1. Why Energy Ratings Matter More Than Ever
Once upon a time, choosing windows was about frames and finishes. Georgian bars or not? Oak or white? Left-hinge or right?
Today, there’s a new question at the top of the list:
“How much energy will these save me?”
With rising energy bills, tighter building regulations, and a growing focus on sustainable living, the thermal performance of your windows has become a financial, environmental, and even legal issue.
💸 The Financial Pressure: Energy Bills Aren’t Dropping
- UK energy prices remain volatile
- Heating accounts for over 60% of domestic energy use
- Heat loss through poor windows = money out the window—literally
- Efficient windows can reduce heat loss by 30% or more, especially in older homes
🌍 The Environmental Pressure: Net Zero & EPC Targets
- By 2030, UK homes must dramatically reduce emissions
- EPC ratings (Energy Performance Certificates) now influence property value, mortgage approval, and lettability
- Poor-performing windows can single-handedly prevent an EPC uplift
📏 The Regulatory Pressure: Building Regs & Renovation Standards
- Updated Part L Building Regulations (2022) set tougher efficiency benchmarks
- Extensions, retrofits, and new builds must now hit tighter U-value limits (e.g., ≤1.2 W/m²K for windows)
- Some councils enforce even stricter local targets, especially in eco-sensitive zones
🧠 What’s Changed in the Conversation?
Performance is no longer a “nice-to-have.”
It’s a non-negotiable.
Homeowners now want windows that:
- Look right for their architecture
- Keep their home warm and dry
- Lower their bills
- Futureproof against coming regulation
🔍 So Where Does Aluclad Fit In?
That’s what the rest of this guide is here to show.
Because aluclad windows—especially triple-glazed systems from Internorm and Rationel—don’t just meet modern energy demands.
They often exceed them.
And yes—they do it while still looking great in conservation areas, barn conversions, and period homes.
Next: Let’s unpack what “Passivhaus” really means—and why your windows are the make-or-break component.
2. What Is Passivhaus – And Why Do Windows Matter So Much?
You’ve probably heard the term. “Passivhaus.”
It conjures ideas of hyper-modern homes sealed tighter than a submarine, sipping energy and humming in comfort.
But what actually is it?
And why do windows—especially aluclad windows—play such a decisive role?
🏠 Passivhaus in a Nutshell
Passivhaus (or Passive House) is a world-leading building standard that focuses on extreme energy efficiency, comfort, and airtightness.
To qualify, a building must achieve:
- Very low heating demand (≤15 kWh/m²/year)
- Exceptional airtightness (≤0.6 air changes/hour at 50Pa)
- Minimal thermal bridging
- Comfortable internal temperatures year-round—with minimal mechanical input
It’s not just for eco-warriors or new builds. More UK architects and renovators are adopting Passivhaus or EnerPHit (the retrofit variant) as future-proof benchmarks.
🪟 Why Windows Are the Make-or-Break Factor
In a Passivhaus build, windows are the thinnest part of the envelope—and the most vulnerable to energy loss.
Bad windows =
- Cold spots
- Overheating in summer
- Draughts
- Condensation
- Failed airtightness tests
That’s why windows must hit exacting performance metrics—especially:
- U-values below 0.8–1.0 W/m²K
- High solar gain control (g-values)
- Deep frame installation to reduce bridging
- Airtight seals + compression gaskets
🔍 The Passivhaus Window Triangle
Every Passivhaus window must strike a balance between:
| Metric | Goal | Why It Matters |
|---|---|---|
| U-value | ↓ Lower = Less heat loss | Keeps warmth in during winter |
| g-value | ↔ Balanced | Avoids overheating from sun in summer |
| Airtightness | ↑ Total seal | Prevents air leakage, moisture, noise |
Aluclad windows—especially those from Internorm and Rationel—are designed with these principles baked in.
They’re not “good enough.”
They’re engineered for precision performance.
✅ Certified Performance (Not Marketing Hype)
- Many aluclad systems are PHI-certified (Passivhaus Institute)
- Triple glazing, thermally broken frames, and layered gaskets come standard
- Options include solar control coatings, insulated spacers, and integrated blinds
With the right installation, aluclad can help even older homes hit EnerPHit targets.
Next: Let’s decode the three technical specs that matter most—U-values, g-values, and airtightness—so you can read window data sheets like a pro.
3. U-Values, g-Values, & Airtightness — Demystified
Window specs often read like engineering homework:
U = 1.0 W/m²K, g = 0.52, Class 4 air permeability…
But these aren’t just numbers. They’re the performance DNA of your windows.
