When you're shopping for windows, you'll encounter a number called U-value (or U-factor). It's the single most important number in window performance — yet most American homeowners have never been told what it actually means, or what the gap between a European and a US-made window costs them each year. This article fixes that.
In This Article
1. What Is U-Value?
U-value (also written as U-factor) measures how much heat passes through a window per unit of time. Lower is better. A U-value of 1.0 means 1 watt of heat escapes through every square meter of window for every degree of temperature difference between inside and outside.
Think of it as a leakage rate. A window with U-value 0.73 leaks heat at less than half the rate of a window with U-value 1.8 — which means the home with 0.73 windows needs far less energy to stay warm in winter and cool in summer.
U-value covers the entire window unit — frame, glass, and the seal between them — not just the glass alone. This is why frame design matters so much. A 5-chamber VEKA frame traps multiple dead-air zones that dramatically slow heat transfer. A standard 1-chamber American frame provides almost none of this resistance.
2. US vs. European U-Value Scales
Here's where it gets confusing for American buyers: the US uses a different unit system than Europe.
BTU/(hr·ft²·°F)
ENERGY STAR max: 0.30
Lower = better
W/(m²·K)
Passive House max: 0.80
Lower = better
To convert: multiply the European Uw value by 0.176 to get the US U-factor. So VEKA Therm-82's Uw of 0.73 W/m²K = approximately 0.13 US U-factor — far below the 0.30 ENERGY STAR threshold. When you see us quoting European Uw values throughout our specs, that's the metric we use internally.
3. VEKA vs. Andersen: The Numbers
Here is the direct comparison between our VEKA uPVC systems and the most commonly installed US window brands:
U-Value Comparison (W/m²K — lower is better)
| Window System | Uw (W/m²K) | US U-Factor | Glazing |
|---|---|---|---|
| VEKA Therm-82 Passive | 0.73 | 0.13 | Triple |
| VEKA Therm-76 | 0.92 | 0.16 | Triple |
| VEKA Therm-70 | 1.10 | 0.19 | Triple |
| Andersen 400 Series | ~1.70 | 0.30 | Double |
| Pella 250 Series | ~1.80 | 0.32 | Double |
| Standard Builder-Grade | ~2.20 | 0.39 | Double |
Our most affordable system — VEKA Therm-70 — already outperforms Andersen 400 Series by 35%. The Therm-82 Passive outperforms it by 57%. This is not a marginal difference; it's a different category of performance entirely.
4. What the Gap Costs You Per Year
Let's translate U-values into dollars. We'll use a typical US scenario: a 2,500 sq ft home with 20 windows averaging 15 sq ft each (300 sq ft total glass area), located in Connecticut where electricity costs $0.25/kWh and the heating season is 6,000 heating degree days.
The formula for annual heat loss through windows: Heat loss (BTU/year) = U-factor × Area (ft²) × Heating Degree Days × 24 hours
Annual Energy Cost Comparison — Connecticut Home, 20 Windows
That's a saving of $410–$510 per year on heating costs alone, just from the windows. Over 20 years (well within the window's 40+ year lifespan), that compounds to $8,200–$10,200 in energy savings — before accounting for cooling savings, rising energy costs, and the avoided cost of replacing standard windows every 15–20 years.
5. Which U-Value Do You Need for Your Climate?
Not every US climate requires the same U-value. Here's a practical guide by climate zone:
- Cold climates (CT, MA, CO mountains, NY Upstate, MN): VEKA Therm-82 Passive (Uw 0.73). Passive House-rated, fastest payback in cold climates with high energy costs.
- Mixed climates (NY, NJ, WA, OR, mid-Atlantic): VEKA Therm-76 (Uw 0.92). Excellent performance for both heating and cooling seasons.
- Mild climates (Southern CA, Pacific NW lowlands, parts of FL): VEKA Therm-70 (Uw 1.10). Still significantly outperforms any US standard window; lower cost appropriate for lower energy demands.
- Hot climates (TX, AZ, FL, GA): For hot climates, Solar Heat Gain Coefficient (SHGC) matters as much as U-value. We specify low-SHGC Saint-Gobain coatings for southern exposures. Ask our team for climate-specific glass specifications.
6. Choosing the Right VEKA System
The best way to choose is to talk to our team — we'll look at your climate zone, the orientation of your home, and your project goals (energy savings, Passive House certification, aesthetics) and recommend the right specification.
As a starting point:
- If you're in Connecticut, Massachusetts, Colorado, or Upstate New York → VEKA Therm-82 Passive
- If you're in New York City, New Jersey, Washington State, or mixed-climate regions → VEKA Therm-76
- If you want aluminum with slim sightlines → Schüco MB-86N or MB-104 Passive
All three VEKA systems include Saint-Gobain PLANITHERM triple glazing as standard, multi-point locking hardware, and a 40+ year profile lifespan. The difference is profile depth and the resulting U-value.
