Weather Proof Louvers: Selection Guide for HVAC Ventilation (2026)
Every HVAC system that draws outside air or exhausts indoor air needs a weather proof louver where the duct penetrates the building envelope. Choose wrong, and you get water intrusion, insect infiltration, or excessive pressure drop that starves the system of airflow. This guide covers louver types, materials, sizing, and performance ratings — with product data from our 150-SKU ventilation catalog.
In This Guide
1. What Is a Weather Proof Louver?
A weather proof louver is an exterior-mounted ventilation grille with angled blades designed to allow air to pass through while blocking rain, snow, and debris from entering the duct system. It sits at the building envelope penetration point — where ductwork passes through an exterior wall or soffit — and serves as the first line of defence against the elements.
The fundamental design challenge is balancing two competing requirements: maximum airflow (which requires large openings) versus maximum rain protection (which requires smaller openings with more blade overlap). Every louver represents a trade-off between these two factors, quantified by two key metrics: free area percentage (how much of the louver face is open for airflow) and water penetration rate (how much water passes through at a given wind speed).
Industry testing standards such as AMCA 500-L and AS 4740 measure water penetration at increasing wind velocities. A louver rated to AMCA Class A allows zero water penetration at 75 km/h (47 mph) wind-driven rain — suitable for most commercial applications. Residential applications in sheltered locations can use louvers with lower ratings.
2. Louver Types
| Type | Rain Protection | Free Area | Material | Typical Application | Our Product |
|---|---|---|---|---|---|
| Full Chevron (rectangular) | Excellent (near zero penetration) | 35-45% | Aluminium | Exposed exterior walls, coastal, high-wind | 4 sizes |
| Half Chevron (rectangular) | Good (some penetration in extreme wind) | 50-60% | Aluminium / Plastic | Semi-sheltered walls, residential | 1 size + 3 plastic |
| Round (wall-mount) | Moderate | 50-55% | Plastic (ABS) | Residential exhaust, bathrooms, dryers | 2 sizes |
| Louver Box (integrated collar) | Good to excellent | 40-50% | Steel / Aluminium | Ducted exhaust, range hoods, bathroom fans | 1 config |
| Gravity Flap Grille | Good (flaps close when fan off) | 60-70% (when open) | Plastic | Residential exhaust, prevents backdraft | 3 sizes |
2.1 Full Chevron Louver
The full chevron design uses blades bent into a complete inverted-V profile, creating overlapping channels that deflect water downward while allowing air to pass through the gaps. This geometry provides the highest rain resistance of any louver type — water penetration is virtually zero even in wind-driven rain at 75 km/h.
The trade-off is pressure drop. Full chevron louvers have the lowest free area (35-45%) of any type, meaning the system fan must work harder to push air through. For a given airflow requirement, you need a larger louver face compared to half chevron or gravity flap designs.
Our full chevron louver is available in 4 rectangular sizes from 225×150 mm to 300×300 mm in natural anodised aluminium — the most durable and maintenance-free finish available.
2.2 Half Chevron Louver
Half chevron louvers use 45° angled blades without the complete V closure. Rain deflection relies on gravity and the blade angle rather than a sealed channel. This provides 50-60% free area — significantly more airflow capacity than full chevron at the same face size — but allows some water penetration in extreme wind-driven rain (above 50 km/h).
We offer half chevron in two materials: aluminium (natural anodised) for commercial applications, and plastic (ABS) in 3 round sizes (100-150 mm) for residential exhaust.
2.3 Round Wall Louver
Round louvers mount directly over a circular wall penetration, typically 150 mm (6") or 200 mm (8") diameter. They are the standard choice for residential bathroom exhaust fans, dryer vents, and kitchen range hood exhaust. Our round white louver is available in both sizes with a clean finish that suits residential exteriors.
2.4 Louver Box
A louver box combines the weather proof louver face with an integrated rear collar for direct duct connection. This eliminates the need for a separate transition fitting between the louver and the duct, simplifying installation and reducing potential air leaks. Our black louver box includes a 6" (150 mm) collar ready for flexible duct connection.
2.5 Gravity Flap Grille
Gravity flap grilles are not louvers in the traditional sense — they use hinged flaps that open when the exhaust fan is running (air pressure pushes them open) and close by gravity when the fan stops. The closed flaps prevent backdraft (cold outside air flowing back through the duct) and provide moderate rain protection. Our gravity flap grille is available in 3 round sizes (100-150 mm) in white plastic.
3. Material Comparison
| Property | Aluminium (Anodised) | Plastic (ABS/ASA) | Galvanised Steel |
|---|---|---|---|
| Corrosion Resistance | Excellent (20+ years) | Good (no metal corrosion) | Moderate (10-15 years) |
| UV Resistance | Excellent | Moderate (ASA better than ABS) | Good (with paint) |
| Structural Strength | High | Low-Medium | High |
| Weight | Light | Very light | Heavy |
| Max Size | Up to 2,000×1,500 mm | Up to 300×300 mm typical | Up to 3,000×2,000 mm |
| Finish Options | Natural anodised, powder coat | White, black, grey | Powder coat (RAL) |
| Cost (relative) | 1.0× (baseline) | 0.4-0.6× | 0.8-1.0× |
| Maintenance | None | None (but may yellow) | Repaint every 7-10 years |
| Best For | Commercial, coastal, premium | Residential, budget, small sizes | Industrial, high-impact areas |
Recommendation: Aluminium with natural anodised finish is the best all-round choice for any application exposed to weather. The anodising process creates a hard oxide layer that is integral to the metal surface — it cannot peel, chip, or flake like paint. Our aluminium louvers carry a 20+ year service life expectation with zero maintenance.
