What Double-Paned or Double-Glazed Windows Are

Double-paned or double-glazed windows are an energy-efficient type of window that is made up of two panes of glass with a sealed air space in between. The double pane construction provides better insulation, noise reduction, and condensation resistance compared to single pane windows. Here is a detailed overview of what double-paned windows are, their pros and cons, how they work, differences from single pane windows, and more.

How Double-Paned Windows Are Constructed

Double-paned windows, also known as insulating glass units (IGUs), have a straightforward construction:

Two panes of glass separated by a space filled with air or gas – This creates an insulating air gap usually around 1/2″ wide.
Spacer bar between the glass panes – This seals the air space and keeps the panes separated. Spacers are made of aluminum, stainless steel, fiberglass, or plastic.
Sealants around spacer bar – Sealants like silicone ensure the air space stays airtight.
Frame materials – Vinyl, wood, aluminum, and fiberglass are common frame materials that hold the glass panes in place.

The two individual panes are made of various types of glass, like annealed, tempered, or laminated glass. The composition of the glass can impact strength, safety, sound insulation, and other factors.

How Double Glazing Works to Improve Energy Efficiency

Double-paned windows work by creating a layer of insulating air or gas between the two panes of glass. This significantly reduces heat transfer in and out of a building.

There are a few reasons why the air gap increases efficiency:

Trapped air is a poor conductor of heat – The motionless air space acts as an insulator against heat flow. Still air does not easily transmit heat from the hot side to the cold side of a window.
Increased thermal resistance – The layer of trapped air adds another barrier that heat must pass through, increasing the window’s total R-value. More barriers means more resistance to heat flow.
Reduced convection – Heat cannot easily flow via air currents between the panes, since the air space is sealed off. Less convection reduces heat transfer.
Radiative heat reflection – Some heat radiating from interior or exterior surfaces can reflect off the glass before being transmitted. This also lowers heat conduction.

These factors combine to significantly slow the rate of heat transfer and prevent indoor air from escaping out through the window or outdoor air infiltrating in. Properly installed double pane windows can reduce heat loss by around 30-50% compared to single pane windows.

Pros and Cons of Double-Paned Windows

There are many benefits to choosing double-glazed windows, along with some potential drawbacks:

Pros

Increased energy efficiency – Less heat loss in winter and heat gain in summer. Can lower energy bills.
Reduced condensation – The extra air gap prevents inner glass surfaces from getting as cold, reducing condensation.
Improved noise reduction – Double the glass increases sound dampening from outside noise sources.
Added durability – Thicker window construction is more durable and resistant to damage.
Reduce fading – Blocks more UV rays that cause fading of interior furnishings.

Cons

Increased cost – Double pane windows cost more than single pane, though energy savings often recover added costs.
Extra weight – Heavier than single pane windows, requiring increased structural support.
Potential seal failure – If air space seal fails, window loses insulating capacity. Proper installation minimizes this.
Visible spacer bar – Depending on construction, the spacer bar between glass panes may be visible. Newer models better hide spacers.

Overall, the benefits of double pane windows usually make them a smart investment despite their higher initial cost. Prioritizing energy efficiency and noise reduction are two key reasons homeowners choose them.

Differences from Single Pane Windows

There are a few key differences that separate double pane and single pane windows:

Number of glass panes – Single pane has one sheet of glass while double pane has two panes separated by a spacer.
Thickness – Double pane glass is around 1″ thick total, while single pane is around 1/4″ to 1/2″ thick.
Energy efficiency – Double pane windows insulate 2-3 times better than single pane, reducing heating and cooling costs.
Noise reduction – Double pane glass blocks more exterior sound due to its thicker double layer construction.
Condensation resistance – Lower chance of inner glass surface condensation with two panes instead of one.
Cost – Double pane costs 50-100% more upfront, but saves energy and costs long-term.
Weight – Double pane windows are around twice as heavy as single pane windows.

While single pane windows are cheaper initially, double pane models are a smart investment for most climates and situations where energy efficiency, noise reduction, and condensation resistance are priorities.

Types of Double Pane Window Constructions

There are several design variations when constructing double pane windows:

Glass types – Different inner and outer glass can be used like laminated glass for security, tempered glass for strength, or Low-E coatings to reflect heat.
Gas fills – Argon, krypton, or other gas between panes provides better insulation than just air.
Spacing – Wider spacing like 1 inch improves insulation but requires thicker frames. Narrower spacing like 1/2 inch allows thinner frames.
Panels – Some double pane windows have more than two panels, like triple pane with three glass sheets.
Thermal breaks – Structural elements reducing heat flow between inner and outer window frames.
Spacer types – Advanced spacers like foam or thermoplastic improve insulation and reduce condensation.
Tints and coatings – Tinted or reflective glass manage solar energy, visibility, and interior fading.

