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How To Thermally Insulate Windows: A Complete Guide

How To Thermally Insulate Windows: A Complete Guide

How To Thermally Insulate Windows: A Complete Guide

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If a room feels cold even when the heater is on, the problem is often not your heating system. It’s heat escaping due to inadequate insulation around your home.

And in most homes, windows are one of the biggest heat-loss points. Glass cools quickly. Small gaps around frames let in too much cold air. Cold surfaces pull warmth out of a room and make the whole space feel less comfortable.

The good news is you don’t need to renovate your entire home to feel a difference. You can improve comfort by upgrading your existing windows and doors.

This guide covers the most effective ways to thermally insulate a window, from quick fixes to longer-term upgrades, plus what to prioritise for the best return.

What Does It Mean to Thermally Insulate a Window?

A well-insulated window does three things:

  1. Reduces heat transfer through the glazing
    Less warmth escapes in winter (and less heat enters in summer).
  2. Minimises air leakage and drafts
    Because if air can move through gaps, your warm air will keep escaping.
  3. Improves comfort near the window
    Even if the room is heated, cold glass can make a space feel uncomfortable.

Most people don’t actually want “a warmer house” in theory. They want warmer bedrooms at night, fewer cold zones near windows and insulation that feels like it’s working all year round.

Step 1: Identify Where The Heat Is Escaping

Before you spend money, do a simple check:

  • Stand near your windows on a cold day. Do you feel a chill near the glass?
  • Run your hand around the frame. Can you feel air movement?
  • Look for visible gaps in seals or around sliding tracks.
  • Notice which rooms feel hardest to keep warm. Those are usually your priority windows.

This is the “no guesses” step. The best outcomes come from improving the windows that are causing the discomfort.

Step 2: Seal Drafts And Air Gaps (The Fastest Win)

Draft sealing is one of the most cost effective first method. If your existing windows leak air, even good heating will feel ineffective.

Start with:

  • Weather seals around opening windows
  • Brush seals for sliding windows where practical
  • Gap sealing in visible frame joints
  • Door seals and threshold gaps (often overlooked but related)

Sealing the drafts will make the next upgrade i.e curtains and glazing perform better.

Step 3: Use Window Coverings That Reduce Heat Loss

Window covering won’t “insulate” the window in a technical sense, but it can improve comfort quickly and at relatively low cost.

Best options include:

  • Thermal curtains (heavy, lined and close to the wall)
  • Honeycomb blinds (designed to trap air in pockets)
  • Pelmet or top seal where possible (helps reduce warm air escaping behind curtains)

The key is fit. A small gap at the sides or top can let warm air circulate against cold glass, reducing the benefit it is meant to provide.

Step 4: Improve The Glazing Performance (The Most Effective Long Term Solution)

Even with curtains and seals, a standard single glazed window will lose heat quickly. If you want meaningful improvement, you need a solution that creates a thermal barrier between the inside and outside of your home and improve overall window performance through frame, glazing and seals.

You generally have three options:

Option A: Window Replacement (New Double Glazing)

This can be effective, but it often involves:

  • Higher cost
  • More disruption
  • Changes to frames and external appearance (which can be an issue for heritage or strata)

Option B: Secondary Glazing / Retrofit Double Glazing (Upgrade the Existing Window)

Retrofit double glazing improves window insulation without having to replace existing window. A secondary glazing system is installed on the inside of the existing window reveal, creating an air cavity between the two window systems. That air cavity acts as a thermal barrier that help to reduce heat transfer and eliminate cold coming through. This is an ideal option for older homes, heritage buildings or strata apartments where full replacement is not allowed.

Option C: Window Films (Limited Thermal Impact)

Some films can help with solar control but they do not solve drafts and don’t provide the same insulation benefit as a secondary system with an optimal air cavity.

If the goal is thermal insulation, the standout options are replacement double glazing or a retrofit double glazing system.

Why Retrofit Double Glazing Existing Windows Is Often The Best Choice

Retrofit double glazing improves window insulation without replacing the existing window.

A retrofit double glazing system creates a sealed air cavity between your existing window and the internal glazing panel. That air cavity is the insulation engine. It reduces heat transfer and helps improve comfort.

A critical detail (especially for setting expectations and avoiding confusion):

Magnetite’s retrofit double glazing is installed on the inside of windowsill or reveal, creating a maximum air cavity for best thermal and acoustic insulation. The existing window is not touched or altered.

That means you keep your existing windows in place and upgrade the performance from the inside. For many homes, that’s the sweet spot: insulation improvement without structural changes.

What You’ll Notice When a Window is Properly Insulated

People often expect insulation upgrades to feel subtle. With windows, the difference can be immediate, especially in the rooms you use most.

Common improvements include:

  • Rooms feel warmer near the window, not just in the centre
  • Less “cold radiating” from glass at night
  • More stable temperatures across rooms
  • Heating doesn’t feel like it’s constantly chasing comfort

The goal is not to turn your home into a sealed box. It’s to reduce heat loss so your heating is working with the home, not fighting it.

Does Window Insulation Help with Condensation?

Condensation is a big concern in colder climates, and it can be common in many Australian homes.