4. Complementary Ventilation Components
A complete ventilation penetration requires more than just the louver. Here are the components that work together with weather proof louvers in our Ventilation & Louvers catalog of 150 SKUs:
| Component | Function | Our Range |
|---|---|---|
| Weather proof louver | Rain/debris protection at wall penetration | 11 products, multiple sizes |
| Inline fan | Moves air through the duct | 3 sizes (6", 8", 10") |
| Roof cowl | Rain protection at roof penetration | 6" size |
| Door grille | Transfer air between rooms (under doors) | 10 sizes (600-1000 mm) |
| Egg crate grilles | Interior return/exhaust openings | 26 sizes (fixed + removable core) |
| Eave/soffit vents | Passive ventilation in roof spaces | Multiple configurations |
The advantage of sourcing all ventilation components from one manufacturer is dimensional consistency. Our louver collar sizes match our duct sizes, our inline fan diameters match our louver collars, and our roof cowl fits our standard duct. No field modifications needed.
5. How to Size a Louver
Louver sizing follows a simple formula, but getting it wrong has outsized consequences. An undersized louver starves the system of airflow, increases energy consumption, and creates noise. An oversized louver wastes wall space and money but has no performance downside — so always round up.
Step 1 — Determine required airflow. This comes from the mechanical engineer's ventilation schedule or the fan specification. Typical values: bathroom exhaust 25-50 L/s (50-100 CFM), kitchen range hood 150-300 L/s (300-600 CFM), commercial fresh air intake 500-5,000 L/s (1,000-10,000 CFM).
Step 2 — Set maximum face velocity. For weather proof louvers, keep face velocity below 2.5 m/s (500 FPM). Above this speed, wind-driven rain can be carried through the louver blades. For critical applications (server rooms, hospitals), reduce to 2.0 m/s.
Step 3 — Calculate required free area. Free area = Airflow ÷ Face velocity. For 500 L/s at 2.5 m/s: Free area = 0.5 m³/s ÷ 2.5 m/s = 0.2 m².
Step 4 — Convert to louver face size. Louver face area = Free area ÷ Free area percentage. For a full chevron louver at 40% free area: Face area = 0.2 ÷ 0.4 = 0.5 m². This means a 700×700 mm louver (0.49 m²) or 500×1000 mm (0.5 m²).
| Airflow | Full Chevron (40% FA) | Half Chevron (55% FA) | Application Example |
|---|---|---|---|
| 25 L/s (50 CFM) | 150×150 mm | 100×100 mm | Small bathroom exhaust |
| 50 L/s (100 CFM) | 225×150 mm | 150×150 mm | Standard bathroom |
| 150 L/s (300 CFM) | 300×300 mm | 250×250 mm | Kitchen range hood |
| 500 L/s (1,000 CFM) | 500×500 mm | 400×400 mm | Commercial fresh air |
| 1,000 L/s (2,000 CFM) | 700×700 mm | 550×550 mm | Large commercial AHU |
6. Installation Guidelines
Mount with a 3-5° outward tilt. Tilting the louver slightly outward (top further from the wall than the bottom) allows water that collects on the blades to drain forward and away from the building, rather than running down the interior of the wall cavity. A 3-5° tilt is sufficient. Some louver frames include drain holes at the bottom for collected water.
Seal the louver-to-wall joint. The gap between the louver frame and the wall opening must be sealed with exterior-grade silicone sealant or a compression gasket. This joint is a common water entry point — even a 2 mm gap can allow significant water intrusion in wind-driven rain. Apply sealant to all four sides and check that the bead is continuous.
Maintain clearance from the ground. Exterior louvers should be mounted at least 300 mm (12 inches) above ground level, or 150 mm above the expected snow line in cold climates. Ground-level mounting leads to snow blockage, splash-back water entry, and debris accumulation.
Consider bird/insect screening. For intake louvers, install a 12.5 mm (1/2") bird screen behind the louver face to prevent birds from nesting in the duct. For insect protection, add a 1.5 mm stainless steel or aluminium woven mesh behind the bird screen. Note that each layer of screening adds pressure drop — account for this in the fan selection.
Provide access for cleaning. Exterior louvers accumulate dirt, leaves, and debris that reduce free area over time. In areas with significant pollen, dust, or leaf fall, plan for annual cleaning. Removable louver faces or hinged designs simplify maintenance access.
7. Sourcing from Airwise
Our Ventilation & Louvers catalog covers 21 product types and 150 SKUs — from full chevron louvers in natural anodised aluminium to gravity flap grilles in plastic to inline fans and roof cowls.
Because ventilation penetrations involve multiple components (louver + duct + fan + grille), sourcing everything from one manufacturer eliminates the compatibility guesswork. Our louver collar sizes match our duct diameters (206 SKUs), and our inline fans fit directly into our duct system without adaptors.
All aluminium louvers are manufactured from extruded 6063-T5 aluminium with natural anodised finish at our Dongyang factory. Standard items ship within 15-25 business days. Custom sizes and powder coat finishes (RAL colours) available with 30-45 day lead time.