What Are the Best Energy Efficient Windows?

What are the best energy efficient windows for homes in Florida? That’s a question that gets plugged into Google at least a few hundred times a month by people who are tired of their homes getting turned into ovens by the hot Florida sun.

A strong air conditioning system can help keep your home cool, but the electricity required to power it can lead to high electric bills. Also, when it does get cold out, as it occasionally does even in Florida, you’ll need windows that can keep the heat in your home from leeching outwards.

On top of that, in Florida, windows need to meet stringent requirements for impact resistance if your home is in a wind borne debris region (WBD), which much of Tampa Bay is.

So, the best energy efficient windows will need to insulate against outside temperatures effectively, block out the heat generated by sunlight coming through their glass, AND be built solidly enough to withstand hurricane conditions per the Florida Building Code (FBC) requirements adopted by the State of Florida.

What Makes the Best Energy Efficient Windows: Framing Material

The frame of the window plays an enormous role in determining the window’s overall energy efficiency. The best window frames for energy efficiency should be durable, corrosion-resistant, and insulative.

Three popular materials for window frames are wood, aluminum, and vinyl. Which of these make the best energy efficient windows?

Aluminum and Energy Efficiency

When compared to wood and vinyl windows, aluminum windows are the worst windows from a pure energy efficiency standpoint, by far.

This is because aluminum has a high thermal conductivity. According to The Engineering Toolbox, aluminum has a thermal conductivity rating of 237 Watts per meter Kelvin (W/m*K). What this means is that 237 watts of thermal energy will penetrate a one meter thick block of aluminum until the temperatures on both sides of the block reach equilibrium.

This is a very high thermal conductivity value. It makes for a great baking sheet or heat sink material, but a very poor insulator. 

However, the main draw of aluminum isn’t its energy efficiency, but its durability. It has an incredibly high tensile strength. Depending on the alloy, aluminum can have a tensile strength ranging from 100 MPa to over 500 MPa (Source: AZO Materials). This makes aluminum a great choice for use in impact windows, as it leads to a very rigid, durable frame… when you don’t have to account for exposure to chemical corrosives, at least.

Wood and Energy Efficiency

It’s hard to generalize about the insulative qualities of wood, because there are so many different varieties that can be used to fill the frame of a window, each with their own properties for thermal conductivity, moisture resistance, hardness, and more.

However, wood does tend to be a good insulator compared to metal. Stanford Advanced Materials notes that wood tends to have a thermal conductivity between roughly 0.1 to 0.2 W/m*K. This makes wood thousands of times better at insulating your home than aluminum.

So, why isn’t wood a more common window frame material in Florida? After all, it’s energy efficient and some hardwoods are very durable.

However, wood isn’t nearly as durable as aluminum for sheer tensile strength and shear modulus. Worse, even the best hardwoods are still somewhat susceptible to wood rot and termites if not near-constantly cleaned and treated.

Things like wood splitting from moisture intrusion or window frames literally rotting away in the window opening are all too common in a hot, humid state like Florida.

So, wood is generally not a preferred framing material for modern windows, especially along the coastline.

Vinyl and Energy Efficiency

When it comes to pure energy efficiency, vinyl is a great choice for your window frame. With a thermal conductivity of just 0.19 W/m*K on the high end (Source: Alfa Chemistry), PVC is, like wood, over a thousand times more energy efficient than aluminum.

In fact, let’s do the math: 237 / 0.19 = 1247.368. 

In other words, for every single watt of thermal energy a vinyl window would let in, an aluminum window would let in 1,247!. 

This makes vinyl vastly better at blocking heat during the summer than aluminum. It’s not even a contest. Like a race between a slug and an F1 dragster.

One area where aluminum has polyvinyl chloride (PVC) beat is in its tensile strength and durability against direct force. Most formulations of vinyl have an ultimate tensile strength between 34 and 62 MPa according to Rollepaal. Even the weaker aluminum alloys tend to have a tensile strength of 100 MPa or more.

So, is the best energy efficient window made from pure aluminum, wood, or vinyl? If energy efficiency was the only concern, then pure wood or vinyl would be the best choice. However, in Florida, we have to contend with periodic hurricanes, frequent salty air coming off the ocean and the Bay, and ever-present pest infestations like termites.

That’s why the best energy efficient windows for Florida use a combination of materials to maximize both energy efficiency and durability.

Modern impact windows have a layered design for their frames. The frame has an outer layer of extruded vinyl that blocks thermal transfer and repels chemical agents. Then, inside the vinyl sealing against both heat and chemical corrosives, there’s a core of aluminum material that provides extra rigidity and toughness in the face of massive wind pressure or flying debris impacts.

