A window can have a strong design-pressure rating without being guaranteed to admit zero water in every storm.
That surprises many Florida homeowners because several different performance terms are often grouped together in sales conversations.
Impact resistance, structural pressure, air leakage and water penetration are related—but they are not the same test.
What does design pressure describe?
Design pressure, often abbreviated DP, relates to wind pressure acting on the window or door.
The required pressure depends on the building and opening conditions.
The product approval then shows whether the exact product and configuration is rated for the required positive and negative pressures.
Positive and negative pressure both matter because wind pushes against some surfaces and creates suction on others.
See our Florida design pressure guide for the broader calculation context.
What is water-penetration resistance?
Water testing evaluates the product’s resistance to water entering through the assembly under defined laboratory conditions.
The North American Fenestration Standard uses water penetration as a distinct performance criterion.
That distinction matters because water behavior depends on:
- Pressure difference
- Drainage design
- Sill construction
- Weatherstripping
- Frame joints
- Glass-to-frame seals
- Installation
- Surrounding wall conditions
A strong structural rating does not mean a product is an aquarium wall.
What does impact resistance test?
Impact testing addresses wind-borne debris and the way the complete assembly behaves after the specified impacts and pressure cycling.
It does not replace water testing.
Likewise, a product that passes a water test is not automatically hurricane-impact approved.
For impact approval, verify the actual Florida Product Approval or Miami-Dade NOA.
What about air leakage?
Air leakage is another separate performance category.
An operable window has joints, weatherstripping and moving components.
Different window styles and products can therefore have different air-leakage performance even when both satisfy the project’s impact and pressure requirements.
For energy comparisons, the NFRC label can also include air-leakage information where reported.
Why NAFS performance terminology matters
The North American Fenestration Standard combines multiple performance requirements into product classification and rating systems.
The 2026 edition of NAFS was published this year, and FGIA says its notable update is a clarification making the secondary designator mandatory.
That secondary information identifies underlying values used in the rating, including structural and water-related performance.
Our separate NAFS-26 guide explains that 2026 change.
Can a hurricane still drive water through a window?
Severe storms can expose a building to conditions that differ from a standardized laboratory test.
Water can also enter from areas around the window, including:
- Wall cracks
- Failed perimeter sealant
- Flashing defects
- Sill or drainage problems
- Roof or façade leaks
- Adjacent doors
- Wind-driven rain entering through another opening
Do not assume visible water automatically proves the glass or frame failed.
A proper diagnosis should identify the actual path.
What should you compare on a proposal?
Ask for:
- Exact manufacturer and series
- Product approval
- Positive and negative design pressures
- Approved maximum size
- Impact classification
- Water-performance data
- Air-leakage data where relevant
- Glass configuration
- Installation and flashing details
- Warranty exclusions and maintenance requirements
A proposal that only says “hurricane rated” leaves too much unanswered.
Can installation affect water performance?
Absolutely.
Even a well-tested window can perform poorly if installed outside the approved or manufacturer-required conditions.
Important installation details can include:
- Square and plumb frame
- Approved shim space
- Correct anchors
- Perimeter sealant
- Sill conditions
- Drainage paths
- Flashing
- Wall condition
Our bucks, anchors and substrate guide explains the structural side of those installation details.
Frequently asked questions
Does a higher DP mean a window is more waterproof?
Not automatically. Structural design pressure and water-penetration resistance are distinct performance characteristics.
Are impact windows waterproof?
No responsible manufacturer or contractor should describe a window as immune to all water intrusion under every storm condition.
Does water around a window always mean the window failed?
No. The path can involve the unit, installation, sealant, drainage, flashing or surrounding wall system.
Should I compare water ratings when buying impact windows?
If water performance is important for the project, yes. Ask for product-specific tested information instead of relying on the impact label alone.
Authoritative sources
- FGIA — 2026 North American Fenestration Standard Published
- ANSI Webstore — NAFS scope and performance categories
- Florida Building Commission — Product Approval Search
- PGT Windows & Doors — Approvals and Certifications
The bottom line
Impact, structural pressure, air leakage and water penetration answer different questions.
A strong Florida window comparison looks at all of them—and then verifies that the exact approved product is installed correctly in the actual opening.
AA Glass & Windows can help you compare impact-window performance documentation before selecting a product. Request a free estimate to discuss your openings.
