Windows and glazing in a hot climate: U-factor and SHGC, double glazing, thermally broken aluminium, and safety glass

Key points

U-factor measures how fast heat passes through the window (lower insulates better). SHGC measures the share of solar heat admitted (lower blocks more heat).
In a hot climate the priority is a low SHGC, together with a low U.
A near-invisible low-e coating reflects infrared and ultraviolet, and gas (argon) between the panes improves insulation.
Aluminium frames conduct heat strongly; a polyamide thermal break cuts that path.
Codes require safety glass (tempered or laminated) in glass doors, shower enclosures, glass near floors, doors and stairs.

Written for:OwnerContractorSupplier

In a hot climate, a window that looks beautiful can be a "heater" letting in the sun's heat all day. Choosing the right window depends on specific figures you can ask your supplier for.

1) Performance figures: what do they mean?

The US Department of Energy's Building America Solution Center (PNNL) explains the window performance label figures1:

Figure	What it measures	Better is
U-factor	Rate of heat transfer — how well the window insulates.	Lower.
SHGC solar heat gain coefficient (0–1)	Share of solar energy passing through the window.	Lower in hot climates.
VT visible transmittance (0–1)	Amount of visible light passing through.	Higher for more daylight.
AL air leakage	Air passing through the joints of the closed window.	Lower (most codes require ≤ 0.3 cfm/ft²).

The challenge in our climate: you want a low SHGC to keep out solar heat, with reasonable VT so the house doesn't become dark. "Spectrally selective" low-e coatings are designed for this balance.

2) Technologies that improve a window

Low-e coating: a near-invisible silver layer on one glass surface that reflects infrared and ultraviolet radiation1.
Gas between the panes: filling the space with argon or krypton insulates better than air1.
Spacers: non-metal insulating spacers reduce heat transfer at the glass edges1.
Double glazing: two panes with a space between them — the base the technologies above build on.

3) Aluminium frames: why a "thermal break"?

Aluminium conducts heat very well, so the frame becomes a "thermal bridge" between outside and inside. The solution is a thermal break: a polyamide (nylon-type) strip separating the inner and outer parts of the frame, cutting the heat path and reducing cooling costs2.

When asking for an aluminium quote, ask explicitly: is the profile thermally broken? And what is the U-value of the whole window (glass and frame together)?

4) Safety glass: where is it mandatory?

There are two types of safety glass3:

Tempered: breaks into small granules instead of sharp shards.
Laminated: two panes with an interlayer that holds the pieces when broken.

The International Residential Code (IRC 2021, R308.4) treats these as "hazardous locations" requiring safety glass, including3:

Glazing in doors.
Glazing next to doors: if the nearest vertical edge is within a 24-inch (≈ 61 cm) arc of the door edge and the bottom edge is less than 60 in (≈ 152 cm) above the floor.
Glazing in bathing areas (shower screens, bathtubs, saunas, hot tubs) where the bottom edge is less than 60 in above the standing surface.
Glazing near stairs, ramps and landings within 36 in (≈ 91 cm) horizontally.
Glazing in guards and railings.

Safety glass must carry a permanent manufacturer's mark proving compliance3.

For owners: a glass shower screen must be tempered or laminated, with the compliance mark etched in the corner. Don't accept ordinary glass, however thick.

5) The window is part of the facade: orientation, shade and leaks

Factor	What it means for your choice	Details
Facade orientation	Facades facing midday and afternoon sun benefit most from low SHGC or external shade (practical advice); figures come from the actual product label1	sun protection
External shade	Stops the sun before the glass and reduces the need for dark glass	shades and screens
Air and dust leakage	The AL figure on the label, plus installation quality and sealing around the frame	window problems, dust
Water leakage	Sill falls, drainage slots and sealant around the frame	sealants
Facade finish around the window	Cracks and efflorescence around frames from leaking water	exterior finishes

Record the properties of the actual product supplied (label or data sheet), not those of "the same type"; performance varies between products.

6) Checklist for a window quote

U, SHGC and VT for the whole window (from a test certificate or performance label).
Glass type: double, low-e, gas fill, pane thicknesses.
Frame: thermally broken aluminium, uPVC or wood.
Safety glass in required locations, with the compliance mark.
Installation details: insulation around the frame, external sealant, water drainage. See window problems.
Compliance with the SBC 602 energy code requirements, calculated by the designer.

Sources

The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.

1High-Performance (ENERGY STAR) Windows — U-factor, SHGC, VT, low-e — Building America Solution Center — Pacific Northwest National Laboratory (PNNL) / U.S. DOE
2Polyamide Thermal Breaks for Aluminum Windows: An Overview — Gabrian International
3IRC 2021 R308.4 — Safety glazing in hazardous locations — Jaspector

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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.