A high-quality double-glazed unit (Insulating Glass Unit, or IGU) is characterized by superior thermal efficiency, airtight construction, and durable, secure materials. Key indicators of a good quality unit include an A or A++ energy rating (WER), a low U-value of 1.2 W/m²K or less, the use of Argon gas, and warm-edge spacer bars. 

Here is a detailed guide on how to evaluate a good double-glazed unit:

1. Technical Performance (Energy & Insulation)

• Window Energy Rating (WER): Look for an A+ or A++ rating verified by the British Fenestration Rating Council (BFRC). A-ranked windows are high quality.
• U-Values: The lower the U-value, the better the insulation. Top-quality windows have a U-value of around 1.2 W/m²K or lower (lower is better, with1.4 W/m²K being the minimum for new replacements).
• Inert Gas Fills: Good units are filled with inert gas (usually Argon) between the panes to provide better thermal insulation than air.
• Low-E Coating: High-performance glass has a Low-Emissivity (Low-E) coating, which is an invisible metal oxide layer that reflects heat back into the room. 

2. Physical Components & Structure

• “Warm Edge” Spacer Bars: Look for spacer bars (the thin strip between the panes) made of composite, plastic, or foam rather than aluminium. These reduce condensation and heat loss at the edges.
• Welded Joints: On uPVC frames, welded joints are stronger, more durable, and provide better thermal insulation than mechanically joined (screwed) corners.
• Internal Beading: High-security windows are glazed from the inside. This means the beading (the strip that holds the glass in place) is on the inside, preventing burglars from removing the glass from the outside.
• Frame Strength: High-quality uPVC frames should be reinforced with galvanized steel for added strength. 

3. Inspection & Quality Control Checks

• No Internal Condensation: There should be absolutely no moisture, fogging, or condensation between the two panes of glass. If you see this, the seal has failed, and the unit is defective.
• No Drafts: Run your hand around the frame on a windy day. A good unit will be completely airtight.
• Visible Markings: Check the corner of the glass for a manufacturer stamp or date. Many high-quality units have this information etched, showing they comply with standards.
• Uniformity: The spacer bar should be straight and evenly placed around the entire unit, with no visible sealant oozing into the viewing area. 

4. Certifications & Warranty

• FENSA/CERTASS: Ensure the installation company is registered with FENSA or CERTASS, which confirms compliance with building regulations.
• Long-Term Guarantee: High-quality units should come with at least a 10-year, insurance-backed guarantee covering frame damage, hardware failure, and seal failures (misting).
• Kitemarks: Look for British Standards Kitemarks, such as BS 7412 for uPVC performance.