Insulating windows correctly: methods for greater energy efficiency

Why window insulation is so important

You may be wondering whether window insulation is really necessary—after all, your windows seem to be airtight and work perfectly well. But that's precisely where you're mistaken: heat, energy, and even moisture often escape unnoticed—with noticeable effects on your living comfort, the environment, and your wallet.

The advantages of good window insulation:

• Energy efficiency: Effective insulation reduces heat loss by up to 30% and ensures that heating energy stays in the room in winter and coolness is retained in summer. This significantly improves the overall energy performance of the building.

• Cost savings: Lower energy consumption reduces heating and air conditioning costs noticeably, an advantage that pays off every year.

• Increased comfort: Insulated windows create a pleasant indoor climate without draughts or cold corners and increase everyday well-being.

• Climate protection: Better energy efficiency also means lower CO₂ emissions. This means you are actively contributing to environmental protection.

• Noise reduction: Modern insulated windows effectively keep out street noise and other outside sounds, ensuring more peace and quiet within your own four walls.

• Preserving and increasing the value of your property: Energy-efficient windows increase the value of your property and make it more attractive when selling or renting.

• Protection against moisture: Less condensation on the windows significantly reduces the risk of mold growth and moisture damage.

• UV protection: High-quality insulating glazing filters out harmful UV rays and protects furniture, carpets, and textiles from fading.

• Longevity of heating and cooling systems: Since less energy is lost, technical systems do not need to run at maximum capacity as often. This extends their service life.

• Year-round comfort: Whether summer or winter, well-insulated windows keep the indoor climate consistently pleasant and help save energy.

Insulating windows against the cold – these methods are available

The advantages have convinced you and you understand how crucial good window insulation is – but what is the best way to proceed? There are several ways to optimize your windows for energy efficiency, depending on how old they are and what effect you want to achieve.

Sealing strips and draft stoppers

Can you feel cold air even though the windows are closed? Then heat is escaping through small gaps between the frame and the window sash. You can easily seal these gaps with self-adhesive sealing strips and keep your home nice and warm again.

• Advantages: inexpensive, easy to install, and immediately reduce drafts

• Disadvantages: wear out over time and need to be replaced regularly

• Suitable for: older windows where small gaps are visible between the frame and sash

• How to proceed: Measure the gap, choose the appropriate type of seal (rubber, foam, or brush), clean the frame thoroughly, and stick the seal on so that it fits perfectly.

Seal joints with silicone or acrylic

Have you noticed drafts or moisture seeping in around the edges of your windows? Then the joints between the wall and the frame are probably leaking. You can restore the seal with silicone or acrylic and protect your home from heat loss and mold.

• Advantages: Sealing is durable, inexpensive, and provides reliable protection against energy loss

• Disadvantages: only suitable for solid joints; the material requires clean preparation

• Suitable for: leaky transitions between window frames and masonry

• How to proceed: Remove old sealant, clean the joint, apply silicone or acrylic evenly, smooth it with a joint smoother and allow it to harden completely.

Insulating film for winter

Does the cold seep through the glass in winter and the room never really warm up? A transparent insulating film creates an additional layer of air in front of the window, improving thermal insulation—ideal if you want to save energy quickly and cost-effectively.

• Advantages: inexpensive, easy to apply, and immediately effective

• Disadvantages: only suitable for the cold season and not permanently attractive

• Suitable for: rental apartments or old single glazing

• How to do it: Cut the film to size, stick it onto the clean frame, and stretch it with a hair dryer until it is wrinkle-free.

Low-E film (heat-reflective window film)

Does your room heat up quickly in summer and stay cool in winter? Low-E film reflects heat radiation back into the room and effectively keeps out the sun's heat, ensuring a pleasant indoor climate all year round.

• •Advantages: reduces heat loss, lowers UV radiation, and protects furniture from fading

• Disadvantages: no protection against drafts; installation must be done carefully

• Suitable for: windows with strong sunlight or south-facing windows

• How to proceed: Clean the glass pane thoroughly, cut the film to size, apply it with a film of water, and carefully smooth out any air bubbles.

Thermal curtains

Are you freezing in your living room despite closed windows? Heavy thermal curtains help keep the cold out and the room cozy and warm—a simple solution that requires no DIY skills.

• Advantages: easy to retrofit, improves the feeling of space and saves heating costs

• Disadvantages: only effective if they are kept closed at all times

• Suitable for: large windows in living rooms or bedrooms

• How to do it: Install curtains close to the window frame, floor-length if possible, and close them regularly when it gets cold.

Honeycomb pleated blinds

Does heat build up in summer and escape in winter? Honeycomb pleated blinds act as an insulating layer thanks to their air-filled structure and ensure a pleasant indoor climate in every season.

• Advantages: year-round thermal insulation, elegant, energy-efficient

• Disadvantages: no substitute for sealing; usually requires customization

• Suitable for: living spaces with large glass surfaces or south-facing windows

• How to proceed: Measure the window accurately, install the pleated blind close to the frame, and close it completely on cold nights or hot days.

Internal storm windows (internal storm windows)

Want to keep your old windows but eliminate energy loss and drafts? Internal storm windows create an additional layer of insulation and significantly reduce heating costs.

