5 Essential Questions to Ask Your Window Supplier

Buying windows is a significant investment that will affect your home’s comfort, energy efficiency, and security for decades. Too often, homeowners focus solely on the price tag, only to regret it later when the windows start to warp, draft, or fail within just a few seasons. In a climate like Plovdiv’s—where scorching summers can stress PVC profiles and humid winters test the limits of insulation—choosing the right components is non-negotiable.

To avoid being just another statistic, you need to go beyond the glossy sales pitch and ask the hard technical questions. An informed buyer is one who gets the best ROI. Here are the 5 critical questions that will help you distinguish a quality product from a cheap imitation and set you apart from the average, uninformed buyer.

1. “What is the wall thickness of the profile?”

In the window industry, not all uPVC is created equal, even if different brands look identical from the outside. Look for “Class A” profiles, which have a minimum outer wall thickness of 3.0 mm (with a tolerance of +/- 0.2 mm). These profiles offer the highest structural stability, superior corner weld strength, and much better screw retention for the heavy hardware.

Why does this matter? Cheaper “Class B” profiles (with wall thickness below 2.7 mm) might look fine immediately after installation, but they are significantly more prone to thermal expansion and warping over time. In the long run, this leads to structural failure, loss of airtightness, and doors or windows that are impossible to adjust. Always ask for a technical specification or a certificate of the profile class.

2. “What kind of reinforcement is inside the profile?”

The uPVC profile alone isn’t strong enough to support the massive weight of modern double or triple glazing. Ensure that there is galvanized steel reinforcement inside both the frame and the sash, with a thickness of at least 1.5 - 2.0 mm.

Some suppliers, aiming for the lowest possible price, use ultra-thin steel (0.8 - 1.0 mm) or skip reinforcement entirely in smaller windows to save costs. This is a “hidden” economy that leads to fatal consequences: sagging sashes, hardware that jams, and even the risk of the entire frame deforming under wind load or building settlement. Ask about the shape of the reinforcement—fully enclosed (square) steel in the frame provides much higher rigidity than U-shaped channels.

3. “What is the spacer width and gas filling in the glass unit?”

The space between your glass panes is where the actual insulation happens. The optimal distance for maximum thermal performance is between 14 and 18 mm. If the spacer is too narrow (under 12 mm), thermal conductivity increases; if it is too wide (over 20 mm), internal convection occurs, which again worsens the insulation.

Furthermore, always ask if the cavities are filled with Argon gas. Argon is an inert gas with much lower thermal conductivity than plain air. You should also check what kind of “spacer bar” is being used. Look for “Warm Edge” technology, which replaces the traditional aluminum spacer with a plastic or composite one. This prevents thermal bridging at the edges of the glass and drastically reduces the risk of indoor condensation.

4. “What is the warranty and what EXACTLY does it cover?”

Don’t be fooled by grand but vague promises like a “10 or 15-year warranty.” A professional supplier should provide a clear breakdown:

• Profile Warranty: Often the longest, covering color stability and resistance to cracking.
• Hardware Warranty: Usually 2 to 5 years. These are the moving parts that experience the most wear and tear.
• Glass Unit Warranty: Covers the loss of hermetic seal (condensation forming between the panes).
• Installation Warranty: Extremely important, as issues from poor fitting usually show up during the first winter.

Ask about the company’s after-sales service policy. If a window needs a minor adjustment after 3 years, can you rely on the same team to return and fix it, or will you be left to search for a third-party technician?

5. “Who will perform the installation and what methods will they use?”

Even the most expensive, high-tech window will fail if it’s installed poorly. Installation accounts for at least 50% of the final performance of the product. Ask if the company uses its own trained, permanent teams or if they rely on temporary external subcontractors.

In-house teams are generally more accountable and follow the manufacturer’s specific installation guidelines. Also, ask about the materials—do they only use foam, or do they apply “RAL Installation” using vapor-permeable and vapor-tight tapes to protect the wall-to-frame joint? Quality installation is the only way to ensure you won’t suffer from leaks, mold, or drafts around the frame in the years to come.

Taking the time to ask these questions might feel like extra work, but it’s the only way to ensure your investment truly pays off. An informed choice is the best insurance for your long-term comfort and peace of mind.