It’s also one of the trickiest issues to manage because it depends on more than the window:

  • indoor humidity
  • ventilation
  • heating habits
  • airflow
  • how cold the glass surface gets

Window insulation such as retrofit double glazing can reduce condensation but it won’t eliminate it entirely. When you improve glazing, it helps the glass surface stay warmer as a well-insulated window reduces the temperature difference that causes moisture to condense. However, humidity control remains the key in managing condensation.

To manage condensation effectively:

  • use exhaust fans in bathrooms and kitchens
  • ventilate when conditions allow
  • avoid drying clothes indoors where possible
  • consider a dehumidifier if moisture is persistent

This sets the right expectation: insulation helps but condensation needs humidity control too.

The Best “Order Of Operations” For Insulating Windows

If you are looking for effective path that balances cost, disruption and results:

  1. Seal drafts and gaps
  2. Add effective window coverings
  3. Upgrade performance with retrofit double glazing or replacement double glazing
  4. Prioritise high-impact rooms first (bedrooms, living areas, home office)

You don’t need to do every window at once. Start with the rooms that bother you most. Experience the difference in comfort level. Then expand to the rest of the house if you want to.

Quick FAQ: Thermal Insulation for Existing Windows

What is the cheapest way to thermally insulate a window?
Draft sealing plus thermal curtains is generally the most cost-effective starting point. Both can deliver noticeable improvements when fitted correctly.

What works best long term?
A well insulated glazing system either retrofit double glazing or full replacement double glazing provides the most durable thermal improvement. The air cavity acts a thermal barrier against the heat, cold and noise.

Do I need to replace my windows to get better insulation?
Not always. Retrofit double glazing windows can improve thermal comfort without replacing the existing windows or doors. It is installed on the inside of your existing window reveal without touching the existing window. This is an ideal solution for homes where full replacement isn’t an option.

Will insulating windows reduce my heating bills?
A well insulated window help to reduce heat loss. This means your heating system will work less to maintain a comfortable indoor temperature. The extend of the savings will vary by a home’s construction, window type and usage – but most homes with a single glazed window can see a measurable improvement.

Does window insulation help in summer too?

Yes. A well insulated window reduces heat transfer from both directions, which helps keep indoor space cooler during hot weather.

Thermally insulating a window is not just about one product, but rather a layered approach to improve performance of entire window. If you want the biggest comfort gains:

  • start by sealing drafts
  • use proper window coverings
  • then upgrade your existing window with a solution that improve your entire window performance – glazing, frame and seals.

If you’d like a free measure and quote to get your home ready for winter, get in touch.

  1. Conduction
    Heat moving through an object (i.e. heat will travel through an aluminium, or steel frame more easily than through a timber or PVC frame).
  2. Radiant heat
    Heat from the direct sun. It can pass through space until reflected or absorbed.
  3. Convection
    Heat flow resulting from air movement e.g. either 'natural' convection - warm air rises, or 'forced' convection - a breeze by a window, both can result in a draught by a window.
  4. Infiltration
    Air leaks around the frame, around the window sash and through the gaps in the movable window parts.

When evaluating your options to improve the thermal performance of your windows, the following double glazing key performance criteria should be considered:

  • U-Value (Uw) represents heat transfer through a window, irrespective of direct sunlight. The lower a window’s U-Value, the better the insulation performance.
  • Solar Heat Gain Coefficient (SHGC) measures the solar energy that passes through glazing. The lower the SHGC, the less solar heat is transmitted through a window.
  • Air Infiltration or leakage measures the rate at which air passes through gaps in window joins and seals. The lower the air infiltration, the better a window’s energy performance.

The Window Energy Rating Scheme (WERS) Thermal Performance

Aluminium Frame Windows

Glazing Type Cooling Stars Heating Stars Cool% Heat% Uw SHGC
Single glazed 3mm clear glass* half blue star red starhalf red star 7.4 0.77
Single glazed 3mm clear glass &
4.5mm Magnetite		 blue starblue starhalf blue star red starred starred starred starred starhalf red star 35% 59% 2.9 0.67

Timber Frame Windows

Glazing Type Cooling Stars Heating Stars Cool% Heat% Uw SHGC
Single glazed 3mm clear glass* blue starhalf blue star red starred starred starhalf red star 20% 28% 5.5 0.69
Single glazed 3mm clear glass &
4.5mm Magnetite		 blue starblue starhalf blue star red starred starred starred starred starred star 37% 61% 2.6 0.66
Single glazed 3mm clear glass with
Solartite** and 4.5mm Magnetite		 blue starblue starblue starblue starblue starblue starhalf blue star red starred starred starred starhalf red star 76% 45% 1.8 0.20

 

*WERS standard typical industry window
**Solartite is a range of high performance window film. The window tested above has Solartite’s low-e film.

Magnetite vs Traditional Double Glazing – What the experts says

Winter - the ideal situation is to let in as much of the sun's heat and then trap it inside to maintain a comfortable environment in your home or office. This will also significantly reduce heating costs and saves you money. The ideal window for winter will have a high SHGC to let the sun's heat in and a low U-Value to prevent the heat from escaping. Clear windows without shading films are better suited for winter climates.

