Building Envelope
What this domain tests
Envelope questions test heat flow, moisture, and the difference between insulation and air sealing. You should be able to compute conduction and infiltration losses.
Core rules
- U-factor = 1 ÷ R-value; assembly U includes surface films and framing, not just cavity insulation.
- Heat loss = U × area × ΔT; annual conduction uses HDD × 24.
- SHGC controls solar gain; VLT controls daylight. They trade off in glazing selection.
- Air leakage is often larger than conduction losses in older buildings; blower-door testing quantifies it.
- Thermal bridges (slabs, balconies, steel studs) reduce effective R-value.
Worked example
A 1,000 ft² wall has U = 0.10 Btu/h·ft²·°F and sees 5,000 HDD. Annual heat loss = 0.10 × 1,000 × 5,000 × 24 = 12,000,000 Btu, or 12 MMBtu. Adding insulation to U = 0.05 halves conduction to 6 MMBtu. At $1.00/therm and 80% boiler efficiency, the saving is 6 MMBtu ÷ 0.1 ÷ 0.80 = 75 therms/yr - small, which is why envelope work must be bundled.
Traps
Using cavity R-value as assembly R; ignoring air leakage; and treating SHGC and U as interchangeable.