Envelope Priorities for a 1968 Elementary School
A 90,000 ft² elementary school built in 1968 has single-pane aluminum windows, a roof insulated to about R-8, and wall cavities at about R-7. The building uses 120 kBtu/ft²-year, well above similar schools. Teachers report cold drafts, condensation on north-facing windows, and mold on the north wall. The boiler and rooftop units are near the end of life but still functional. The district has $500,000 for envelope work and wants a defensible priority list. A blower-door test has not been done, and the maintenance staff already caulked the most obvious cracks. The climate has 4,500 heating degree-days and 900 cooling degree-days. Natural gas costs $0.80/therm and electricity $0.10/kWh. The roof is due for replacement in three years, but the windows are not. The district asks whether to replace windows first because they look like the obvious problem.
1. Propose a diagnostic plan before selecting envelope measures. What would you measure, and how would you interpret the results together?
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Before selecting measures, test the building. A blower-door test with zonal pressure diagnostics locates the leakage paths; infrared imaging during a cold snap shows missing insulation and thermal bridges; smoke pencils confirm bypasses at attic hatches, penetrations, and vestibules. Use moisture meters and a roof moisture scan on the north wall and roof. Log indoor CO2, temperature, and relative humidity to check ventilation and moisture. Review window operation and existing caulking.
Interpret the results together. Air leakage can dominate heating and comfort, but sealing a wet wall or roof can trap moisture. A window that feels cold may be a draft problem, not a glass problem. The test-in/test-out plan should include a second blower-door test after air sealing and a moisture check after the heating season.
2. Rank the envelope options and justify the order. Address the district's window replacement proposal.
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Rank air sealing first: roof deck and attic penetrations, wall penetrations, vestibules, and window weatherstripping. It is the lowest-cost measure, improves comfort and moisture control, and reduces both heating and cooling loads. Second, add roof insulation when the roof is replaced, because the marginal cost is much lower than a stand-alone project. Third, repair and weatherstrip windows and consider interior storm panels; full window replacement is last because the installed cost is high and the R-value gain is modest. Wall insulation should wait until air sealing and moisture control are verified, especially on the moldy north wall.
The visible window problem is not necessarily the best investment. Use blower-door results to target air sealing and reserve window replacement for failed units or safety and egress issues.
3. Calculate the approximate heating saving and simple payback for roof insulation alone, and explain why timing changes the decision.
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Using a 90,000 ft² roof area, going from R-8 to R-30 reduces roof UA by 90,000 x (1/8 - 1/30) = 8,250 Btu/h-°F. With 4,500 heating degree-days, annual heat loss falls by 8,250 x 4,500 x 24 = 891 million Btu. At 80% boiler efficiency, that is about 11,140 therms, or $8,900 per year at $0.80/therm. A stand-alone $270,000 roof insulation job has a 30-year simple payback, so it fails a normal screen. If done as the marginal cost during a planned re-roof, say $60,000, the payback is about 7 years and becomes attractive.
This is why the decision is timing-based, not just R-value-based. The calculation excludes cooling and demand savings, so it is conservative, but it still shows that window replacement alone is unlikely to be cost-effective. Present the range and the re-roof option.