Window & Door Ratings & Terms, Explained

Shopping for new windows or doors? You’ll see a blizzard of acronyms and numbers. This guide explains the ratings and key terms that homeowners often encounter when buying windows and doors, in simple terms, and how to use them to compare quotes – especially for cold Prairie winters.

Quick Cheat Sheet: Ratings & Terms

• U‑Factor: lower = better insulation
• R‑Value (doors, some glass): higher = better insulation
• Energy Rating (ER): higher = better overall efficiency
• SHGC: lower = blocks more heat from sunlight; higher = captures more winter sun
• Visible Transmittance (VT): higher = brighter glass
• Air Leakage (AL): lower = fewer drafts
• Condensation Resistance (CR/CI): higher = less window sweating
• Design Pressure / Performance Grade (DP/PG): higher = stronger against wind/rain
• STC / OITC: higher = quieter
• Low‑E + Gas Fill: modern efficiency must‑haves

Energy & Comfort Ratings

U‑Factor

• What it is: How easily heat escapes through the entire window or door assembly.
• Units/scale: ~0.20 to 1.20 Btu/hr·ft²·°F on NFRC (National Fenestration Rating Council) labels.
• Better when: Lower.
• Why it matters: Your #1 number for winter comfort and energy bills in Saskatchewan.

R‑Value

• What it is: Thermal resistance to heat flow; the reciprocal of U-Factor.
• Units/scale: m²·K/W (metric) or hr·ft²·°F/Btu (imperial).
• Better when: Higher.
• Why it matters: Commonly used for doors and some glass packages; compare like-for-like thickness and construction.

Energy Rating (ER)

• What it is: Canadian composite score combining U-Factor, solar gain, and air leakage.
• Units/scale: Unitless composite score.
• Better when: Higher.
• Why it matters: ENERGY STAR Canada recognition is typically U-Factor ≤ 1.22 W/m²·K or ER ≥ 34.

Solar Heat Gain Coefficient (SHGC)

• What it is: Fraction of the sun’s heat that passes through the glass.
• Units/scale: 0 to 1.
• Better when: Lower for summer comfort; moderate on south-facing glass to capture winter sun.
• Why it matters: Helps balance overheating in summer vs. free solar heat in winter.

Visible Transmittance (VT)

• What it is: How much visible light passes through the glass.
• Units/scale: 0 to 1; efficient windows typically ~0.30–0.70.
• Better when: Higher for brighter rooms.
• Why it matters: Extra coatings and panes can lower VT a bit.

Air Leakage (AL)

• What it is: Air that moves through the unit at a set pressure.
• Units/scale: cfm/ft² at 1.57 psf (75 Pa); many residential classes target ≤ 0.30 cfm/ft².
• Better when: Lower.
• Why it matters: Installation quality also drives drafts.

Condensation Resistance (CR/CI)

• What it is: Index estimating resistance to interior condensation.
• Units/scale: 0 to 100 (CR older method, CI newer method).
• Better when: Higher.
• Why it matters: Useful in kitchens, bedrooms, and other higher-humidity rooms.

Glass & Frame Tech

Low‑E Coatings

• What it is: Microscopically thin metallic layers on the glass that reflect heat. Multiple coatings can be tuned for insulation and/or solar control. The effects show up as U‑Factor and SHGC values on the label.
• Why it matters: Improves winter insulation and summer comfort without majorly darkening glass.
• Ask for: Which glass surfaces are coated (for triple-pane, e.g., #2 and #4).

Argon / Krypton Gas Fill

• What it is: Inert gas between panes slows heat transfer. Argon is common and cost‑effective; krypton is used in thinner cavities and is a premium option.
• Why it matters: The right gas plus the right gap boosts insulation.
• Ask for: Gas type and fill percentage at manufacture.

Spacer Type

• What it is: The material separating the panes at the edges.
• Why it matters: Warm-edge spacers reduce edge-of-glass heat loss and condensation.
• Ask for: Spacer material and brand; prefer non-aluminum warm-edge.

Pane Count & Cavity Width

• What it is: Number of glass layers and the gap between them.
• Why it matters: Triple-pane boosts insulation and reduces cold-glass drafts. Cavity width matters; for argon, around 11 to 12 mm is typically optimal; krypton performs better in narrower gaps around 8 mm. Very wide gaps can allow convection that reduces insulation. For noise, laminated or asymmetrical glass is usually more effective than adding a third pane alone – check STC/OITC below.
• Ask for: Actual cavity width and gas type for your configuration.