• Advantages: very good insulation, better soundproofing, ideal for old buildings

• Disadvantages: slightly more expensive to purchase, cleaning between the panes is more time-consuming

• Suitable for: listed buildings or wooden windows worth preserving

• How to proceed: Measure the window accurately, choose a suitable design with a seal, and install the storm window airtight on the inside.

External storm windows

Does the room often feel cool and damp when there is strong wind or rain? External storm windows protect the windows from the outside against the weather and prevent heat loss – ideal for buildings exposed to the elements.

• Advantages: effective weather protection, significant energy savings

• Disadvantages: more complex installation, changes the external appearance

• Suitable for: houses directly exposed to the weather or old wooden windows

• How to proceed: Have storm windows made to measure, install them on the outside and seal them carefully to prevent moisture from penetrating.

Secondary glazing

Are your windows still in good condition but outdated in terms of energy efficiency? A second layer of glass on the inside significantly improves insulation without having to replace the entire window.

• Advantages: significant thermal insulation, cheaper than new windows

• Disadvantages: slightly more complicated to install, heavier

• Suitable for: windows with a solid structure and frames worth preserving

• How to proceed: Insert a second pane of glass made to measure, seal it airtight, and ensure condensation can drain away.

Window replacement / energy-saving windows

If your windows are badly damaged, warped, or simply outdated, the only long-term solution is to replace them with modern energy-saving windows—this will give you peace and quiet, comfort, and sustainability all at once.

• Advantages: maximum energy efficiency, long-term solution, noticeably improves living comfort

• Disadvantages: more expensive and labor-intensive than retrofitting

• Suitable for: old, leaky, or severely damaged window frames

• How to proceed: Choose windows with triple glazing and Low-E coating, have them installed by professionals, and ensure an airtight fit.

Insulating windows from the inside or outside – which is better?

Whether you insulate your windows from the inside or outside depends on what you want to achieve:

• Internal insulation is particularly suitable if you want to save energy quickly, flexibly, and without much effort – for example, with seals, films, or secondary glazing. It is ideal for rental apartments and older buildings, as no structural changes are necessary.

• External insulation, on the other hand, has a more lasting effect. It protects window frames and facades from the weather in the long term, reduces heat loss more effectively, and increases the energy efficiency of the entire building. However, it is more costly and more suitable for owners or renovations. In short: insulating internally means quick results with little effort, while insulating externally means lasting protection and maximum energy savings.

Our expert tip: If you are thinking long term, it is worth combining both approaches. Good interior insulation can stop energy loss in the short term, while a professional exterior solution optimizes thermal insulation in the long term – allowing you to get the most out of your windows.

Insulate your windows yourself – step-by-step instructions

Want to insulate your windows yourself to save energy and prevent drafts? With a little DIY skill and the right materials, it's easy to do. These instructions will show you step by step how to effectively seal your windows against the cold and make them permanently airtight.

1. Check for leaks (draught test): Start with a simple test by holding a candle flame along the frame. If it flickers, air is entering. Mark these areas for later sealing.

2. Measure the gap: Measure the gap between the sash and the frame at several points. The gap determines the correct thickness of the window sealing tape.

3. Clean the surface: Thoroughly remove old seals, dust, and dirt. The new sealing material will only adhere optimally to a clean, dry surface.

4. Apply window sealing tape: Cut the sealing tape to size and stick it evenly into the groove of the frame without pulling or stretching it. Press it firmly all around.

5. Seal joints: Seal cracks and transitions between the window frame and wall with silicone or acrylic. Mask off the joint, apply the sealant evenly, smooth it out, and allow it to dry. This will ensure that the connection remains airtight.

6. Apply window film: Clean the window thoroughly and apply insulating window film. Secure it with double-sided adhesive tape and then stretch it with a hair dryer. This method is ideal for retrofitting window insulation, especially in rental apartments.

7. Check and readjust fittings: If the window no longer closes evenly, you can easily readjust the hinges or locking pins. This ensures even contact pressure and prevents draughts.

8. Perform a leak test: After insulating, check again with a candle flame or by hand to see if the window is really airtight. Detect any drafts and correct them if necessary.

9. Maintain and ventilate regularly: Ventilate regularly for a short time to prevent condensation and maintain the seals once or twice a year with rubber care products. This will keep the insulation effective in the long term.

If you would prefer to have your insulation planned professionally or are unsure which method is right for your windows, please contact our expert advisors, who will provide you with personalized support and tailor-made solutions for your home.

Avoid common mistakes when insulating windows

Avoid these common mistakes when insulating your windows:

• Incorrect window sealing tape selected and applied: Tape that is too thin will not seal properly, while tape that is too thick will prevent the window from closing. If it is also applied unevenly, small leaks will reappear.

➡ How to avoid this: Measure the gap accurately and choose the right material (rubber for large gaps, foam for small gaps). When applying the tape, make sure it is attached evenly and without stretching along the entire frame.

• Film not attached airtight: If air bubbles or wrinkles form, heat will continue to escape.

➡ How to avoid this: Stick the film onto a clean, dry surface and smooth it out with a hair dryer until it is taut.

• Silicone joints applied uncleanly: Holes or cracks in the sealant allow draughts and moisture to penetrate.

➡ How to avoid this: Completely remove old joints, degrease the surface, and apply new joint evenly with a smoothing tool.

• Irregular contact pressure during installation: If the seal is pressed unevenly, leaks will occur.

➡ How to avoid this: Apply even pressure when sticking and then check the closing resistance of the window.