Choosing the Model for a House With a Big Addition
A 1,500 sq ft 1968 ranch had a 900 sq ft great-room addition built in 2010 with 2x6 walls, R-21 cavity insulation, and low-e double-pane windows. The original house has 2x4 walls with R-11 and single-pane windows. The auditor has a full blower door (3,100 CFM50), duct leakage to outside (18% of fan flow), and utility bills showing 1,050 therms/year for gas heat and 9,800 kWh/year for everything else. The client wants to know whether to spend $12,000 on new windows for the whole house or on attic and wall insulation plus air sealing. The auditor must produce a model that separates the two building sections and predicts savings for each option.
1. Describe how you would set up the energy model to represent the two sections, and list the inputs you must not guess.
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Model the house as two thermal zones (original 1968 section and 2010 addition) because their envelope performance differs by roughly a factor of two, and a single-zone model would average away the very difference the decision depends on.
Inputs that must be measured, not guessed:
- Conditioned floor area and volume for each zone (1,500 and 900 sq ft).
- Envelope areas by orientation for each zone: wall, window, door, ceiling, floor. Measure or take from plans; do not assume.
- Assembly U-values for each zone's walls, windows, and ceiling. Use the actual construction (R-11 2x4 vs R-21 2x6; single-pane vs low-e double-pane) and include framing fraction.
- Infiltration: the 3,100 CFM50 must be apportioned between zones. Use a zone pressure test or a guarded blower door if possible; otherwise split by volume and document the assumption.
- Duct leakage to outside (18% of fan flow) and duct location (attic? crawl?) - this is a large loss and must be modeled explicitly, not folded into infiltration.
- Internal gains and setpoints from the client, and baseline consumption from the bills (1,050 therms, 9,800 kWh).
Calibrate the model so its predicted annual gas and electric use match the bills within about 10% before running any measure. A model that does not reproduce the bills cannot be trusted to rank measures.
2. How would you use the calibrated model to compare the $12,000 window package against insulation and air sealing?
Read the worked answer
Run each package as a separate measure and then a combined run, and compare annual savings, simple payback, and cost per unit of saved energy.
Windows (whole house): model U-value improvement from single-pane (~U-1.0) to low-e double-pane (~U-0.30) on the original section, and from existing low-e to better low-e on the addition. Expect the model to show modest savings because windows are a small share of total envelope area and the addition's windows are already decent. A $12,000 package that saves, say, 60-90 therms/year has a very long payback.
Insulation and air sealing: model attic insulation to R-49, dense-pack or exterior insulation on the original 2x4 walls, and air sealing to cut the 3,100 CFM50. Also model duct sealing (the 18% leakage to outside is a big, cheap win). Expect larger savings per dollar, especially in the original section.
Decision rule: rank by savings per dollar invested, not by total savings. Report both annual dollar savings and the comfort co-benefit (the original section's cold rooms). The model will almost certainly favor insulation, air sealing, and duct sealing over whole-house windows; present the numbers and let the client see the trade-off, including that windows may still be justified for aesthetics or the worst single-pane units.