Insulating Glass Unit (IGU)

• What it is: The sealed glass sandwich made of glass, spacer and gas.
• Why it matters: Quality IGUs with robust edge seals resist fogging and extend service life.
• Ask for: Seal warranty term against IGU fogging.

Thermal Break

• What it is: An insulating barrier inside aluminum frames to block heat/cold conduction.
• Why it matters: Essential in cold climates to prevent chilly frames and condensation.
• Ask for: Thermally broken frames if choosing aluminum.

Strength, Weather, And Code‑Type Items

Design Pressure (DP) & Performance Grade (PG)

• What it is: DP is a wind load rating; PG is a broader performance grade covering wind, water, and air.
• Units/scale: Numeric performance grade or pressure rating.
• Better when: Higher.
• Why it matters: Important for tall units and exposed, windy sites.

Water Penetration Rating

• What it is: Pressure at which water is forced past the unit during testing.
• Units/scale: Test pressure (Pa or psf).
• Better when: Higher.
• Why it matters: Reduces wind-driven rain leaks; good flashing still matters.

Egress

• What it is: Minimum clear opening sizes and heights for bedroom windows.
• Why it matters: For safety. Bedrooms must provide a clear escape route for occupants and access for firefighters. When replacing a bedroom window, confirm the new style and size meet local egress requirements.
• Ask for: Confirmation the replacement meets local egress rules.

Safety Glass (Tempered vs Laminated)

• What it is: Tempered breaks into pellets; laminated holds together via an interlayer.
• Why it matters: Required in specific locations (near doors, floors, tubs/showers, and large low‑sill openings). Laminated also improves security and sound control.
• Ask for: Tempered or laminated where required; laminated also helps with sound and security.

Sound Control

STC (Sound Transmission Class)

• What it is: Blocks mid to high-frequency noise like voices and TV.
• Units/scale: STC points; typical windows ~25–40+.
• Better when: Higher.
• Why it matters: Quieter interior in street-side rooms.

OITC (Outdoor‑Indoor Transmission Class)

• What it is: Blocks lower-frequency outdoor noise like traffic and aircraft.
• Units/scale: OITC points; typical windows ~20–35+.
• Better when: Higher.
• Why it matters: Better for road rumble and other bass-heavy noise.

Door‑Specific Tidbits

Slab Core & Insulation

• What it is: Steel/fiberglass doors often have insulated cores.
• Why it matters: Higher R‑values mean warmer doors; glass inserts lower overall performance.

Handing & Swing (Inswing vs Outswing)

• What it is: Which way the door opens (and hinge side).
• Why it matters: Outswing can seal well in strong winds but may complicate storm‑door use; verify space and code/step clearance.

Sill/Threshold & Pan Flashing

• What it is: The bottom piece and waterproofing under the door.
• Why it matters: A well‑flashed sill pan is your insurance against leaks.

How To Read Labels

• Canada: Look for the ER score plus U‑Factor, SHGC, VT, AL (and sometimes CR/CI) on accredited labels. ENERGY STAR® Canada recognizes models via U‑Factor or ER, as noted above.
• NFRC label (National Fenestration Rating Council): Shows U‑Factor, SHGC, VT, and may include AL and CR/CI. Remember: installation quality affects real‑world air tightness.
• Quick tip: Start with U‑Factor (winter bills), then ER, then AL/CR for comfort, then SHGC/VT for sunlight/brightness.

Common Trade‑Offs

• Triple‑pane vs brightness: Better insulation/comfort, slightly dimmer glass (VT drops a bit).
• Lower SHGC vs winter gains: Great for summer comfort, but captures less free winter sun – balance by orientation.
• Huge glass walls vs energy: Bigger openings mean more heat loss; pick high‑performance IGUs/frames and check PG/DP for wind.
• Sound vs cost/weight: For traffic noise, prioritize laminated or asymmetrical glass with higher OITC/STC.

Questions to Ask Your Installer

1. What are the U‑Factor and ER for the exact configuration I’m buying?
2. Where are the Low‑E coatings placed, and what gas fill do you use?
3. What are the Air Leakage and Condensation Resistance (CR/CI) ratings?
4. What DP/PG is this unit, and is that appropriate for my exposure?
5. How do you handle air sealing and pan flashing so the install backs up the factory ratings?
6. For bedrooms: will the new window maintain egress?

For the Prairies

Choosing windows and doors is easier when you know what to ask. Use this guide to frame the conversation with your vendor, compare options fairly, and match products to your home, climate, and exposure. Ask to see the product’s ratings in plain English, such as what they mean for comfort, drafts, and energy costs, and you’ll be able to choose confidently.