Cavity — series resistance

Total thermal resistance is the sum of the interior surface film, every material layer, and the exterior surface film. The U-value is the inverse of that total.

• Material conductivities (λ, W/m·K) come from a built-in library; pick Custom λ… to enter your own.
• Air gaps use fixed cavity resistances (not thickness ÷ λ) — horizontal 0.18, vertical 0.15 m²·K/W.
• Surface films are selectable for heat-flow direction and wind exposure; set to None for a surface-to-surface (R-value-only) result.
• Valid only for non-bridged, layered assemblies. For repeating thermal bridges use the Clear Field tab.

Assumes steady-state conditions and no moisture, convection, or 2-D/3-D bridging effects.

Clear Field — parallel-path correction

When an assembly contains repeating thermal bridges (studs, Z-girts, fasteners), each distinct cross-section is treated as an independent series path. Path U-values are area-weighted by their fractional areas, which must sum to 1.0.

• This is the upper limit of resistance — it assumes layers are isothermal and tends to over-predict U (conservative for bridged assemblies).
• The U-value increase reports how much the corrected U rises versus the cavity path (path 1): (U_corrected − U_cav) ÷ U_cav.
• Layer rows align across paths so the R-table can be read row-by-row.

The full method averages the upper and lower limits; this tool reports the upper limit only. For high-conductivity bridges (steel) consider a detailed 2-D/3-D calculation or χ/ψ point/linear transmittances.

Glazing — window assembly

Two interchangeable methods are offered via the toggle; both share the same geometry (glazing and frame areas derived from the diagram).

• Area-weighted EOG. The glazing is split into a centre-of-glass zone and an edge-of-glass band (a share of glazing area, default 10%). The edge U-value is the regression U_eog = a + b·U_cog with spacer coefficients — aluminium a=0.359/b=1.14, stainless a=0.182/b=1.04, warm-edge/TGI a=0.136/b=1.02. Each zone (COG, EOG, frame) is weighted by its fraction of the whole window. Pick Custom… to enter a measured U_eog directly.
• Geometry. A_pf = width × height (projected fenestration area). The glazing rectangle is the window inset by the frame border width on all four sides, so A_g = (width − 2·border)·(height − 2·border) and A_f = A_pf − A_g (frame fraction is derived and shown as a card). The glazing perimeter L_g = 2·(glazing width + glazing height) — so a tall, narrow window carries more edge length per unit area than a square one.
• Spacer ψ_g. Linear thermal transmittance in W/m·K. Typical values (non-metal frames): aluminium/metal ≈ 0.08, stainless ≈ 0.06, warm-edge/insulated ≈ 0.04; single glazing has no spacer (ψ = 0). Choose Custom ψ… (or drag the ψ slider) to enter a value from a spacer datasheet or a 2-D edge calculation.
• Window install ψ. An optional linear thermal bridge around the window-to-wall junction (W/m·K), graded very good → worse than typical, or Custom. Shown as a separate install ΔU = ψ·P ÷ A_pf over the outer window perimeter P — it is not folded into the headline U-window.
• Spacer edge ΔU reports ψ_g·L_g ÷ A_pf — the amount the spacer adds to the whole-window U-value.
• Selecting a frame preset fills the frame U-value; editing the U-value directly switches the type to Custom.

Fenestration benchmarks are whole-window U-factor targets. The climate-zone selector filters the zonal code-minimum target to the chosen zone; the non-zonal High-Performance Target is always shown. A simplified method — for ratings use certified whole-window values.

2D Detail — quasi-2D conduction

Solves the steady-state temperature field through a cross-section drawn as rectangles or polygons (orthogonal or angled). Shapes are rasterised onto a regular grid; each cell carries one conductivity, and a five-point control-volume balance (harmonic-mean face conductances) is assembled and solved by conjugate gradient. This computes the real 2-D heat flow that the Clear Field tab can only bound. A material background fill tiles the whole bounding box for simple layered sections; choosing Outside / void makes only the drawn shapes solid, so corners and angled exposed faces can be modelled.