Here’s how to understand them—without a physics degree.
🌡️ What’s a U-Value?
- Measures how much heat a window loses
- Expressed as W/m²K = watts lost per square metre per degree of temperature difference
- Lower is better
| Type of Window | Typical U-Value |
|---|---|
| Single-glazed sash | ~5.0 W/m²K |
| Double-glazed uPVC | ~1.4–1.6 W/m²K |
| Aluclad triple-glazed | 0.8–1.0 W/m²K ✅ |
Passivhaus target: ≤0.8
Building regs (2022): ≤1.2
☀️ What’s a g-Value?
- Measures how much solar heat gain the glass allows in
- Scale from 0 to 1 (or 0% to 100%)
- Higher = more free warmth from the sun
- Lower = better shading / cooling
| Glass Type | g-Value Range |
|---|---|
| Standard clear triple-glaze | ~0.60–0.70 |
| Solar control glazing | ~0.40–0.55 |
| Passive balance glazing | ~0.50–0.65 ✅ |
Balance is key: too low and you lose winter heat gains; too high and you overheat in summer.
💨 Airtightness: The Invisible Hero
- Often overlooked—but critical for energy savings
- Airtight windows mean no draughts, whistling, or heat escape
- Tested by “air permeability classes” (BS EN 12207 standard)
| Airtightness Class | Air Leakage (m³/h·m²) | Performance |
|---|---|---|
| Class 1 | ≤50 | Basic |
| Class 3 | ≤12 | Good |
| Class 4 | ≤3 | Best ✅ |
Most aluclad systems from Internorm and Rationel are Class 4 certified—sealed tight against UK weather.
🔧 What Affects These Numbers?
- Glass type (low-E coatings, thickness)
- Frame material and depth
- Spacers between glass panes
- Seals, gaskets, and installation quality
- Position in the wall (deep vs flush)
✅ TL;DR: The Ideal Spec
For Passivhaus-aligned, high-performance homes:
- U-value: ≤0.8 W/m²K
- g-value: 0.5–0.6
- Airtightness: Class 4 (≤3 m³/h·m²)
And guess what?
Well-specified aluclad windows hit all three—while looking stunning.
Next: Let’s dive deeper into how aluclad windows actually achieve these numbers—and why they outperform traditional timber, PVC, and basic aluminium in every metric.
4. How Aluclad Windows Excel in Energy Ratings
Let’s now connect the dots.
You know the benchmarks: low U-values, balanced g-values, and airtightness that keeps your heating in and the cold out.
But how do aluclad windows specifically deliver all three—while outperforming traditional PVC, timber, and aluminium frames?
🧬 The Material Synergy: Timber + Aluminium
- Timber core: Excellent natural insulator, reduces thermal bridging
- Aluminium shell: Provides weatherproofing, protects from UV and moisture, extends lifespan
- Bonded together with pressure-laminated technology for strength and stability
This combo means you get the thermal efficiency of wood, without the rot, warping, or high maintenance.
🧊 Glazing That Works Harder
Most aluclad systems come triple-glazed as standard:
- 3 panes of low-E coated glass
- Argon or krypton gas-filled cavities
- Warm-edge spacer bars (to reduce edge loss)
- U-values as low as 0.8 W/m²K — Passivhaus-grade
And the best part?
It all fits in sleek, modern profiles—often with more glass, less frame.
🛡️ Frame Design That Seals Everything Tight
- Multi-layer seals between sash and frame
- Compression gaskets to eliminate air gaps
- Hidden drainage systems that avoid thermal breaks
- Ventilated frame design to control condensation
These aren’t bolt-on features—they’re baked into aluclad from the blueprint level.
💪 Stability That Keeps Its Numbers
PVC can sag. Timber can swell. Aluminium can warp under heat.
Aluclad stays rigid. The aluminium skin protects the timber from environmental movement, meaning:
- No performance drift
- No seasonal swelling
- No weakening of seals or joints
- Long-term retention of U-value & airtightness ratings
🧰 Examples of Real-World Performance
| Brand | U-value (W/m²K) | Airtightness | Passivhaus Ready? |
|---|---|---|---|
| Internorm | 0.62 – 0.79 ✅ | Class 4 ✅ | ✅ Yes |
| Rationel | 0.78 – 1.0 ✅ | Class 3–4 ✅ | ⚠️ Depends on spec |
| Velfac | 0.85 – 1.1 | Class 3 | ❌ Not certified |
Note: All available via Cherwell showroom
🎯 Bottom Line
Aluclad doesn’t just “do the job.”
