 3,900) x 0.00000476] 0.026 0.026 - [(HDD - 16,000) x 0.000002] 0.025

0.1

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

  • 0

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2

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4

5

16

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Figure 5. Roof/ceiling assembly U-value (excerpted from ICC 2003)

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7 8 9 10 11 12 13 14 Annual Heating Degree Days, Base 65

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  • No external shading at any time from roof projections, awnings, adjacent buildings, trees, etc. Basic architectural features such as attached garages and enclosed porches shall be included in the Benchmark model, but the model shall not include window shading effects from these features.

  • No self-shading shall be modeled for the Benchmark.

  • The area and location of opaque exterior doors shall be the same as the Prototype, with door U-value equal to 0.20 Btu/h·ft2·°F (air-to-air).

  • Solar absorptivity is equal to 0.50 for opaque areas of exterior walls, and 0.75 for opaque areas of roofs.

  • Total emittance of exterior walls and roofs is equal to 0.90.

  • The above-grade exterior walls shall be light-frame 2 × 4 or 2 × 6 wood construction with sufficient insulation to achieve the correct overall U-value. The framing factors in Table 4 are representative of typical construction practices, and shall be used as inputs for the Benchmark model.

Table 4. Benchmark Framing Factors

Frame

Framing

Spacing

Fraction

(in. o.c.)

(% area)

16

23%

16

13%

24

11%

Enclosure Element

Walls Floors Ceilings Below Unconditioned Space

8

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