Blind Solutions Academy · C18 · A4 Quick Reference

Facade & Solar-Shading Cheat Sheet

Shadow angles, SHGC and decision trees on four pages — the working numbers for the desk and the site.

Page 1 · Sun geometry & shadow angles

Angles & device sizing

sin(alt) = sinφ·sinδ + cosφ·cosδ·cos(H)

Solar altitude. φ=latitude, δ=declination (±23.45°), H=hour angle (15°/hr from noon).

VSA: tan(VSA) = tan(alt) / cos(γ)

Vertical Shadow Angle (profile angle). γ = wall-solar azimuth. Governs horizontal devices.

HSA = solar azimuth − wall azimuth

Horizontal Shadow Angle = wall-solar azimuth. Governs vertical fins.

Overhang: P = h / tan(VSA)

P = projection, h = glass height below overhang. Full beam cut-off at the design hour.

Fins: d = s / tan(HSA)

d = fin depth, s = clear spacing. Full beam cut-off for vertical devices.

If P > h ⇒ change device family

Low-altitude E/W sun → use vertical fins / egg-crate / operable, never a deeper overhang.

Facade (S. hemisphere)	Design sun	Device
North	High altitude, near noon	Shallow horizontal overhang / louvres
South	Diffuse + brief low summer sun	Glare/light control; edge fins
East	Low am, wide azimuth	Vertical fins / operable venetian
West	Low pm + ambient peak (worst)	Deep fins / egg-crate / motorised + SHGC≤0.25 glass

Page 2 · Glazing & solar gain

SHGC, glazing & load

Q = A × SHGC(combined) × I

Solar gain (W). A=glazed area, I=facade-plane irradiance (beam+diffuse).

I_total = I_beam(1−f_shade) + I_diffuse(1−f_diff)

Device removes beam fully when cut-off, but only part of diffuse — never drop the diffuse term.

LSG = VLT / SHGC

>1.25 = spectrally selective (daylight in, near-IR heat out).

Combined SHGC ≈ SHGC(glass) × F(device)

External devices only; internal devices use tested combined value.

Glazing	SHGC	VLT	U
Clear 6mm single	0.82	0.88	5.7
Clear DGU	0.70	0.78	2.8
Reflective DGU	0.34	0.40	1.7
Selective low-e DGU	0.28	0.62	1.6

SA altitude note: Highveld clear-sky DNI ≈ 950–1000 W/m² (10–15% above sea level). Size shading on SA data, not imported assumptions.

Fabric openness: ~3% = strong glare control + residual view; 10% = more view, more heat/glare; feeds F in Q.

Page 3 · Decision trees

Choosing the strategy

Device-family decision tree

Which facade?

North → high sun → horizontal overhang/louvres

Need winter sun? → keep fixed overhang (auto-selective)

South → mostly diffuse → glare/light control, not heat rejection

East / West → low sun

Compute P = h/tan(VSA). P > h? → vertical fins (d=s/tan(HSA))

View/daylight critical? → operable / motorised venetian

No external device allowed? → SHGC≤0.25 glass + internal screen

Compliance decision tree

Facade passes SANS 204 fenestration on glazing alone?

Yes → document & proceed (prescriptive route)

No → add external shading

Lower effective combined SHGC in the calc

Recompute fenestration energy (rational route)

Within limit? → document trail (sun-path, shadow angles, SHGC)

Still over? → reduce WWR or upgrade glazing, re-run

Comfort check

Complaint near glazing?

Measure inner glass surface temp + air temp

Glass hot, air cool → radiant asymmetry → external device (controls MRT)

Glare on screens → operable screen / low-openness fabric / light shelf

Page 4 · Worked snapshots & standards

Numbers to remember

West curtain wall (Gauteng), 120 m²

	Unshaded	Engineered
SHGC	0.70	0.28 + fins
I (W/m²)	800	800
Peak Q	67.2 kW	4.2 kW
W/m² facade	560	35

≈ 94% peak-gain cut · ~18–19 kW chiller saved.

North window (Bloemfontein), h=1.8m

Summer VSA 80° → P = 1.8/tan80 = 0.32 m. Winter VSA 35° → 0.22 m winter sun admitted.

SA standards quick map

Standard	Use
SANS 10400-XA	Mandatory energy-usage regulation
SANS 204	Fenestration SHGC/area limits by zone
SANS 10160-3	Wind actions (device structure / auto-retract)
Green Star SA	Energy + IEQ credits for shading
EDGE	20% energy-reduction threshold

External > internal: an external device with SC 0.15 beats an internal blind of identical fabric — it stops radiation outside the envelope before 85% gets trapped indoors.