It does the job better, longer, and with more architectural flexibility.
You can hit your energy goals—without compromising on aesthetics or future-proofing.
Next: Let’s compare how different aluclad brands stack up in Passivhaus or high-efficiency projects—and where Cherwell fits in the process.
5. Comparing Aluclad Brands for Passivhaus Projects
Not all aluclad windows are created equal.
While many brands offer “triple-glazing” and “low U-values,” very few are engineered with true Passivhaus or EnerPHit performance in mind.
Let’s break down how the leading aluclad systems compare—so you can align spec with ambition.
🥇 Internorm: The Passivhaus Benchmark
- Certified by the Passivhaus Institute for multiple window systems
- U-values down to 0.62 W/m²K
- Exceptional airtightness (Class 4), even at large scale
- Integrated blinds between glazing for solar control
- Highly modular: choose frame depths, finishes, handles, and even automation
Best for:
- Self-build Passivhaus
- New builds or high-spec retrofits
- Architects needing hard numbers & proven results
Available via Cherwell – Full systems on display in Banbury showroom.
🥈 Rationel: Balanced Performance + Heritage Flexibility
- U-values as low as 0.78 W/m²K
- Class 3–4 airtightness
- Suits both modern and traditional builds (FormaPlus range ideal for heritage)
- FSC-certified timber core + aluminium outer
- Broad palette of RAL finishes
Best for:
- Retrofits with traditional aesthetic needs
- Homeowners wanting performance without sacrificing style
- Listed buildings or conservation zones (with planning-friendly visuals)
🥉 Velfac: Design-Led, Modestly Efficient
- U-values typically around 0.85–1.1 W/m²K
- Slim frames = more light, but less insulation
- Not Passivhaus-certified
- Air permeability = Class 3
- Elegant aesthetics, but lighter on airtightness sealing tech
Best for:
- Homeowners prioritising natural light and sleek design
- Mid-tier energy performance goals
- Projects where Passivhaus is not a core requirement
🧠 Cherwell’s Role in the Brand Decision
We’re not just a reseller—we’re a specification partner.
At Cherwell, we help you:
- Compare exact window systems side-by-side
- Break down which frames hit your target U-values
- Match finishes to your home’s character
- Understand lead times, planning implications, and thermal modelling
⚠️ Important: Performance Is Spec-Dependent
You can have a “triple-glazed aluclad window” that performs poorly—if specified or installed wrong.
That’s why our in-showroom consultations walk through:
- Glazing unit depth
- Spacer bar types
- Frame thickness & installation position
- Ventilation strategy (including trickle vents & MVHR compatibility)
Next: Let’s explore how aluclad performs in real-world new builds and retrofits—and what you should watch out for depending on your property type.
6. Aluclad in Retrofit vs New Build — What Changes?
Whether you’re building your dream Passivhaus from scratch or upgrading a draughty Victorian semi, aluclad windows can deliver world-class performance.
But the way you specify, install, and seal them will vary—dramatically—between retrofit and new build.
Let’s break it down.
🧱 In New Builds: Maximum Control, Minimal Compromise
Advantages:
- Walls, reveals, and frames designed around window dimensions
- Deep installation possible (insulating over frame edges)
- Seamless integration with airtight membranes, tapes, and MVHR
- Freedom to optimise solar orientation and glazing ratios
- Pre-certified Passivhaus components fit easily into workflow
How Aluclad Performs:
- Internorm systems like HF 410 or KF 520 are ideal
- U-values can hit 0.62 W/m²K with full envelope coordination
- Triple-glazed units enhance acoustic comfort for suburban sites
- Architects can match RAL colours to the design palette in advance
🧰 In Retrofits: More Constraints, Smarter Workarounds
Challenges:
- Limited reveal depth or wall thickness
- Brick or stone openings with irregular shapes
- Existing lintels, sills, or damp membranes may not align
- Listed buildings = external appearance can’t change
Solutions with Aluclad:
- Use slimline or heritage-profiled frames (Rationel FormaPlus)
- Opt for secondary glazing setups in listed homes
- Install within existing frames (if sound) to reduce disruption
- Add airtight tapes + expanding foam for improved seal
Performance Boosts:
- U-values around 0.78–1.0 W/m²K still possible
- Airtightness improved 2–3× over old casements
- Energy rating uplift can raise EPC bands in period properties
🏡 EnerPHit Retrofit Examples (Cherwell Installs)
- 1950s Oxford bungalow: Internorm windows + external insulation → EPC from D to B
- Stone cottage, Cotswolds: Rationel triple-glaze + airtight install → 30% heat loss reduction
- 1980s detached, Banbury: Aluclad upgrade with deep reveals + solar control glass → summer overheating mitigated
🧠 Takeaway:
New build? Aluclad lets you optimise.
