Most new windows sold in the US carry an NFRC label listing their U factor, Solar Heat Gain Coefficient (SHGC) and visible transmittance, and sometimes air leakage and condensation resistance. U factor measures how well the window keeps heat in, and lower is better. SHGC measures how much solar heat comes through the glass. For Pittsburgh, which sits in the ENERGY STAR Northern zone, U factor is the number that matters most, and you want it around 0.22 to 0.25 or lower.
What is on an NFRC window label?
The National Fenestration Rating Council is the independent nonprofit that runs the rating program window manufacturers use. Its label rates the whole window, glass and frame together, not just the center of the glass. That matters because the frame and spacer are where a lot of heat escapes.
Here is what each number means and what to look for in western Pennsylvania.
| Rating | What it measures | Better is | What to look for here |
|---|---|---|---|
| U factor | Rate of heat loss through the whole window | Lower | About 0.22 to 0.25 or lower |
| Solar Heat Gain Coefficient | Share of solar heat that passes through | Depends on climate | Moderate to higher can help in winter |
| Visible Transmittance | Share of visible light that passes through | Higher | Personal preference for brightness |
| Air Leakage | Air passing through the window assembly | Lower | 0.3 cfm per square foot or lower |
| Condensation Resistance | Ability to resist interior condensation | Higher | Helpful for humid rooms and cold snaps |
Air leakage and condensation resistance are optional on the label, so some windows will not show them. If they are missing, ask the manufacturer or your installer for the numbers.
What does U factor mean and why is it the key number in Pittsburgh?
U factor is the rate of heat flow through the window, measured in Btu per hour per square foot per degree Fahrenheit. Most modern windows fall somewhere between about 0.20 and 0.40, while an old single pane window can be above 1.0. The lower the number, the less heat leaves your house.
Our heating season runs from October into April, with long gray stretches and plenty of nights in the teens and twenties. That makes heat loss the main energy cost a window adds to your home. When you compare two windows for a western PA house, start with U factor.
What is SHGC and should it be high or low here?
Solar Heat Gain Coefficient is a number between 0 and 1 that tells you how much of the sun’s heat comes through the glass. A low SHGC blocks more heat, which is great in Phoenix. In a heating dominated climate like ours, a moderate or higher SHGC lets winter sun warm south facing rooms for free.
The tradeoff is summer comfort. A west facing family room with a big picture window can overheat in July. Many homeowners pick one glass package for the whole house, but you can choose different SHGC values for different sides of the home when a room is a known problem.
What are the ENERGY STAR requirements for the Northern zone?
ENERGY STAR divides the country into climate zones, and Pittsburgh and all of western Pennsylvania fall in the Northern zone. Under the current version 7.0 criteria, a window qualifies in the Northern zone if it meets one of these combinations:
- U factor 0.22 or lower qualifies with any SHGC
- A slightly higher U factor, in the 0.23 to 0.25 range, can still qualify through an equivalent performance path, but only when the window also has a higher SHGC
The sliding scale lets a window with a slightly higher U factor qualify if it also lets in more free solar heat. Criteria get updated over time, so check energystar.gov for the current numbers before you buy. For a closer look at what better windows can do to your heating bill, see our post on energy efficient windows and your heating bill.
How do Low E coatings and argon gas work?
Low E stands for low emissivity. It is a microscopically thin metal oxide coating on the glass that reflects heat back toward its source. In winter it bounces your furnace heat back into the room instead of letting it radiate out through the glass. Different Low E formulations are tuned for different climates, which is why two windows with similar frames can have very different SHGC values.
Argon is an inert, colorless gas sealed between the panes of an insulated glass unit. It is denser than air, so it slows heat transfer across the gap. Krypton does the same job even better in the narrower gaps of triple pane glass, at a higher cost.
The biggest jumps in performance usually come from these upgrades:
- Moving from clear glass to a Low E coating
- Adding argon fill to the insulated glass
- Warm edge spacers instead of aluminum spacers
- Triple pane glass for the coldest exposures or noisy streets
Why does installation matter as much as the label?
The NFRC label rates the window in a lab. It says nothing about the gap between the window and your wall. A window with a great U factor that is set in an opening full of drafts, with no insulation around it and poor flashing, will still feel cold and can still leak water.
This is especially true in older Pittsburgh homes. Original wood windows often have empty sash weight pockets in the jambs, and those need to be insulated during replacement. Brick homes need careful sealing where the new frame meets the masonry. A quality install uses low expansion foam or backer rod and sealant around the unit, proper flashing at the sill and head, and trim that is sealed against wind driven rain.
If you are planning a project, our guide to window replacement in Pittsburgh covers what to expect from start to finish.
Frequently Asked Questions
Is a lower U factor always better?
For heat loss, yes. Lower U factor means less heat escaping. The gains get smaller as you go lower, so compare the price difference against how many windows and which exposures you are upgrading.
Does the NFRC label rate just the glass?
No. NFRC ratings cover the whole window, including frame, sash and spacer. Some product brochures list center of glass numbers, which look better, so make sure you are comparing whole window ratings.
Are triple pane windows worth it in western PA?
They can be, especially on north facing walls, in bedrooms on busy roads, or in rooms that never feel warm. Triple pane is heavier and costs more. For many homes a good double pane Low E argon unit already meets ENERGY STAR Northern zone criteria.
What does condensation resistance tell me?
It is a score from 1 to 100 that rates how well a window resists condensation on the interior surface. Higher is better. It helps for bathrooms, kitchens and homes with humidifiers running all winter.
Where do I find the label on a new window?
The NFRC label is a temporary sticker on the glass or frame of a new window. Ask your installer to leave it on until you have seen it, or ask for the product’s NFRC ratings with your quote.
Want help comparing labels on real quotes? Call Bella at (724) 515 5163 for a free estimate, and we will walk you through the ratings for every window we propose. Schedules fill up before winter, so now is a good time to book. Learn more about our window replacement services and vinyl replacement windows.