When choosing a new window, it’s not a simple matter of one aspect of the window’s performance determining everything. The best windows for your home need to satisfy all of your needs as much as possible. 

What Makes the Best Energy Efficient Windows: Window Glass

The frame isn’t the only thing affecting the energy efficiency of your window. The glass used for your window also has a huge impact on how much heat gets into your home. Not just the material of the glass pack itself, but whatever coatings it has, any thermal inhibiting gases it’s filled with, and the number of layers of glass present as well!

When you’re shopping for an energy efficient window, it’s important to know all of the factors that affect the thermal performance of the glass as well as the frame.

Some key improvements found in the best energy efficient windows for homes in Florida include:

• Solar Control Low-E Glass. This is a film of material applied to glass in the factory that inhibits ultraviolet and infrared light from getting through the glass. The nanometer-thick layer of silver catches the UV and IR wavelengths and reflects them away from your window. As UV and IR rays carry most of the thermal energy of the sun’s light, this helps keep your home cool without blocking out visible light from the sun. Often makes windows have a slight sheen or a very mild greenish tinge to them (varies depending on the film formulation). Typically applied to the interior of the glass pack to prevent chipping or wear from exposure to the outside.
• Multiple Layers of Glass Separated by an Air Gap. Energy efficient windows tend to have more than one layer of glass. Having two (or more) panes of glass separated by air gaps helps ensure that as the top layer of glass heats up from direct sunlight exposure, that heat doesn’t immediately transfer to the interior of the home. 
• Argon Gas Fill. An inert, heavy gas that is pumped into the glass pack between the panes to replace oxygen. Argon gas is heavier than regular air and inhibits the flow of thermal energy between your house and the outside. This helps improve the insulation already present from having multiple layers of glass in the glass pack separated by an air gap.

The best windows for energy efficiency tend to have sashes that incorporate multiple panes of glass with air gaps that have inert gas fills like argon gas.

What about Window Tints? Do They Improve Energy Efficiency?

It’s a common misconception that dark window tints improve a home’s energy efficiency. However, this couldn’t be further from the truth. Many dark tints for windows are energy efficient, but not because they’re dark. It’s because they’re different formulations of an energy efficiency-enhancing film.

The only thing a darker tint color does is inhibit the transmission of visible light. VLT, or visible light transmission, doesn’t impact the thermal performance of a window overly much. Dark tints are best for meeting two goals:

1. Compliance with Municipal Turtle Glass Codes. If you have a home near the beach, you might be required to use Turtle Glass tinted windows that inhibit VLT between your home and the outside. These Turtle Glass codes are meant to help protect vulnerable marine life that might get confused by bright lights coming from houses near their nesting sites.
2. To Enhance Privacy. Dark, obscuring window tints can help protect your privacy by making it harder to see into your home. However, there are obscuring glass patterns that let in light while making it nearly impossible to see through the window itself without being darkly tinted.

If anything, dark window tints tend to trap heat in the home, making it warmer. Darker colors tend to absorb and retain heat from sunlight more than lighter colors do.

What Makes for the Best Energy Efficient Windows: Installation

Event the best-designed windows with vinyl frames, low-e glass, and every energy efficiency-enhancing trick in the book won’t make a noticeable difference if they’re installed in a grossly incorrect manner.

Installation quality absolutely matters for energy efficiency! There’s a lot that goes into ensuring that a window opening doesn’t have leaks of either air or moisture following the installation of a new window.

Things like the use of sealing tape, foam insulation, shims, bucks, screws, caulking, and even the techniques used during the installation… Everything that is part of your window installation process affects the energy efficiency of the window.

If the window needs shims (or a complete window buck) and doesn’t have them, it won’t sit level in the opening, which can create gaps that let air pass through the window opening. If there isn’t insulative tape, foam, and caulking around the opening, that too could let hot air into the home and cold air out, impairing energy efficiency.

In extreme cases, an improperly installed window can simply fail outright, not closing properly or even coming out of the window opening.

That’s why it’s important to only work with a window installation company that uses licensed, highly-qualified, and trained professionals for every install they perform. You need installers who have the right tools and know-how to do a perfect job, every time. 

If you need a perfect window installation, be sure to reach out to Karoly Windows & Doors! Karoly’s installation team has decades of training and experience, and are held accountable to doing things the right way, the Karoly way!

We only ever use in-house installers on every job. We never outsource work to third-party contractors. We use top-shelf insulation, caulk, shims, and tools to ensure maximum energy efficiency and useful life for every window we install. 

Get a free in-home consultation with a window specialist from Karoly Windows & Doors: