Window

Glazing with its frame: panes, gas gaps, optics of the glazing (Buildings), sky and wind outside

FastSmoothBuildings.Envelope.Window, model (source)

Extends PartialSurface.

Purpose

A window of area A: the glazing of Data.Glazing on the glass area A_gla = A (1 − frac), with a temperature state per pane (T[1] outside to T[nPan] room side), the gas gaps at the temperatures of their panes, the angular optics of the glazing, an optional overhang and side fins, and a frame without capacity on the rest of the area.

Equations

Balance of pane i, of capacity C_i = A_gla x_i d_i c_i (cGla when the glazing gives no c_i):

C_i dT_i/dt = Q_abs,i + Q_in,i − Q_in,i+1,

where Q_in,i flows inwards across interface i: Q_in,1 = −Q_ext from the outside, Q_in,nPan+1 = Q_int − Q_rad to the room (convection to the room air (interiorConvection) minus the radiation the zone gives to the inner pane), and between panes i and i + 1

Q_in,i+1 = G_gap,i R_gap0,i / (R_i/2 + 1/h_gap,i + R_i+1/2) (T_i − T_i+1),

with R_i = x_i/k_i the resistance of a pane and h_gap,i the conductance of the gap at the current temperatures of its panes (gapConductance, with the height hWin). R_gap0,i is the resistance between the centres of the two panes at the reference state of EN 673 (mean 10 °C, 15 K across the glazing) and G_gap,i = A_gla/R_gap0,i: the factor G_gap R_gap0 is A_gla, and G_gap sets the scale of the exchange for a calibration.

Outside, the outer pane exchanges with the outdoor air and with sky and ground (skyExchange, emissivity absIR_a):

Q_ext = A_gla (4 + 4 √(v² + 0.01)) (T₁ − T_out) + Q_sky,

v the wind speed. The frame conducts Q_fra = G_fra (T_air − T_out) from the room air to the outdoor air, with G_fra = A frac / (1/hFraExt + 1/UFra + 1/hFraInt).

Optics: the transmittance and the absorptance of each pane for the direct sun are quartics in cos θ, fitted by least squares to the values of the glazing at 0, 10, ..., 90° (polynomialFit); the diffuse sun takes the hemispherical values. With f_sun the sunlit fraction of the glass and the irradiance H_dir, H_dif of TiltedSurface,

Q_abs,i = A_gla (f_sun H_dir α_i(cos θ) + H_dif α_i,hem),   QTraDir = A_gla f_sun H_dir τ(cos θ),   QTraDif = A_gla H_dif τ_hem,

cos θ bounded below by 0 with positivePart (width 0.01). The transmitted sun goes to the zone through the bus, which sees the room face of the glazing with the emissivity absIR_b, the hemispherical absorptance of the inner pane and the hemispherical transmittance.

Shading

From the weather bus the window computes the solar altitude β = asin(cos ζ), the angle γ between the horizontal projection of the sun's rays and the normal, cos γ = cos θ/cos β (the relation of a vertical window), and the side of the sun. The sunlit fraction is

Shading	fsun
none	1
haveOverhang	fove from overhangSunlitFraction
haveSideFins	ffin from sideFinsSunlitFraction
both	smoothMax(fove + ffin − 1, 10−3, 5·10−4)

Only the direct sun is shaded; the diffuse sun is not.

Parameters

• frac: the share of the frame in A; 0 by default, no frame.
• hWin, wWin: the height and width of the window, √A by default; the height enters the convection of the gaps (the aspect ratio of the cavity) and both enter the shading.
• haveOverhang, haveSideFins: evaluated; a window without them keeps f_sun = 1 and none of the shading functions. The geometry (depOve, gapOve, wLOve, wROve, depFin, gapFin, hFin) is that of Buildings' Overhang and SideFins; left and right are seen from outside.

Relation to Buildings

The glazing data and its angular optics are those of Buildings (Buildings.HeatTransfer.Data.GlazingSystems.Generic), the gaps follow Buildings.HeatTransfer.Windows.BaseClasses.GasConvection (see gapConductance), the overhang and the side fins Buildings.HeatTransfer.Windows.BaseClasses.Overhang and SideFins with smooth minima, maxima and steps, and both combine as Buildings combines them. As in Buildings, the diffuse sun is not shaded.

Validation

BESTEST cases 610, 630, 910 and 930 (overhangs, and overhangs with side fins): every result inside its band, within 2.2 % of Buildings.

Use

The windows of Examples.FreeFloatingRoom and Examples.TwoZones.

Parameters

Parameter	Type	Unit	Default	Description
glazing	Data.Glazing			Glazing, panes from the outside
frac	Real		0	Frame share of the window area
hFraExt	Real	W/(m2.K)	25	Exterior film coefficient of the frame
hFraInt	Real	W/(m2.K)	7.7	Interior film coefficient of the frame
cGla	Real	J/(kg.K)	720	Specific heat capacity of glass when the glazing gives none
hWin	Real	m	sqrt(A)	Height of the window: the aspect ratio of its gas gaps
wWin	Real	m	A/hWin	Width of the window
haveOverhang	Boolean		false	true: an overhang shades the window (Buildings Overhang)
depOve	Real	m	0	Depth of the overhang
gapOve	Real	m	0	Distance between the top of the window and the overhang
wLOve	Real	m	0	Extension of the overhang beyond the left edge, seen from outside
wROve	Real	m	0	Extension of the overhang beyond the right edge, seen from outside
haveSideFins	Boolean		false	true: side fins shade the window (Buildings SideFins)
depFin	Real	m	0	Depth of the side fins
gapFin	Real	m	0	Distance between the window edges and the fins
hFin	Real	m	0	Extension of the fins above the top of the window

Revisions

• October 2026: moved from trano's FastModel.Envelope.