Envelope and HVAC Interaction in a Humid Climate Retrofit
A 1970s elementary school in a hot-humid climate has single-pane aluminum windows, an uninsulated masonry wall (U-0.35), and a roof with R-11 batt (U-0.07). Outside air is 95°F/78°F WB in summer; indoor setpoint is 75°F/50% RH. The 200-ton packaged VAV system short-cycles and struggles to hold humidity, with space RH drifting to 62% on peak days. The district proposes air-sealing and adding R-30 roof insulation ($420,000), window replacement with low-e double pane ($900,000), or a dedicated outdoor air system (DOAS) with dehumidification ($650,000). Energy cost is $0.10/kWh; the building uses 2.1 million kWh/yr, of which cooling is 45%. Peak demand is 620 kW with a $14/kW-month charge.
1. Diagnose why the space humidity is high and explain why envelope measures alone may not fix it.
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High RH with short cycling points to sensible-only cooling and uncontrolled latent load. The packaged VAV cools to setpoint quickly, then the compressor stages down; at part load the coil stays wet but the leaving-air dew point rises, so it removes less moisture per unit of sensible cooling. Meanwhile outdoor air at 78°F WB carries a large latent load, and infiltration through single-pane windows and unsealed envelope adds more. The result is a space that meets temperature but not humidity.
Envelope measures reduce both sensible and latent infiltration, but they also shrink the sensible load faster than the latent load in a humid climate. A tighter, better-insulated building short-cycles even more, so the coil runs less and dehumidifies less - humidity can get worse. Roof insulation and low-e glass help energy but do not provide controlled dehumidification. The root fix is decoupling ventilation and moisture removal from sensible cooling, which is what a DOAS does: it conditions outdoor air to neutral or dry and lets the VAV system handle only sensible space load, running longer at part load and controlling RH.
2. Compare rough annual savings and first cost of the three options, and recommend a sequencing strategy.
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Cooling energy is 45% of 2.1 million kWh = 945,000 kWh = $94,500/yr, plus demand. Roof/air-sealing might cut cooling 15% = 142,000 kWh ($14,200) plus ~30 kW demand ($5,000) = ~$19,200/yr; payback 420,000/19,200 = 22 yr. Windows might cut cooling 12% but add no latent control; ~$15,000/yr; payback 60 yr. DOAS cuts ventilation load on the main system and enables better part-load control; assume 20% cooling reduction plus latent correction, ~$19,000-$25,000/yr; payback 26-34 yr.
None passes a short payback, so this is a life-cycle and health decision. Recommended sequence: (1) commission the VAV and fix minimum outdoor-air and coil control - cheap, immediate RH improvement; (2) air-seal the envelope, the lowest-cost envelope step; (3) add the DOAS to guarantee humidity control and ventilation; (4) defer full window replacement to a planned capital cycle, since it is the weakest return. Present life-cycle cost and occupant comfort/health benefits, not simple payback alone.