Energy Savings from Double Pane Windows

Installing energy efficient double pane windows can save considerable money on utility bills by reducing energy loss from a building. Here are some typical energy and cost savings:

Heating and cooling savings of 10-30% annually by better retaining conditioned indoor air. This reduces energy bills by hundreds of dollars per year.
Shorter air conditioner runtimes in summer since less heat is gained through windows. Each window can save around 100-500 kWh of summer cooling energy.
Lower furnace runtimes in winter as double pane windows lose 30-50% less heat. Savings of 2-8 therms of natural gas per window are common each winter.
Lower carbon footprint by using less fuel for heating and cooling. This creates a greener home.
Reduced drafts and hot/cold spots from windows. This increases comfort.
Shorter payback period of 5-10 years for the higher upfront cost. This results in thousands of dollars in lifetime savings.

Replacing all single pane windows with energy efficient double pane windows is one of the best investments homeowners and builders can make to save energy and costs long-term.

Optimal Spacing Between Double Pane Glass

The width of the space between the two panes of glass impacts the overall insulating value of a double pane window. Some guidelines for optimal spacing include:

Standard spacing is 1/2″ to 5/8″ – This provides a good balance of insulating value and a thin frame.
Wider gaps like 1″ improve efficiency – More trapped air gives higher R-value, but requires thicker frames.
Narrower gaps of 3/8″ – These provide less insulation but allow thinner window frames.
Ideal gas fills have wider gaps – Argon or krypton filled units need 1″ spacing to contain enough gas.
Consider energy codes – Many building codes now require minimum double pane R-values, influencing ideal spacing.

In general, wider spacing is better for maximum energy efficiency, while narrower gaps allow for thinner, sleeker frames. 1/2” to 5/8” spacing is common as a compromise between insulation and frame thickness for many double pane windows.

Common Gas Fills for Double Pane Units

Various gases can be used between the glass panes of double pane windows instead of just air:

Argon – Argon is an inexpensive inert gas often used for better insulation than air. It is denser than air which slows heat transfer.
Krypton – Krypton is a pricier but very effective insulating gas, around 20% better than argon.
Xenon – Xenon is also an insulating gas sometimes used but is very expensive. It offers minimal gains over argon or krypton.
Air – Plain air is the most affordable option, though it provides the lowest insulation value.
Mixtures – Combinations of argon, krypton, and air are used to balance cost and performance.
SF6 – Sulfur hexafluoride was used in the past but discontinued due to environmental concerns.

Argon and krypton are most common, as they significantly boost window insulation for a reasonable added cost. Air-filled units are less expensive but sacrifice some energy efficiency.

Benefits of Argon vs Air Double Pane Windows

Argon gas-filled double pane windows offer some notable improvements over air-filled units:

Better insulation – Argon gas is denser than air and slows conductive and convective heat flow, improving insulation around 10-15%.
Less solar heat gain – Argon is less transparent to infrared radiation from the sun, reducing solar heat gain.
Lower operating costs – Increased insulation and lower solar gain lead to less energy usage for heating and cooling.
Reduced condensation – Warmer inner glass surface temperatures minimize inner pane condensation.
Minimal added cost – Argon only costs a few dollars more per window compared to air-filled units.
Long-term stability – Argon will not dissipate from properly sealed windows for decades.

For a marginal extra cost, opting for argon-filled double pane windows provides noticeably improved thermal performance in most climates compared to air-filled units.

Installing Double Pane Windows for Best Performance

Proper installation is key to ensure double pane windows deliver their full benefits. Here are some tips:

Seal perimeter gaps – Use caulk and weatherstripping to seal any openings around the window frame to stop air leakage.
Insulate surrounding frame – Adding insulation around the window frame boosts efficiency.
Apply flashing – Flashing helps water drain effectively and keeps gaps watertight.
Check spacer bar seal – Inspect the primary seal around the spacer bar to ensure the argon/air gap remains airtight.
Use shims – Shims properly position the window in the opening to minimize gaps and improve friction fit.
Level and square window – Make sure the window is evenly placed in the opening without twist or bow.
Secure mounting – Use adequate screws or anchors for a snug and rigid installation.

Taking extra care during window installation maximizes the real-world performance of any double or triple pane window. A quality installation greatly improves air sealing, insulation, and structural integrity.

Home Features That Enhance Double Pane Windows

Certain complementary home features and design choices can further enhance the benefits of energy efficient double pane windows:

Airtight construction – Tight building envelopes reduce air leakage that can compromise double pane performance.
High-performance HVAC – An efficient furnace, air conditioner, and ductwork optimize indoor comfort and energy savings.
Insulated walls/attic – Well-insulated exterior walls and attics prevent heat loss through other elements.
Window overhangs – Eaves and overhangs control sun exposure on windows to manage solar heat gain.
Low-E coatings – Low-emissivity coatings boost efficiency by blocking radiative heat transfer through the glass.
Smart glass – Electrochromic, photochromic, or thermochromic windows intelligently respond to solar conditions.
Automated shades – Motorized interior or exterior shades managed by home automation optimize solar gain.
Proper orientation – Positioning larger windows to face north and south can provide ideal solar gain in winter and minimize it in summer.

