Start with the label and model
Request the manufacturer, model, operating style, size and glass specification. Ratings should describe the proposed assembly and use comparable units and certification methods. A centre-of-glass claim is not the same as a whole-window rating, and a brochure’s best-performing configuration may differ from the unit in the quote.
Separate insulation from solar gain
U-factor describes heat transfer through the assembly; lower values indicate less transfer under the rating conditions. Solar heat gain coefficient describes the portion of solar energy admitted. Visible transmittance concerns daylight. These measures answer different questions, so a low solar-gain choice should be discussed alongside room orientation, shading and the desired amount of light.
Put air leakage in the right place
A product’s tested air leakage and the finished installation’s perimeter air sealing are related but separate topics. Ask how the crew connects the new window to the surrounding air-control layer. A strong product rating cannot certify the workmanship at a specific property. Keep product documentation and installation scope together when comparing proposals.
Use certification carefully
ENERGY STAR certification applies to eligible products and documented configurations. Verify the exact product through the current Canadian program rather than relying on a logo in an advertisement. Certification does not establish an installer’s credentials, a permit exemption, a promised bill reduction or eligibility for a particular rebate. Confirm each of those questions separately.
Questions worth asking
- Whole-window or centre-of-glass figure?
- Same model, style, glass and rating units?
- What does the room need from solar gain and daylight?
- How is perimeter air sealing specified?
Further reading
Natural Resources Canada: ratings and certification ↗Source checked October 16, 2026. Current requirements and eligibility need separate project-specific verification. This guide is an original planning explanation, not a product certification or installation instruction.
