Composite vs. Fiberglass Windows

Composite vs. fiberglass windows overview

Composite windows can either be made of multiple materials that are formed into a new, unified material or they can include multiple distinct materials that are attached to one another. A common example is a wood frame with an aluminum exterior for added protection from moisture.

Fiberglass window frames are made from small glass fibers and a mixture of resins. They’re technically composite frames, too, but most people call this particular mixture of materials fiberglass and refer to other material combinations as composite.

Key differences at a glance

Both composite and fiberglass windows have excellent energy efficiency, durability, longevity and value, but there are some important differences to be aware of before deciding. You can use the table below to compare these materials side by side to help you decide which is likely best for your home and budget.

Composite vs. fiberglass windows: performance

Composite and fiberglass windows both perform well in just about any climate. Both materials resist expansion and contraction with changes in temperature, and neither is prone to warping or cracking. Both also have excellent structural integrity that’s suitable for windows in large wall openings, even if they have heavier triple-pane glass.

Neither material is universally better, so you should compare the performance specifications below and decide which suits your particular needs better.

Strength

Fiberglass and composite windows are both extremely strong and can boast up to eight times more structural strength and stability than vinyl windows. This makes them both great candidates for oversized picture windows, sprawling bow windows and floor-to-ceiling windows that can face heavier structural loads.

Either material will perform extremely well in these scenarios, and differences in strength are more likely to stem from the specific manufacturer or manufacturing process rather than the material itself.

Thermal expansion and air leakage

Composite and fiberglass windows both resist thermal expansion better than vinyl and wood, but fiberglass is the better option if you live in an area that sees dramatic temperature swings.

Since a main component of fiberglass frames is glass fiber, the frame will expand and contract at a rate that’s nearly identical to that of the window glass itself. That means less pressure on the seals between the frame and the glass, which equals a lower risk of broken seals and a loss of energy efficiency.

Broken seals and air leaks are not terribly common on composite windows, but they are slightly more likely, as the frame material expands and contracts at a different rate than the glass inside the frame.

Water resistance

Once again, both materials have excellent water resistance, but fiberglass is slightly better in this regard. Fiberglass is impervious, so it will never absorb water or moisture, making it ideal for areas that see heavy rainfall or high humidity levels.

Although fiberglass windows cost anywhere from $100 to $300 more per window than composite, they can last up to 40 years.

Some composite frames have an equal level of water resistance, but it depends on the construction.

Composite frames that have unified materials, like Andersen’s Fibrex frames, boast a similar water resistance to fiberglass. Composite windows that have a wooden frame with an aluminum exterior, often called aluminum-clad windows, still have a wooden core that can absorb moisture. The aluminum protects it from rain and snow, but humidity can be an issue.

Lifespan

Fiberglass is a better option for longevity and long-term value, with an average lifespan of 20 to 40 years. Composite frames can last for just as long in some cases, but aluminum-clad windows only last for 15 to 20 years, as the aluminum exterior is still prone to corrosion.

Composite vs. fiberglass windows: energy efficiency

Energy efficiency is an increasingly popular consideration for homeowners choosing replacement windows or windows for new construction, so it’s a good idea to consider the following factors when deciding between composite and fiberglass windows.

U-factor

U-factor is a measurement of how well your window insulates, so it’s an important factor to consider, especially in cold climates and for north-facing windows. Generally speaking, fiberglass is the most energy-efficient window frame material available, so you’re likely to see better (i.e., lower) U-factor ratings and overall energy efficiency with fiberglass frames.

U-factor measures how well a window insulates, while the solar heat gain coefficient measures heat transfer to your home.

When comparing windows, make sure you confirm whether the manufacturer reports a center-of-window U-factor or the whole-window U-factor. The former only rates the insulative properties of the glass, while the latter is more indicative of how your entire window will perform.

Solar heat gain coefficient

The solar heat gain coefficient (SHGC) is another rating that tells you how much solar heat your window will transfer to the interior of your home. Unlike U-factor, SHGC is usually a whole-window rating, so it gives you a snapshot of the thermal conductivity of all materials.

Your window’s SHGC value will depend more on the glass type than the frame material, but fiberglass frames tend to have lower SHGC values than composite windows with equivalent glass inserts.

You should prioritize a low SHGC value in hot, sunny climates, but a high SHGC is better in cold climates, since it allows you to capitalize on natural heat from the sun. As such, fiberglass is likely best in hot climates, but you may be better off with composite windows in cold climates, depending on other factors.

Air infiltration

As mentioned above, fiberglass expands and contracts at about the same rate as your window glass, which puts less pressure on the seals between the materials. This means a lower risk of air leaks.

In reality, airtight seals and air infiltration ratings will affect energy efficiency more than insulation properties, so for maximum efficiency, it’s a good idea to choose fiberglass for the lower risk of air leaks over time.

Other considerations

Frame material is a crucial decision to make when installing new windows, but there are many other things you should consider that can have an even greater impact on your window’s performance.

If you’re thinking about composite or fiberglass windows for improved efficiency, you should also think about the following factors carefully. The best window companies will be able to couple either composite or fiberglass frames with any of these upgrades:

• Double-pane vs. triple-pane glass: Triple-pane glass is significantly more energy-efficient and ideal in extreme climates. Upgrading to triple-pane glass will add an average of $100 to $200 per window to your costs, but the investment could pay for itself in energy bill savings.
• Argon vs. krypton gas fill: The specific gas in the voids between glass panes is usually argon, but you can upgrade to krypton for better performance and thermal insulation. The bump in cost will depend on your manufacturer and the size of your window.
• Low-emissivity (low-E) coating: Low-E coatings can reduce heat transfer through your windows by up to 70%, meaning this addition can have more of an impact on overall window efficiency than your frame choice. A low-E coating will add an average of $50 to $150 per window to your total.
• Spacer type: Spacers sit between window panes to hold them at a constant distance. Fiberglass spacers have some of the best R-values, so they’re the most insulative, but a mix of materials might be best depending on the climate. Once you choose a window installer, ask about spacer options and which best suits your home.
• Installation quality: The quality of the installation can end up having more of an impact on your window’s performance than the materials themselves, as gaps and air leaks can quickly lead to energy losses, regardless of frame and glass materials. Always hire a professional installer with a good track record to avoid issues.

Composite vs. fiberglass windows: cost and value

On average, a composite window will cost between $600 and $1,200, while fiberglass windows are more expensive, at $700 to $1,500. You could pay significantly less for smaller window types, like hopper windows or transom windows, while prices for each can climb beyond $7,000 for larger windows, like bay windows or oversized picture windows.

Here are a few things that can affect your window replacement costs, including things to consider for each frame type:

• Window size: The larger your window, the more you’ll pay for the materials. Labor fees can also increase for oversized windows that require multiple techs or specialized window lifts to move the unit into place. Both composite and fiberglass frames can support large windows, so size will be a major factor for either.
• Window type: Some window types, like casement and bay windows, also cost more due to increased manufacturing complexity. The energy efficiency of fiberglass makes it ideal for casement windows, but because they use complex opening mechanisms, you’re more likely to pay a premium for this high-efficiency window type with fiberglass frames.
• Window location: Installing a window on the second floor requires the use of safety equipment and is more likely to demand a window lift, which can add an average of $50 to $150 to your total for the added labor.
• Energy-efficiency upgrades: Since fiberglass frames are preferable for greater energy efficiency, homeowners are more likely to upgrade other aspects of fiberglass windows, too. Things like triple-pane glass, low-E coatings and window tinting are more likely to run up your window costs.
• Long-term maintenance: Composite and fiberglass windows require little maintenance, but repairs can be more expensive for fiberglass, leading to higher long-term costs. This is especially true when comparing composite frames that have a wooden interior and an aluminum facade, as wood is the easiest frame material to repair.

FAQ

Bottom line

Choose composite windows if you need a high level of energy efficiency on a budget, but consider splurging for fiberglass windows if you want maximum energy efficiency, a longer lifespan and more durable frames. If you’re not sure which is right for your situation, ask a local window installer for help.