Nine mistakes developers make when buying windows.
Where window packages go wrong at bid stage, and the questions to put to any supplier, including us.
Vistaza Engineering · Apr 20, 2023 · Updated Sep 1, 2026 · 6 min read
Most window problems on a project are bought, not built. They start at bid stage, when quotes that describe different things are compared on price alone. The mistakes below are common and cheap to avoid. The questions at the end work on any supplier. Put them to Vistaza as well.
1. Comparing prices before comparing scopes.
Two quotes for "the window package" rarely cover the same things. One includes limiters, screens, sills, mullions, trims and delivery to site. The other lists them as extras, or leaves them out. One prices the acoustic glass on the street elevation. The other prices standard glass everywhere. Line the quotes up item by item before looking at the totals.
If two quotes are far apart, they are probably not for the same window.
2. Taking a simulated number for a certified one.
Every U-factor begins as a simulation. A certified value has been through NFRC and carries a CPD number that anyone can look up. A simulated or center-of-glass value on a quote is not the number the energy code counts. Ask for the certificate number. If there is none, the value is a claim. The check is set out in certified or simulated.
3. Accepting a rating for a different type or size.
Structural ratings belong to one window type at one tested size. The highest rating in a series is usually held by one type, and it does not carry over to the others. The same holds for sound. A casement and a fixed window with identical glass do not test alike. Ask for the report for the type you are buying, and read the tested size on it. For bigger openings, ask how the rating is carried over.
4. Buying the letters and ignoring the number.
CW and AW are classes. The grade after them is the design pressure. It must meet the wind load for your building, at your corner zones and your upper floors. A high class with too low a grade does not help you. See what a window's class really tells you.
5. One sound rating for the whole building.
Buy acoustic glass for every elevation and you overpay. Buy none and the street-side apartments are loud. Get the requirement by elevation from the acoustical consultant. Ask for OITC when the noise comes from outside.
6. Choosing the material by habit.
Each frame material has work that suits it.
| Material | Where it is the better choice | What to check |
|---|---|---|
| Aluminum | Very large openings, very slim frames, curtain wall and storefront, tall towers | The thermal break, and the certified U-factor of the whole window |
| Wood, or wood with cladding | Historic work, and interiors where the look of wood matters | How the finish will be maintained, and the exposure |
| Residential vinyl | Houses and small residential buildings | The performance class and grade, against the building's design pressure |
| Reinforced uPVC built for commercial ratings | Mid-rise residential work that needs low U-factors, high sound ratings and little upkeep | The class and grade of each type, at your sizes |
Residential vinyl and commercial-rated uPVC belong to the same family of material. The difference lies in the profile design, the steel reinforcement and the ratings. Judge any frame by its test reports, not by the name of the material.
7. Reading the lead time as one number.
A lead time is two clocks. The first is shop drawings and their approval. The second is fabrication and shipping, and it normally starts only when the drawings are approved. A slow review on your side costs as much time as a slow factory.
Vistaza's current figures are 1–2 weeks for the first set of shop drawings, 1–3 days for each revision, and 8–10 weeks for delivery. Ask every supplier the same three things. Ask what event starts the clock, and whether the date holds for phased deliveries.
8. Treating shop drawings as paperwork.
Approving the shop drawings is the last moment at which a change is free. A good set shows every opening with a tag, its size, how it opens, its glass, its colors and its options. Look at a sample set from a past job before you sign with anyone. Drawings that are vague turn into disputes on site. What a full set contains is on the shop drawings page.
9. Not asking who installs, and who serves the building afterwards.
Some suppliers install. Others, Vistaza among them, supply only. Both arrangements work. A single contract for supply and installation puts the responsibility in one place. A supply-only contract lets you choose the installer. Then the perimeter joint, the flashing and the field test belong to that installer, and the contracts must say so. Ask a supply-only supplier what the installer gets: instructions, details, training, someone on site when questions come up.
Then look past turnover. The windows will be in the building long after the project team has left. The super will need handles, gaskets, hinges and sometimes a glass unit. Ask how parts are ordered, how one unit is identified among hundreds, and whether the care instructions exist in the languages your staff read.
Questions for any supplier.
| Ask | A good answer |
|---|---|
| What is the NFRC CPD number for this type with this glass? | A number you can look up yourself |
| Can I see the structural report for this type, with the tested size? | A report from an accredited lab, for the type you are buying |
| How are my larger sizes covered? | A comparative analysis, a mock-up test, or both |
| What are the STC and the OITC for the whole window? | A lab report for that type and that glass. Not a glass-only figure, and not a percentage |
| What exactly is in the price? | A list: glass by elevation, limiters, screens, sills, trims, mullions, delivery, shop drawings |
| What is the lead time, and from what event? | Separate figures for drawings, revisions and delivery |
| May I see a full shop drawing set from a past job? | Yes |
| Who installs, and what support does the installer get? | A clear split of responsibility, in writing |
| How does my super order a part years from now? | A parts list, a named contact, and a way to identify the unit |
| What does the warranty cover, and who handles a claim? | The written terms, before you sign |
A supplier who answers all ten in writing is worth comparing on price. One who cannot has told you something useful too.
When you are ready to compare, send plans.