Round Rock ADU Builders: Best Windows for an ADU in Texas Heat: SHGC & U-Factor

Windows for a Texas ADU

Windows are where a small building either stays comfortable or does not. On an ADU the stakes are higher than on a house, because the floor area is small enough that one badly specified west-facing window can dominate the cooling load of the entire unit.

The specifications are not complicated, but the ones that matter here are not the ones marketing emphasizes.

The Number That Matters Here

Direct answer: Solar heat gain coefficient measures how much solar energy passes through the glass, from 0 to 1. In Central Texas, where cooling dominates the year, a lower SHGC keeps more heat out and matters more than U-factor. Northern climates optimize the opposite way, which is why generic window advice frequently misleads here.

U-factor measures conductive heat transfer. It still matters, particularly for the handful of genuinely cold nights, but it is the second priority in a cooling climate.

Visible transmittance measures how much daylight comes through. The tension is real: aggressive solar control coatings can make a small unit feel dim. On an ADU with limited wall area, that tradeoff deserves attention rather than defaulting to the lowest available SHGC.

Orientation Changes the Answer

The same window is a different problem depending on which wall it sits in.

West-facing is the hardest condition in Central Texas. Afternoon sun arrives low and direct, exactly when outdoor temperature peaks. This elevation justifies the most aggressive solar control, and where the floor plan allows, the fewest and smallest openings.

South-facing takes high-angle sun that a modest overhang can shade effectively for most of the cooling season. This is the easiest elevation to handle well with geometry rather than glass technology.

East-facing takes morning sun, which is real but arrives when ambient temperature is still low. Moderate specification is usually sufficient.

North-facing receives little direct sun. This is where you can afford generous glazing for daylight without a heat penalty, and on a small unit it is often the best place to buy the light that makes the space feel larger.

Specifying every window in a unit to the same numbers ignores all of this. Varying the specification by elevation costs nothing extra and produces a noticeably more comfortable building.

Low-E Coatings

Low-emissivity coatings are microscopically thin metallic layers that reflect infrared while passing visible light. Which surface of the glass unit carries the coating determines whether it primarily blocks incoming solar heat or retains interior heat. In a cooling climate you want the former.

This is a detail worth confirming rather than assuming, because the same manufacturer typically offers coatings optimized for both, and the northern-optimized version is not what you want here.

Frames

Vinyl is the common default. Adequate thermally, but sustained heat on a west elevation is its weak point over time.

Fiberglass expands and contracts at nearly the same rate as glass, which reduces seal stress through heat cycling. It costs more and performs better in this specific climate.

Aluminum conducts heat readily. Thermally broken versions mitigate this, but it is working against the climate rather than with it.

Wood clad performs well and looks excellent. It is the highest-maintenance option in Central Texas UV.

Where to Spend

If the budget is constrained, put it into the west elevation. Specify aggressive solar control there, moderate elsewhere, and buy daylight on the north wall where it is free. Add an overhang on the south elevation, which is cheaper than glass technology and works permanently.

That approach produces a more comfortable ADU than spending the same money spreading premium glazing evenly across all four walls.