THIS POST IS UP TO DATE
This post reflects the ENERGY STAR program changes which became effective October 23, 2023.
Buying an energy efficient door can feel like a challenge to even the savviest, most data-loving homeowners. Our customers who want to buy new doors in the Cary, NC area usually have lots of questions about what all of the energy ratings and certifications mean.
That’s why we wrote this comprehensive, easy-to-understand guide to buying energy efficient doors.
Why buying energy efficient doors matters
According to the National Fenestration Rating Council, the average American household spends $1,500 – $2,500 on energy costs per year. Around 45% of that cost is for heating and cooling.
So, when buying new doors for your home, it makes sense that you want them to be energy efficient. That way, you can keep your home more comfortable and avoid overspending on energy costs.
The basics of buying an energy efficient door: NFRC ratings
The National Fenestration* Rating Council (NFRC) is a non-profit organization in the United States. The NFRC maintains an energy efficiency certification and labeling program for windows, doors, and skylights.
*Fenestration refers to all things related to windows, doors, and skylights. It derives from the Latin word “fenestra,” meaning “opening.”
The NFRC provide performance standards for manufacturers’ ratings for these products in five categories:
- U-Factor
- Solar Heat Gain Coefficient
- Visible Transmittance
- Air Leakage
- Condensation Resistance
Of these, only U-Factor and Solar Heat Gain Coefficient are only required to be provided by door manufacturers. The remaining three ratings are optional for door manufacturers’ to provide.
These ratings serve several purposes.
They help architects, builders, and homeowners compare products in regards to their energy efficiency. They also help builders and home designers ensure that the products they are considering meet local building codes.
Understanding NFRC ratings when shopping for energy efficient doors
U-factor
U-factor is the rate at which a door allows heat from inside the home to escape outside. The lower the U-factor, the more energy-efficient the door.
U-factor ratings range from 0.10-2.00.
Solar Heat Gain Coefficient (SHGC)
Solar heat gain coefficient (SHGC) is a measure of heat gain inside your home from the sun.
The lower a door’s SHGC, the less solar heat it allows into your home. A door with a high SHGC rating is more effective at collecting solar heat during the winter.
SHGC ratings range from 0.00-1.00.
Visible Transmittance (VT)
Visible transmittance (VT) refers to the natural light (but not heat) that a door’s glass components allow to enter your home.
The higher the number, the more light will come through the door.
VT ratings range from 0.00-1.00.
Air Leakage
Air Leakage measures the rate of air movement around a door.
Specifically, the measure indicates how many cubic feet of air can pass through a door in sixty seconds. For this rating, it’s assumed that the airflow is a constant 25mph. That number is then divided by the total area of the door to calculate the air leakage rating.
A door with a low air leakage rating is tighter, and will let in less air, than a door with a higher air leakage rating.
It’s important to note that the craftsmanship and products used during installation can also impact a door’s air-tightness. The NFRC rating assumes ideal installation conditions. Shoddy installation will, in effect, raise your door’s air leakage rating.
Air leakage ratings range from 0.10 – 0.30
Condensation Resistance
Condensation resistance indicates the degree to which a door resists condensation buildup. It’s important to note that this rating is more applicable to purchasing windows, and is rarely (if ever) provided by door manufacturers about doors.
The higher the number, the better a door resists condensation buildup.
Condensation resistance ratings range from 0 – 100.
One additional measure of a door’s energy efficiency: Light-to-solar-gain (LSG)
Light-to-solar gain (LSG) is the ratio between the VT and SHGC in a door. This number indicates how efficient the glass and glazing in a door are at allowing natural light into your home, while preventing solar heat from entering. The higher the number, the more natural light you can expect to get in your home without having heat transfer through the door.
This number is not always provided by manufacturers, but it can be calculated by dividing VT by SHGC.
Light-to-solar-gain calculation: LSG = VT/SHGC
LSG ratings range from 0.00 – 100
How are NFRC Rating and ENERGY STAR Rating related?
Many homeowners have heard of ENERGY STAR ratings.
ENERGY STAR is a program run by the U.S. Department of Energy and the U.S. Environmental Protection Agency. The program promotes energy efficient by labeling a wide range of products as meeting ENERGY STAR requirements.
In addition to windows, doors, and skylights, ENERGY STAR rates household appliances, computers, lightbulbs, and more.
A product having an ENERGY STAR certification means that it meets a minimum standard of energy efficiency. An ENERGY STAR certification indicates that a door:
- is manufactured by an ENERGY STAR partner,
- is independently tested, certified, and verified by the National Fenestration Rating Council (NFRC), and
- has NFRC ratings that meet strict energy efficiency guidelines set by the US Environmental Protection Agency (EPA).
Recommended door energy efficiency Ratings by Climate Zone
Now you understand what the NFRC performance ratings mean in each category.
You also know how these ratings and other factors are used to determine if a product meets ENERGY STAR standards for certification.
But, before you can decide if a door is really energy efficient for your home, you must also know which climate zone you are in to determine which rating criteria your doors should meet.
ENERGY STAR provides a climate zone map that splits the United States into four climate zones:
- Northern
- North Central
- South Central
- Southern
Here in Wake County, North Carolina, we are in the South-Central Climate Zone for ENERGY STAR ratings.*
*Prior to October 23, 2023, Wake County was in the North-Central Climate Zone.
If you happen to be in a different area, you can use this website to search for your state/county and determine which ENERGY STAR climate zone you are in.
In the South-Central climate zone, in order to receive ENERGY STAR certification, doors must meet the following criteria:
- Opaque doors (doors with no glass) must have a U-factor of 0.17 or less (no SHGC requirement)
- 1/4- or 1/2-Lite doors (doors with about 50% glass or less) must have a U-factor of 0.25 or less and an SHGC of 0.25 or less
- 3/4- or Full-Lite doors (doors with more than 50% glass) must have a U-factor of 0.30 or less and an SHGC of 0.25 or less
Putting it all together to find the best Energy Efficient doors for your home
So, now you:
- Know the 5 NFRC rating, and understand what each measures
- See how NFRC ratings (plus a few other factors) contribute to a door receiving an ENERGY STAR. This will also depend on which climate zone you are in.
Armed with this information, you can evaluate the doors you’re considering purchasing. You can be sure that they will not only look great, but that they will also help you cut back on your home’s energy expenditure.
Summary
When you’re looking to replace the doors in your home, you want to make the best choice for your family and your wallet. We hope that this easy-to-read guide will help you do just that.
The Window Works Co. has helped thousands of families in the Raleigh area find the energy efficient doors they’re looking for.
When you’re ready to replace your old doors with new, energy efficient doors, we’d love to help you.
Use the form below to tell us about your project and schedule your free estimate appointment today.
