Window Solar Heat Gain and Conduction Cooling Load
The two ways a window adds to a cooling load.
Example
You enter
- Glazing area (ft²) 40
- SHGC (NFRC label, 0-1) 0.3
- Peak solar factor (Btu/h/ft²) 200
- U-factor (Btu/h/ft²/F) 0.3
- Glass CLTD (°F) 14
You get
- Solar gain 2400 Btu/h
- Conduction 168 Btu/h
- Total cooling load 2568 Btu/h
Details, formula, and sources
The solar radiation through the glass (area x SHGC x peak solar factor) and the conduction driven by the indoor-outdoor difference (area x U x CLTD). On a west wall in summer the solar term dwarfs the conduction by an order of magnitude, which is why orientation and SHGC, not U-value, drive a glass cooling load - the number to diagnose a hot room or justify a shading or low-SHGC retrofit. The peak solar factor is read from the ASHRAE/ACCA table for the orientation and latitude. One Manual J component, not the stamped load sheet.
q_solar = area_ft2 x shgc x psf; q_cond = area_ft2 x u_factor x cltd_f; q_total = q_solar + q_cond.
ASHRAE / ACCA Manual J fenestration cooling load (Q_solar = A x SHGC x PSF, Q_cond = A x U x CLTD), by name; the relations are public.
The fenestration solar and conduction cooling-load relations are public ASHRAE / ACCA Manual J equations; the SHGC and U come from the NFRC label.
Estimate. AHJ and licensed professional govern.
Field names used by the API: area_ft2, shgc, psf, u_factor, cltd_f, q_solar, q_cond, q_total
- Solar gain Q_solar = area x SHGC x peak solar factor; orientation and SHGC, not U, drive a glass cooling loadASHRAE / ACCA Manual J
- Conduction Q_cond = area x U x CLTD; on a west wall in summer the solar term dwarfs this by an order of magnitudeASHRAE / ACCA Manual J
- Component only one window line of a cooling load, not the whole Manual Jfirst principles