Writing the window spec for a mid-rise building.

A window spec does one job. It makes every bidder price the same window. A spec that says "high performance windows, U-0.30" lets each bidder decide what that means, and the cheapest reading wins. A good spec states numbers that can be checked. It states them by opening where they vary. And it names the evidence that comes with each one.

The specification sets the performance and the proof. The window schedule says what goes in each opening.

What to state, and what to ask for.

Item	State this	Evidence to require
Structure	Performance class and grade for each window type, at the project's sizes	Lab test report for that type, with the tested size
Water and air	Water test pressure and maximum air leakage	The same report
Energy	Whole-window U-factor and SHGC for each type	NFRC certification, with the CPD number
Sound	STC and OITC, by elevation	Lab report for the whole window, in that type and glass
Glass	Makeup for each opening, with safety, acoustic and bird-safe glass marked	Glass schedule in the shop drawings
Operation	Type, handing, hardware, limiters	Shop drawings and a hardware sample
Color	Outside and inside, by name	Physical samples
Proof on the project	Mock-up and field test, with method, number and pressure	Test reports, signed

Performance class and grade.

The structural engineer's wind load calculation gives a design pressure. It is higher at the corners and on the upper floors. The spec turns that into a class and a grade, such as CW-PG50. Two points are often missed:

• State it for each type. A fixed window and an operable one from the same series rarely share a rating. Vistaza's Vi6 casement is certified AW-PG100, and the Vi6 fixed window AW-PG75. A spec that gives one rating for "all windows" invites the better number to be quoted for both.
• State the size. A rating applies up to the tested size. For larger openings, require a comparative analysis or a mock-up. See what a factory mock-up test proves.

The letters are explained in R, LC, CW, AW.

U-factor and SHGC, with certification.

Take the targets from the energy model, not from a product sheet. Then:

• Ask for whole-window values. A center-of-glass U-factor is nearly always the lower number, and it is not what the energy code counts.
• Ask for NFRC-certified values with the CPD number. A simulated value is a calculation that nobody has checked. The check is described in certified or simulated.
• State them for each type. With the same standard insulated double-pane glass, a Vi5 casement is certified at U 0.30 and a Vi5 fixed window at U 0.28.
• Set SHGC by orientation if the model needs it. Add a minimum visible transmittance where daylight matters, because a low SHGC usually darkens the glass.

STC and OITC, by elevation.

Take the requirement from the acoustical consultant. Check which rating it is. For noise from outside the building, it should be OITC, or both. Then put it on the drawings by elevation. The side facing the rail line may need laminated acoustic glass. The courtyard side may not. One number for the whole building costs too much or protects too little. See STC and OITC are not the same number.

Glass, by opening.

The schedule should give the glass makeup for every opening, and flag:

• Safety glazing, tempered or laminated, where the code calls for it: in and beside doors, low glass, wet areas and guards.
• Acoustic glass, by elevation.
• Bird-safe glass, where the local law requires it, with the zone it applies to. See the guide to bird-safe glass.
• Obscure glass in bathrooms, and any spandrel or opaque panels.

Operation, hardware and limiters.

• The operating type for each opening: fixed, in-swing casement, hopper, terrace door or sliding door. Give the handing, seen from a stated side.
• The handle finish, any locking handles, and for accessible units the handle height and operating force.
• Limiters. State which windows carry a four-inch limiter, and whether it is fixed or releasable. Mark the emergency escape windows first, because a fixed limiter cannot go on those. See the four-inch rule.
• Trickle vents, with the open area taken from the mechanical engineer. See trickle vents, explained.
• Insect screens, PTAC louvers, sills, and exterior trims or nailing fins. For terrace doors, give the threshold type.

Color.

State the outside color and the inside color separately. They can differ. Name the color and approve it from a physical sample, never from a screen. Say whether the gaskets, handles, hinge covers and weep hole covers are to match.

The wall around the window.

The spec should say how the window is anchored, how the perimeter joint is sealed, and how it ties into the air and water barrier. It should also say who is responsible for each. A supplier who does not install, as Vistaza does not, provides the instructions and the details. The installer owns the joint. Write that down, so that a leak at the perimeter has an owner.

Submittals.

1. Test reports for each type, with the tested size.
2. NFRC certification, with CPD numbers, for each type and glass.
3. Acoustic lab reports for the rated elevations.
4. Shop drawings with every opening tagged: plans, elevations, types and sections.
5. Samples of color and hardware.
6. Installation instructions, care instructions and warranty terms.

Mock-up and field testing.

Say which configurations get a factory mock-up. Pick the largest, the most complex and the most repeated. Say that production is released only after the mock-up is approved. For the field test, name the method, such as AAMA 502, along with the number of specimens, the test pressure and who pays for a retest. An independent agency runs it.

Writing "or equal" so that it means something.

An "or equal" clause with no criteria is an invitation to substitute on price.

Define equal. It means the same class and grade at the project's sizes, a certified U-factor and SHGC no worse than specified, tested STC and OITC for the rated elevations, and the same submittals. Anything less is a different window and needs the design team's sign-off.

To see the choices with their tested numbers, use choose yours. When the schedule and the plans are ready, send plans.