Tools › Building Systems › Codes and Compliance › Envelope UA Compliance › Example

2,000 ft² single-storey house in Chicago (climate zone 5): total-UA trade-off against the DOE-reprinted IECC U-factors, a 2x6 R-21 + R-5 wall from its layers, degree-day energy and a Glaser condensation check

Tool	Envelope UA Compliance
Source	U.S. Department of Energy, Building Energy Codes Program (2019). Energy Code Compliance Paths. Presentation slides, Pacific Northwest National Laboratory, Richland, WA. Slide 15 (IECC Table R402.1.4 U-factor alternative as reprinted, and the zone-5 framed-wall example); Pacific Northwest National Laboratory (2005). REScheck Technical Support Document. PNNL-14864-1-2, Appendix A eq. (A.1) and Table A.9; Stovall, T. K. (2008). Supporting Documentation for the 2008 Insulation Fact Sheet Update. ORNL/TM-2008/017, Tables 1 and 9; National Centers for Environmental Information (2021). U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991–2020), station USW00094846.
Level	Undergraduate

Problem

A 40 × 50 ft single-storey house (2,000 ft² ceiling and floor, 1,280 ft² of net wall, 300 ft² of windows, a 40 ft² door, 18,000 ft³ at 0.35 ACH) in Chicago, IECC climate zone 5, has a ceiling at U 0.026, a floor over a crawl space at U 0.047 (above the zone's 0.033), windows at U 0.30 / SHGC 0.30 and a 2x6 wall of latex-painted gypsum, polyethylene, R-21 cavity insulation, plywood, R-5 foam sheathing and siding with 25 % framing. Find the wall U-factor by the parallel-path method, decide total-UA compliance against the zone-5 U-factors of the DOE reprint, the window-to-wall ratio, the annual heating energy and gas use at 92 % AFUE with the NCEI 1991–2020 degree days, the design heat loss at 0 °F, the balance point with 3,000 Btu/h of internal gains, and whether the wall condenses at 70 °F / 35 % indoors and 0 °F / 80 % outdoors.

Inputs to Enter

Assemblies (A ft², U proposed, U required Btu/h·ft²·°F): ceiling 2,000 / 0.026 / 0.026; walls 1,280 / from layers / 0.060; floor 2,000 / 0.047 / 0.033; windows 300 / 0.30 / 0.32 (SHGC 0.30 vs 0.40); door 40 / 0.30 / 0.32; zone 5; no slab term
Layers inside first: inside film 0.68; 1/2 in gypsum 0.45 (10 perm); polyethylene (0.06 perm); cavity R-21, 5.5 in; plywood 1.32 (0.7 perm); foam R-5, 1 in (1.1 perm); siding 0.81; outside film 0.17; framing fraction 0.25 at 1.25 per inch (TSD 6.88 for a 2x6); wood framing (ORNL 0.75)
Chicago O'Hare NCEI normals HDD 5,974.3 / CDD 1,003.3 (base 65 °F); set point 70 °F, design 0 °F, 0.35 ACH × 18,000 ft³, internal gains 3,000 Btu/h, AFUE 0.92, COP 3.5, natural gas 100,000 Btu/therm at 1.20, electricity 0.15 per kWh; indoor RH 35 %, outdoor 80 %

Open the Envelope UA Compliance tool (loaded with this example)

Answers

Wall: series R = 29.43 (U 0.0340 without framing); common path 8.43, framing path 15.31, cavity path 29.43 → U = 0.25/15.31 + 0.75/29.43 = 0.04182 Btu/h·ft²·°F (R 23.9); ORNL reduced clear wall 0.75 × 24.43 + 5 = R 23.3, U 0.0429 (the fact sheet's Table 9 prints 0.0429 for R-21 + R-5)
UA proposed 301.53 vs required 303.60 Btu/h·°F: complies by total UA with a 0.68 % margin; the floor (94.0 vs 66.0) fails its own value and is carried by the walls (53.5 vs 76.8); WWR 21.0 %, area-weighted U 0.0537, SHGC 0.30 ≤ 0.40
Infiltration UA 0.018 × 0.35 × 18,000 = 113.4; total UA 414.9 Btu/h·°F; design heat loss 29,045 Btu/h at 0 °F; Q_h = 24 × 414.9 × 5,974.3 = 59.49 MMBtu, 646.7 therms (776 per year); cooling conduction + infiltration 9.99 MMBtu, 837 kWh; balance point 70 − 3,000/414.9 = 62.8 °F
Glaser: indoor p_v 876.6 Pa (dew point 41.1 °F), outdoor 102.1 Pa over ice; the largest p_v / p_sat at an interior interface is 0.78 (after the siding), so no condensation; the polyethylene drops the vapour pressure to 196.8 Pa behind the cavity where the plywood face is 17.4 °F (p_sat 306 Pa)
SI: UA 159.06 vs 160.2 W/K, wall U 0.2375 W/m²·K, design loss 8.51 kW, Q_h 62.77 GJ, balance point 17.1 °C

Notes

The tool's default inputs are this case; press Submit. The house is illustrative; every number is reproduced by the independent numpy script References/Envelope-UA/checks/envelope_check.py (test_envelope_ua_compliance.php, 327 checks), which also reproduces the ORNL fact-sheet Table 9 composite U-values, the REScheck TSD Table A.10 and A.2 sample U-factors, the DOE slide's zone-5 example and the Wood Handbook Table 4-7 conductivities. The required U-factors are the DOE reprint of IECC Table R402.1.4; the adopted edition's values are entered in the table.