Building Envelope and Thermal Performance
Heat transfer fundamentals
The envelope controls transmission and infiltration losses. Know U-value (W/m²K) as the inverse of thermal resistance; the assembly R is the sum of layer resistances plus surface films (Rsi, Rse). Thermal bridging raises effective U - linear thermal transmittance ψ (W/mK) times bridge length is added to the transmission total. Thermal mass and phase affect dynamic behaviour: a heavy wall delays and dampens peaks, which matters for intermittently heated buildings.
Moisture and airtightness
Condensation risk is assessed by comparing surface temperature with dew point; interstitial condensation by the Glaser method (vapour pressure vs. saturation pressure through layers). Vapour barriers go on the warm side. Airtightness (n50, blower-door test) is separate from U-value - a well-insulated but leaky building still loses heat and can suffer moisture damage.
Typical measures and traps
Insulation of walls, roofs, and floors; window replacement with low-e glazing and thermally broken frames; shading and solar control; elimination of thermal bridges at balconies and lintels. Traps: assuming thicker insulation always pays back (diminishing returns); ignoring the effective U-value after thermal bridges; forgetting that glazing U-value and g-value trade off (solar gains help in winter, overheat in summer); confusing U-value units or inverting R incorrectly.
Worked example
A wall: 20 cm brick (R≈0.25), 10 cm EPS (λ=0.035 → R≈2.86), films 0.13+0.04. Total R≈3.28 → U≈0.30 W/m²K. Adding 5 cm more EPS (R≈1.43) gives U≈0.21 - a 30% improvement, not 50%: diminishing returns in action.
Rule of thumb
Each doubling of insulation thickness roughly halves the insulation resistance but only modestly improves total U once other layers dominate.