• U-value (repeating section). The heat per square metre of wall face per degree (W/m²·K). Use it when the slice is a repeating wall section — it is the proper replacement for a Clear Field estimate, and it is what flows into the Library/Envelope. For a one-off corner or junction (which doesn't tile), the heat-flow figure is the more honest output.
• Heat flow. The actual heat (in watts) passing through a one-metre length of the drawn section at the indoor/outdoor temperatures you set — a tangible heat-loss number for the junction. Unlike the U-value it scales with the temperature difference, so re-solve after changing the temperatures. (This is the quantity the solver integrates directly; the U-value is derived from it by dividing through the section's face area.)
• What f_Rsi means. The temperature factor rates how warm the coldest interior surface stays, on a 0–1 scale: (coldest surface − outdoor) ÷ (indoor − outdoor). 1 means the surface is as warm as the indoor air (no bridge); 0 means it is as cold as outdoors. Higher is safer — a low value marks a cold spot where condensation or mould is most likely. As a rule of thumb, values below about 0.7 are a concern in cold climates.
• Draw, then paint. Shapes start material-less (hatched). Pick a material in the top bar, click the 🖌 Paint brush, then click each shape to fill it. The background fill (in Boundary conditions) defaults to an air gap; choose Outside / void to model only the drawn shapes (corners, angled faces).
• Boundary conditions. Surfaces exposed to air are convective (air temperature applied through R_si/R_se); everything else is an adiabatic symmetry cut — place those where no heat flows sideways (mid-stud, mid-cavity). Draw the interior and exterior boundaries as snapping lines (click vertices along the faces, double-click or Enter to finish), so a warm surface can wrap a corner or follow an angled face; the Eraser removes a line. The heat-flow direction (used for the area 2-D U, the air-cavity thickness, and the default interior film R_si) is read from where you placed interior vs. exterior — there is no separate axis setting.
• Air cavities are modelled as an equivalent conductivity derived from the cavity's drawn thickness and a fixed cavity resistance — internal convection and radiation are not modelled. Vapour-control / barrier layers are treated as thermally transparent.
• Condensation — climate-aware. The dew point (and an 80%-RH mould line) is found from the interior temperature and RH by the Magnus approximation, then placed in the field. The vapour-drive direction is read from the temperatures: interior warmer (T_in > T_out) drives vapour outward — risk toward the exterior; interior cooler drives it inward — risk toward the interior. The climate zone sets the severity baseline (Zone 4 and above → Condensation risk, below 4 → Low condensation risk). Draw a vapour-control line on the warm side and the verdict becomes Risk mitigated as long as the dew-point / mould isotherm never reaches the warm side of it; if it does cross, the baseline risk stands. With no climate zone selected the tool falls back to the direct coldest-surface-vs-dew-point check.

A simplified educational 2-D method (orthogonal geometry, temperature-independent conductivity, no radiation/convection in cavities). The condensation logic is a steady-state screening aid following PNNL BASC cold-weather guidance — not a transient hygrothermal (WUFI-style) analysis. For certified thermal-bridge values use a validated 2-D/3-D thermal-bridge solver.

Envelope

Combines individual assemblies into a single envelope figure. Each component contributes U×A to the total heat-loss coefficient; the area-weighted envelope U is that total divided by the total area.

• Add components by loading a saved construction (series / parallel / window) or pulling the assembly currently live in a tab. The U-value is taken from that construction; enter the area it covers.
• Thermal bridges are optional. Linear bridges use a linear transmittance ψ (W/m·K) over a length; point bridges use a point transmittance χ (W/K) over a count. Both add to UA but not to ΣA, so they raise the effective envelope U.
• U-values are snapshots taken when the component is added — re-add or edit a row if the source assembly changes.

A first-order fabric heat-loss estimate. It excludes ground-coupling adjustments, air infiltration/ventilation, and dynamic effects — for compliance use a full envelope/energy model.

Library

The library stores any construction you save from the Cavity, Clear Field, 2D Detail or Glazing tabs. It persists in this browser only (nothing is uploaded), so clearing site data or switching machines starts it fresh — use the per-tab JSON Save/Load to move a construction between browsers.

• Each entry shows its live-recomputed U-value and can be renamed inline.
• Projects are simple labels for grouping. Create one, then assign constructions to it with the per-row dropdown; deleting a project returns its constructions to Unsorted.
• + Env adds a construction to the Envelope tab as a component (a U snapshot — editing the library entry later won't change components already added).