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) | ≤ 0.32 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 requirement (the zone Florida is in): U-Factor ≤ 0.32 and SHGC ≤ 0.23. 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
- ✓Inside the HVHZ — HVHZ-approved product required
- ✓Miami-Dade NOA is the standard proof of HVHZ approval
- ✓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
- ✓Inside the HVHZ — HVHZ-approved product 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
Which Impact Window Ratings Matter Most? A Priority Checklist
Impact window ratings are the numbers on the product approval and label that describe storm strength, energy performance, sound, and light. Not all carry the same weight. Check them in this order.
| Priority | Rating | What it decides |
|---|---|---|
| 1. Must-have | NOA / Florida Product Approval | Whether the product can be permitted at all |
| 1. Must-have | Design pressure (DP / PG) | Whether it meets the wind load your permit requires |
| 1. Must-have | Missile impact level | Whether it meets the debris-impact requirement for the opening |
| 2. Comfort and cost | SHGC and U-factor | Heat gain and energy use with your A/C |
| 3. Nice to have | STC | How much outside noise is blocked |
| 3. Nice to have | VT | How much daylight gets through the glass |
Common Mistakes When Comparing Window Ratings
- Comparing ratings across different sizes. Ratings are approved per model and size range. A rating on a small unit does not automatically apply to a larger one.
- Treating energy ratings as storm ratings. A low SHGC says nothing about hurricane strength.
- Trusting a marketing claim without an approval number. Ask for the NOA or FPA and check the model against it.
- Buying more DP than your permit requires. Match the requirement; extra margin can add cost.
- Ignoring glass options. Different glass packages can change SHGC, STC, and price on the same frame.
Compare Real Products and Prices
Windows Stock Market lists in-stock PGT, ESW, and Mr. Glass impact windows with a price per item by size, color, and glass. Use the instant price tool or browse the marketplace to line up the ratings against the price. Related guides: impact window permits (we coordinate the permit process), PGT vs ESW vs Mr. Glass, 2026 cost guide, and hurricane vs impact windows.
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?↓
What ratings matter most on an impact window?↓
How do I read the label on an impact window?↓
Is a higher DP rating always better?↓
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.