14.2 Thermal Bridging, Continuous Insulation & Thermal Mass

Thermal Bridging & Continuous Insulation (ci)

The Physics of Thermal Bridging

A thermal bridge occurs when an envelope component with high thermal conductivity penetrates or interrupts an insulating layer, creating a localized path of least thermal resistance. Thermal bridging causes three severe building deficiencies: accelerated heat loss/gain, localized cold interior surface temperatures that trigger condensation and microbial mold growth, and premature envelope material degradation.

Quantifying Steel Stud Degradation (ASHRAE Standard 90.1)

Structural steel has a thermal conductivity of approximately $k \approx 314 \text{ Btu}\cdot\text{in}/(\text{hr}\cdot\text{ft}^2\cdot^\circ\text{F})$, compared to fiberglass batt insulation which has a conductivity of only $k \approx 0.27$. Because steel conducts heat more than 1,100 times faster than cavity insulation, light-gauge steel studs act as massive parallel thermal short-circuits across the wall assembly.

Under ASHRAE Standard 90.1 (Appendix A, Table A9.2-2), the severe thermal bridging of steel studs drastically degrades nominal batt insulation ratings:

Critical Exam Fact: Installing an R-19 batt inside a 2x6 steel stud wall at 16 inches on center yields an effective cavity insulation value of only R-7.1. Attempting to satisfy modern commercial energy codes solely by packing thicker batt insulation between steel studs is physically impossible due to the governing laws of parallel heat flow.

Prescriptive Continuous Insulation (ci) Requirements

To overcome thermal bridging, the International Energy Conservation Code (IECC) and ASHRAE Standard 90.1 mandate the installation of Continuous Insulation (ci), defined as:

"Insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope."

Prescriptive energy code compliance for steel-framed above-grade exterior walls enforces hybrid insulation minimums across climate zones:

• Climate Zone 4: Mandates R-13 cavity + R-7.5 ci (or R-19 cavity + R-3.8 ci, or maximum assembly $U \le 0.064$).
• Climate Zone 5 & 6: Mandates R-13 cavity + R-10 ci (or maximum assembly $U \le 0.055$).

Placing continuous rigid insulation (such as polyisocyanurate [R-6.0 to R-6.5/inch], extruded polystyrene XPS [R-5.0/inch], or rigid mineral wool [R-4.0 to R-4.2/inch]) outboard of the steel studs completely blankets the structural framing. Crucially, exterior ci elevates the temperature of the interior gypsum sheathing and steel stud cavity above the indoor dew point, shifting the condensation plane entirely outside the structural wall and preventing interstitial moisture damage.

Cantilever Concrete Balcony Thermal Breaks

Uninsulated cast-in-place concrete slab projections (such as cantilever balconies or perimeter eyebrow shading devices) act as massive cooling fins. A continuous 8-inch concrete floor slab extending outdoors provides a thermal conductance pathway comparable to an uninsulated single-pane window running the full length of the facade. In winter, this draws heat rapidly from the interior floor, chilling the floor slab near the perimeter, causing occupant complaints, and inducing condensation beneath carpets and along baseboards. Modern envelope integration requires structural thermal break modules consisting of structural stainless steel rebar cages and high-density polyisocyanurate blocks cast directly into the slab plane, delivering an uninterrupted R-10 to R-15 thermal envelope boundary while transferring full shear and flexural structural loads.

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Thermal Mass & Diurnal Cycling

Volumetric Heat Capacity & Diurnal Temperature Swings

Thermal mass refers to the capacity of a building material to absorb, store, and subsequently release sensible heat. A material's thermal storage capability is governed by its Volumetric Heat Capacity ($VHC$):

$VHC = \rho \cdot c_p$

Where $\rho$ is material density ($\text{lb/cu ft}$) and $c_p$ is specific heat capacity ($\text{Btu}/(\text{lb}\cdot^\circ\text{F})$). Dense materials—such as poured concrete ($VHC \approx 28 - 32$), solid brick masonry ($VHC \approx 24 - 28$), rammed earth, stone, and liquid water ($VHC = 62.4 \text{ Btu}/(\text{cu ft}\cdot^\circ\text{F})$)—absorb large quantities of thermal energy with minimal rise in internal temperature.

Thermal mass is effective exclusively in climatic zones characterized by large diurnal temperature swings—where the difference between daytime peak temperatures and nighttime lows exceeds $20^\circ\text{F}$ to $30^\circ\text{F}$ (common in hot-arid and Mediterranean climates, ASHRAE Climate Zones 2B, 3B, and 4B). In hot-humid climates with small diurnal swings ($< 10^\circ\text{F} - 15^\circ\text{F}$), thermal mass fails to cool down overnight, trapping heat and radiating warmth into the building during sleeping hours, which exacerbates cooling discomfort.

Temperature (°F)
      ▲
 100° ┤           Outdoor Air Temperature Cycle (High Amplitude)
      │               /---\ 
  80° ┤ ─ ─ ─ ─ ─ ─  /     \ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
      │             /  /---\ \       Interior Surface Temp
  70° ┤────────────/──/─────\─\────────────────────── Comfort Zone
      │           /  /       \ \     (Damped Amplitude: Decrement Factor)
  60° ┤          /  /         \ \
      │         /  /           \---\ 
  50° ┤        /                   \ 
      └────────┼───────┼───────┼───────┼────────────────────────► Time (Hours)
             12:00   16:00   20:00   24:00 (Peak delayed by 8–10 hr Time Lag)

Thermal Time Lag & Decrement Factor

When solar radiation and ambient heat strike the exterior face of a thick masonry or concrete wall, heat travels slowly through the assembly via conduction:

• Thermal Time Lag (Phase Shift): The time delay between peak exterior temperature (typically occurring around 2:00 PM to 4:00 PM) and the arrival of peak temperature at the interior surface. A properly detailed 10- to 14-inch solid masonry or rammed earth wall generates an 8- to 12-hour thermal time lag, ensuring that peak daytime solar heat does not penetrate into the occupied space until 10:00 PM to midnight, when outdoor ambient air has cooled and space heating may actually be welcomed.
• Decrement Factor (Amplitude Damping): The mathematical ratio of the interior surface temperature amplitude relative to the exterior surface temperature swing. Heavy thermal mass flattens daily temperature spikes, stabilizing interior temperatures within the human comfort zone without active mechanical refrigeration.

Passive Cooling via Night Flush Ventilation

In arid regions with cool night temperatures ($< 65^\circ\text{F}$), buildings with exposed interior thermal mass (uncovered concrete floor slabs, exposed CMU partition walls) utilize night flush ventilation. At night, large operable windows, motorized louvers, or low-power mechanical exhaust fans draw high volumes of cool ambient air across the building interior. This cool air washes over the exposed thermal mass, extracting the sensible heat stored during the previous day and discharging it outdoors. By sunrise, the thermal mass is cooled down to 60–65°F. During the hot daytime hours, windows and dampers are sealed shut; the chilled mass functions as an internal "heat sponge," absorbing occupant, plug, and solar heat gains and maintaining indoor thermal comfort throughout the workday without running chillers.

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