Double pane windows perform best when complemented by these other energy-smart home features and design strategies. A whole-house approach is ideal.

Do Newly Installed Windows Need Time to “Cure”?

When windows are newly installed, you may hear claims that they need time to “cure” before realizing their full energy efficiency and performance. Is this accurate?

There are a few factors that lend some truth to this curing timeframe for new windows:

Gasket compression – It can take weeks or months for window gaskets to fully compress and seal tightly.
Offgassing – Chemical offgassing dissipates over the first ~6 months, potentially improving argon content.
Window adjustment – New windows settle into position as the building adjusts, improving seals.
Learning curve – Homeowners learn over time how to best operate new windows and features.

However, any curing period for energy savings from new, properly installed windows is relatively minor – likely less than a 10% difference. If new windows are not performing as expected, it is likely due to an underlying issue, not just curing. The majority of performance gains are realized immediately upon proper installation.

Signs of a Failing Double Pane Window Seal

Over decades of use, the seal between double pane glass layers can eventually fail, causing lower thermal performance and inner glass condensation between panes. Signs of a failing window seal include:

Condensation or fogging – Moisture between the panes signals lost air seal.
Cloudy film on glass – Chemical deposit on inner glass also indicates a compromised air gap.
Lower energy efficiency – More heat loss in winter and solar gain in summer.
Difficult to see out – Impaired visibility as condensation blocks light transmission.
Draftiness – Lost insulation enables more cold air infiltration and convection.
Discolored or corroded spacer – A deteriorating metal spacer bar can cause seal failure.
Bubbles or ripples in sealant – Visible distortion in the spacer sealant can precede air gap leakage.

At the first signs of window seal failure, replacement of the entire insulating glass unit is usually needed, as the original air tightness cannot be restored.

How Long Double Pane Windows Last

With proper maintenance and barring any manufacturing defects, most double pane windows can be expected to last 20-30 years or more. Here are some factors affecting lifespan:

Frame material – Vinyl and fiberglass have the longest lifespan of 30-50 years. Wood lasts 20-30 years and will require regular repainting. Aluminum lasts around 25 years.
Seal durability – Primary seals last 15-30 years before risk of failure. Secondary seals add redundancy.
Glass quality – Insulating glass made with tempered, laminated, or low-E coated glass will be most durable.
Ultraviolet exposure – Excessive unprotected sun exposure degrades window components faster.
Mechanical damage – Cracks, frame movement, faulty installation, etc. shorten lifespan.
Regular maintenance – Keeping up with repainting, caulking, gasket inspection, etc. maximizes longevity.
Manufacturing quality – Well-made windows have tighter tolerances and sturdier construction.

With regular maintenance and repair as needed, quality double pane windows should provide excellent performance for at least 20-25 years, if not longer. Upgrading to triple pane can extend lifespan further.

Do Double Pane Windows Need Storm Windows?

Modern double pane windows provide good insulation on their own. External storm windows offer a few added benefits:

Potential advantages of adding storm windows:

Extra protection from weathering elements prolongs main window life
Added energy efficiency from triple pane construction
Lower external glass temperature reduces condensation
Another barrier for enhanced soundproofing
Can accommodate screen inserts to allow natural ventilation
Less costly than total window replacement

Reasons they may not be necessary:

Already efficient without significant added value
Can make opening and cleaning windows more difficult
Appearance may not match main windows
Added complexity to install and remove seasonally
Lower visibility and light transmission

In colder climates or with single pane main windows, storm windows likely make sense. For most modern double pane windows, storm windows provide incremental benefits that may not justify the extra costs in many situations.

Visible Spacer Bars on Double Pane Windows

Some double pane windows have a visible metal spacer bar between the glass panes while others have more hidden spacers. Here are some key points on spacer visibility:

Metal spacers – Aluminum and stainless steel spacers often have visible edges. Popular in the past for affordability.
Warm edge spacers – Insulated spacers made of silicone foam, thermoplastic, or fiberglass reduce condensation and increase comfort.
Desiccant fill – All spacers contain drying desiccant to maintain the air gap. Beads may be visible.
Improved aesthetics – Many modern spacers use black color and minimized thickness for a more discreet spacer.
Compromise features – Less visible spacers can sacrifice longevity and gas retention capability.
Frame design – Some frames overlap the spacer edge more to hide it.

There are valid tradeoffs between nearly hidden spacers providing a sleek look and more visible spacers offering structural integrity and durability. Many customers accept minor visible spacer edges to ensure optimal window performance.