Retrofit? Aluclad lets you adapt.
Either way—you get comfort, control, and compliance.
Next: Let’s bust a few myths. We’ll answer common homeowner and builder questions about aluclad, Passivhaus windows, and what really matters in a spec.
7. Common Energy Rating Questions – Answered Clearly
The window specification world can feel like a maze of acronyms, numbers, and conflicting advice. Let’s cut through the noise.
Here are the most frequently asked questions Cherwell gets—answered in plain English.
❓ “Is triple glazing always better than double?”
Mostly, yes.
Triple glazing dramatically reduces heat loss (lower U-value) and improves acoustic insulation.
But:
- It’s heavier — may not suit all frames or sashes
- Can reduce g-value (less solar gain)
- You must specify it correctly for your orientation and goals
For Passivhaus or near-Passivhaus targets: triple glazing is standard.
❓ “Why doesn’t my window U-value match the brochure?”
Because brochures often quote the centre-pane U-value, not the full window.
True performance = Frame + Spacer + Glazing
That’s called the whole-window U-value — and it’s usually 0.2–0.3 higher than centre-pane figures.
At Cherwell, we always clarify actual installed values to avoid misleading expectations.
❓ “Can I get Passivhaus performance in a listed or conservation home?”
Yes—with nuance.
While full Passivhaus certification may be off the table, you can absolutely:
- Use high-performance aluclad windows with heritage detailing
- Improve airtightness & insulation discreetly
- Hit EnerPHit or near-Passivhaus targets without external visual change
We’ve helped listed homes lift EPC ratings without compromising planning rules.
❓ “Do I need trickle vents?”
Maybe.
Trickle vents allow background ventilation, now often required by Building Regs Part F. But they reduce airtightness and can conflict with Passivhaus principles.
Alternatives:
- MVHR systems (mechanical ventilation with heat recovery)
- Passive ventilation strategies with controlled flow
Cherwell will advise per-project, based on ventilation strategy and compliance zone.
❓ “How long do aluclad windows last?”
With correct installation and minimal maintenance:
- Timber core protected from sun/rain = no rot
- Aluminium shell = weatherproof for decades
- Lifespan: 40–60 years
- Finish guarantee: Up to 30 years with premium brands (e.g. Internorm)
💬 Bonus Q: “Why choose aluclad over PVC or aluminium?”
- PVC: lower upfront cost, lower lifespan, can yellow
- Aluminium: sleek, but poor insulator unless thermally broken
- Timber: warm, beautiful—but high maintenance
Aluclad = the best of all three.
Insulating timber + durable aluminium = comfort and longevity.
Next: We wrap it all up. If you’re serious about performance, comfort, and futureproofing, Cherwell’s showroom is your first—and smartest—stop.
8. Want True Performance? Book a Spec Consultation at Cherwell
By now, you understand the science:
- U-values matter.
- Airtightness isn’t optional.
- Not all “triple glazing” is created equal.
- And aluclad windows—when properly specified—can take your project from “compliant” to cutting-edge.
But performance on paper means nothing without the right advice, product, and install strategy.
That’s where Cherwell steps in.
🛠️ We Don’t Just Sell Windows. We Engineer Comfort.
At Cherwell, we help homeowners, architects, and developers:
- Compare Internorm, Rationel, and other aluclad systems side-by-side
- Match window spec to your exact energy goals
- Navigate tricky regulations in conservation or heritage settings
- Select the right glazing, frame depth, sealants, and install details
- Coordinate with your builder, retrofit coordinator, or Passivhaus consultant
🪟 Book Your Consultation at Our Banbury Showroom
Step inside the Cherwell Banbury showroom, where:
- You can open, close, and inspect full-size Internorm + Rationel systems
- View frame cross-sections, gasket details, and hardware options
- Learn what U-values actually feel like
- Get honest, engineering-backed advice with zero pressure
📞 Get In Touch
📍 Address:
Unit G3, Marley Way, Banbury OX16 2RL
📞 Phone:
01295 270938
📧 Email:
[email protected]
🚀 Ready to Futureproof Your Project?
Whether you’re deep in a Passivhaus spec or just starting to explore high-performance windows, one conversation at Cherwell could transform your trajectory.
Come see how energy efficiency and architectural integrity can coexist—in style.