Compared with traditional double-glazing, Magnetite’s double-glazing solution can provide better results during winter or for cold climate areas such as Canberra, the Adelaide Hills and Blue Mountains.

Magnetite's clear acrylic window panels allow the direct heat from the sun's rays to enter the building and once inside, Magnetite's superior magnetic seals retain the heat, giving Magnetite an advantage over conventional operating windows.

Glazing Type Cooling Stars Heating Stars Cool% Heat% Uw SHGC AI
Double Glazed 3mm clear glass/12mm air
gap/3mm clear glass IGU (Insulated Glass Unit)*		 blue starblue starhalf blue star red starred starred starred starred starhalf red star 40% 55% 3.0 0.61 5.0
Single glazed 3mm clear glass &
4.5mm Magnetite		 blue starblue starhalf blue star red starred starred starred starred starred star 37% 61% 2.6 0.66 0.5

 

*WERS standard typical industry window

Summer - the ideal situation is to minimize the heat coming in from outside and prevent the cool air inside from escaping through the window. This will make your home more comfortable and reduce your air-conditioning costs. The ideal window for summer, or for a large west-facing window, will have a low SHGC to limit the heat from the sun's direct rays, as well as, a low U-Value to prevent the heat in the air outside from moving through the window and warming the cool conditioned air inside. It is better to have a shading film on the first (outer) glass pane to limit the heat that enters the double glazed unit.

The combination of Magnetite retrofit double glazing and Solartite outperforms the traditional insulated glass unit for hot climate. Magnetite fit over an existing window with reflective film will give better summer heat control than a double-glazed window with supertoned glass.

The Magnetite system stops the conduction and convection of heat better than glass windows giving superior results for summer conditions. Combined with a film or a shading device, Magnetite is the ideal solution for the hot Australian conditions.

Glazing Type Cooling Stars Heating Stars Cool% Heat% Uw SHGC AI
Double Glazed 5mm Supertoned glass/6mm
air gap/5mm clear glass IGU (Insulated Glass Unit)*		 blue starblue starblue starhalf blue star red starred starred starred starhalf red star 54% 425% 3.30 0.461 5.0
Single glazed 3mm glass Solartite**
and 4.5mm Magnetite		 blue starblue starblue starblue starblue starhalf blue star red starred starred starred star 73% 38% 2.4 0.19 0.5

 

*WERS standard typical industry window
**Solartite is a range of high performance window film. The window tested above has Solartite’s low-e film.

Click here to view pdf version of Magnetite WERS test result.

Noise Reduction Technical Data and Results

Proven Results

Noise pollution is one the biggest problems faced in the cities today, particularly in a high-density living. Unwanted noise can be responsible for sleep loss and lack of concentration. This can lead to a deterioration of health through increased stress levels, headaches, high blood pressure and loss of hearing.

Noise penetrates windows through leaky seals or vibrations through the window structure. To reduce noise, a window should minimise air infiltration, hold enough weight to limit vibration and allow a large air cavity. This cavity, or air gap, is the key to secondary glazing.

Expert Advice:

CSIRO Research by John Davy shown an air cavity can effectively enhance sound insulation performance. The larger the air cavity the more noise the window system can reduce. Results show a wide air gap of 100-125mm will improve sound insulation by an average of 7dB(A) more than windows with a narrow gap of 8-13mm.

Window or Glass Door Type Total Glass Thickness (mm) Air Cavity Thickness (mm) Sound Insulation (dB)
Single Glaze 3mm 3 0 23
Single Glaze 6mm 6 0 27
Double Glaze Narrow Gap 9-12 8-13 31
Double Glaze Wide Gap 9-12 100-125 38

 

The human ear is less sensitive to low frequency sound. According to a study released by the CSIRO, the human ear needs a change of 3dB to detect a difference in noise levels and the human ear perceives a change of 10dB as a doubling or halving of the noise level.

If you double the thickness of glass in a window the noise level will decrease by 3dB the least amount possible to make a difference. If the glass is laminated there is no perceived difference in noise reduction.

National Acoustic Laboratory Testing:

Working with the principles of noise transfer, Magnetite® and Soundtite are designed for high performance, which is achieved by combining air tight seals with the ability to maximize an air cavity and the flexibility to vary glazing thickness to suit. Magnetite® is excellent at reducing general noise such as busy roads, commuter trains and noisy neighbours. Whereas, Soundtite works well to combat low frequencies noise such as garbage truck, freight trains or airplane during take-off and landing.

Test Code Material Tested Rw* Improvement
ATF 707 (test window) Aluminium sliding window, 3mm float glass with mohair seals 23 N/A
ATF 709 Test window, 100mm air gap and Magnetite System (4.5mm acrylic panel) 35 52%
ATF 1507 Test window, 100mm air gap and Magnetite System (10.0mm acrylic panel) 39 70%
ATF 817 Test window, 100mm air gap and Soundtite (6.38mm laminated glass) 41 78%

 

*The Weighted Sound Reduction Index (Rw) is a measure for sound insulation performance of a single building element such as windows. The higher the Rw rating, the better the sound insulation.