Building Envelope
U, R, SHGC, and air
Envelope analysis uses four properties. R-value (ft²·°F·h/Btu) measures thermal resistance; U-factor = 1/R and describes the whole assembly including thermal bridging, so a framed wall's U is worse than its cavity insulation suggests (framing typically occupies 15–25% of the area). SHGC measures solar heat gain admitted through glazing; VLT measures visible light - they are different numbers, and daylighting depends on VLT while cooling load depends on SHGC. Air leakage is quantified by ACH50 from a blower-door test and is separate from conduction.
Measures include added insulation at roof and walls, cool and reflective roofing, window films and glazing replacement, air sealing and air-barrier repair, vestibules and dock seals, and solar shading. Always check moisture: adding interior insulation can move the dew point into the assembly, so vapor retarder placement and condensation risk must be assessed.
Traps. Using cavity R-value as assembly U-factor; confusing SHGC with VLT; assuming infiltration is constant across the year (it is driven by wind and stack effect); ignoring winter solar gain when evaluating glazing; and quoting code-minimum U-factors as if they were actual conditions.
Worked example
A 10,000 ft² wall at U = 0.10 has a conductance of 1,000 Btu/h·°F. Upgrading to U = 0.05 halves it, saving 500 Btu/h·°F. Over 4,000 heating degree days: 500 × 4,000 × 24 = 48,000,000 Btu = 48 MBtu saved at the wall. At 80% boiler efficiency the fuel input is 60 MBtu ≈ 600 therms, or about $600/yr at $1.00/therm - a number that usually argues for targeting air sealing and roof insulation first.