Impact Window Ratings Explained: DP, NOA, SHGC, STC — What Every Florida Homeowner Needs to Know
The label on an impact window product sheet is full of acronyms. This guide decodes every number — in plain English — so you know exactly what you need for your county, your home's height, and your climate.
Quick Reference: What Each Rating Means
| Rating / Term | What It Measures | What's Good for Florida | Required? |
|---|---|---|---|
| DP (Design Pressure) | Wind load the window can handle (psf) | DP+40 minimum for HVHZ; DP+30 inland | Yes |
| NOA (Notice of Acceptance) | Miami-Dade HVHZ product certification | Required in Miami-Dade; accepted everywhere | In Miami-Dade |
| FPA (FL Product Approval) | Statewide impact product certification | Accepted in all FL counties except Miami-Dade alone | Yes (statewide) |
| SHGC | Solar heat passing through glass (0–1) | SHGC ≤ 0.23 for Energy Star; lower = cooler | For ENERGY STAR |
| U-Factor | Heat transfer rate (lower = more insulating) | ≤ 0.40 for Energy Star Southern Zone | For ENERGY STAR |
| STC | Sound blocking ability (higher = quieter) | STC 38+ near airports/highways; 32+ baseline | No |
| VT (Visible Transmittance) | How much visible light passes through (0–1) | ≥ 0.40 for bright interiors; tinted = lower VT | No |
| PG (Performance Grade) | Same as DP — used in AAMA standards | PG50 = DP+50; same interpretation | Yes |
| Large Missile / Small Missile | Impact test type (debris simulation) | Large missile for windows within 30 ft of grade | By height |
DP (Design Pressure) Rating — The Most Important Number
Often labeled as DP, PG, or PSF on product sheets
Design Pressure is the maximum wind load a window can handle without structural failure — measured in pounds per square force (psf). Think of it as the window's wind resistance rating. The higher the DP number, the stronger the window.
How DP Numbers Are Tested
- → Window is mounted in a test frame
- → Air pressure is applied in positive and negative cycles simulating hurricane wind loads
- → Then a 9-lb 2×4 plank is fired at it (large missile test)
- → Then cyclic pressure continues to simulate the full storm event
- → Window passes if it remains in frame without penetration
What DP Rating You Need in Florida
Your contractor will calculate site-specific design pressures using FBC tables for your exact address and opening.
Important: DP ratings are given separately for positive pressure (wind pushing in) and negative pressure (suction pulling out). You need both to be sufficient. Product labels show them as DP+40/-50, for example — the window handles +40 psf positive and 50 psf negative pressure.
Miami-Dade NOA vs Florida Product Approval: The Definitive Explanation
Miami-Dade NOA
(Notice of Acceptance)
Florida Product Approval
(FPA / FL Number)
| County | NOA sufficient? | FPA sufficient? | Both accepted? |
|---|---|---|---|
| Miami-Dade County | ✅ Yes | ❌ No (NOA required) | ✅ Yes (NOA + FPA both fine) |
| Broward County | ✅ Yes | ✅ Yes | ✅ Yes |
| Palm Beach County | ✅ Yes | ✅ Yes | ✅ Yes |
| All other FL counties | ✅ Yes | ✅ Yes | ✅ Yes |
SHGC and U-Factor: The Energy Ratings That Affect Your Electric Bill
SHGC — Solar Heat Gain Coefficient
Measures what fraction of solar radiation passes through the glass as heat. Scale: 0 to 1. Lower = less solar heat entering your home.
ENERGY STAR 2026 Southern Zone requires SHGC ≤ 0.23. Most quality impact glass is 0.23–0.27.
U-Factor — Thermal Insulation
Measures how quickly heat transfers through the glass. Lower = better insulation. In Florida, AC efficiency matters more than heating — but U-Factor still affects overall energy bills.
ENERGY STAR Southern Zone U-Factor requirement: ≤ 0.40. Most standard impact glass does NOT qualify — must use Low-E.
Florida Rule: Never buy impact windows without Low-E glass in South Florida. The extra cost ($15–$30/window) pays back in 1–2 years through lower AC bills and qualifies you for the federal 25C tax credit.
STC Rating — Sound Transmission Class
STC measures how many decibels of sound a window blocks. It's not a code requirement in Florida — but it matters enormously if you're near FLL airport, I-95, or Tri-Rail.
| Glass Type | Typical STC | What You'll Hear | Good For |
|---|---|---|---|
| Regular single-pane glass | STC 26–28 | Loud speech audible | Quiet suburban areas only |
| Standard laminated impact glass | STC 32–38 | Speech muffled; traffic subdued | Most Florida neighborhoods |
| Laminated Low-E impact glass | STC 35–42 | Speech barely audible; traffic quiet | Near busy roads, transit |
| Laminated IGU (insulated unit) | STC 42–50 | Only loud trucks barely heard | Near airports, major highways |
| Acoustic laminated IGU (specialty) | STC 48–55 | Near-silence indoors | FLL/MIA approach path, I-95 frontage |
Note: IGU = Insulated Glass Unit (two panes with air or argon space). IGU impact windows cost more but deliver significantly better STC and U-Factor performance. Most in-stock WSM inventory is laminated single-pane — specify IGU if noise is a priority.
VT (Visible Transmittance) — How Bright Is Your Home?
VT measures what fraction of visible light passes through glass. Scale: 0 to 1. Higher = brighter interior. It's often the rating homeowners forget about — then complain their home feels dark after window installation.
Bronze, gray, charcoal tints. Very low solar heat gain but dim interiors. Better for west-facing windows in hot climates.
Light bronze or blue-green tint. Good energy performance + reasonably bright interior. Most standard Low-E impact glass falls here.
Nearly clear glass. Maximum natural light. Higher SHGC than tinted options but best aesthetics. Good for north-facing and interior rooms.
What You Actually Need: By County
Miami-Dade County
- ✓Miami-Dade NOA (required, not optional)
- ✓DP+40 minimum ground floor
- ✓DP+50+ for upper floors
- ✓Large missile test for ground/near-grade
- ✓SHGC ≤ 0.27 recommended (Low-E)
- ✓Licensed FL contractor required
- ✓Building permit required — no exceptions
Broward County
- ✓Florida Product Approval (FPA) required
- ✓NOA also accepted (both valid)
- ✓DP+40 minimum for coastal areas
- ✓DP+35 acceptable for some inland areas
- ✓SHGC ≤ 0.27 recommended
- ✓Licensed FL contractor required
- ✓Building permit required
Palm Beach County
- ✓Florida Product Approval (FPA) required
- ✓NOA also accepted
- ✓DP+40 coastal / DP+30–35 inland
- ✓SHGC ≤ 0.27 for Energy Star
- ✓Licensed FL contractor required
- ✓Building permit required
- ✓Wind mitigation inspection for insurance discount
Frequently Asked Questions
What DP rating do I need for impact windows in Florida?↓
What does SHGC mean and what SHGC should I choose in Florida?↓
What is an STC rating for impact windows?↓
What is the difference between Miami-Dade NOA and Florida Product Approval?↓
What is a large missile vs small missile impact test?↓
What does 'PG' (Performance Grade) mean on impact windows?↓
Need Help Choosing the Right Rated Impact Windows?
Vieser Construction provides free estimates with full specification details — DP ratings, NOA/FPA documentation, SHGC values, and STC performance. We take care of the technical side so you don't have to.