Mistake 1: Thermal bridges — the invisible enemy
A thermal bridge is an area of the building envelope where heat escapes much more easily than elsewhere. In window installation, thermal bridges form at the joint between the window and the wall when it is not properly sealed.
The problem is that a thermal bridge is invisible to the naked eye. The window may look perfectly installed, but in winter that area cools down, accumulates condensation, and over time black mould appears around the frame. A thermal bridge of just 5mm around the entire perimeter can increase heat loss by 15-25%.
The only reliable way to detect thermal bridges is with a thermal camera verification. Cold zones appear as blue or violet spots on the infrared image.
Mistake 2: Installing a premium window with a terrible joint
This is the most frustrating and most common mistake: spending €600-800 on a 6-chamber PVC window with the latest low-emissivity glass, then installing it with a cheap silicone bead that degrades in 3-5 years.
The perimeter joint (the gap between the window and the wall) is the weakest point of the entire envelope. If the window has a Uw value of 0.8 W/m²K but the joint lets air through, the real performance can drop to an equivalent of 1.5-2.0 W/m²K — worse than a mid-range window properly installed.
As we explain in our article on window installation prices and techniques, the solution is to use self-expanding tape or, at minimum, low-expansion foam designed for windows. The cost difference is just €3-5 per linear metre.
Mistake 3: Energy loss through the joint — real data
According to studies by the Passive House Institute, up to 35% of a home's energy losses occur through windows and their installation joints.
In a typical 80m² home with 8-10 windows, poor installation can mean:
- €150-300 extra per year in heating due to cold air infiltration.
- Condensation and mould in window corners, posing health risks.
- External noise filtering through poorly sealed joints, negating the glass's acoustic insulation.
Investment in professional installation with quality materials pays for itself in 2-3 years through heating savings alone. See our guide on window installation prices.
Mistake 4: Not properly sealing the subframe
The subframe is the intermediate element between the wall and the window. Its sealing is critical. Many installers fix the subframe with mechanical anchors and apply foam inside, but forget to seal the exterior face.
The result: wind-driven rain infiltrates the exterior face of the subframe, accumulates between subframe and wall, causing damp, efflorescence and wall deterioration.
The correct solution is to apply a triple seal: waterproof tape on the exterior (against rain), foam or tape in the core (thermal insulation), and vapour-permeable tape on the interior. Leading brands like Veka and Kömmerling include assembly instructions with this triple sealing system.
Mistake 5: Forgetting the vapour barrier
The vapour barrier is a concept many installers are unaware of. In a window joint, a fundamental principle must be met: more airtight on the inside, more permeable on the outside.
Why? Because warm interior air contains more moisture than cold exterior air. If the joint is equally airtight on both sides, moisture that penetrates the foam from inside cannot escape outward and condenses within the joint, degrading the foam and creating a breeding ground for mould.
The solution is to use vapour control tape (or vapour brake membrane) on the interior face of the joint. This material costs less than €2 per linear metre and makes the difference between a correct installation and one that will generate moisture problems in 2-3 years.
Mistake 6: Not levelling or plumbing the window
A window that is not perfectly levelled (horizontal) and plumbed (vertical) will have closing problems from day one. The sashes won't close properly, hardware will suffer premature wear, and airtightness will be compromised.
The symptoms of a poorly levelled window are obvious:
- The sash rubs against the frame when opening or closing.
- The hardware doesn't engage properly, forcing the handle.
- You can feel a draught from one corner even with the window closed.
Mistake 7: Reusing the old frame without checking it
In partial renovations, many installers place the new window on the old frame to save time and cost. This can work if the frame is in perfect condition, but it's a serious mistake if not verified first.
An old wooden frame may have woodworm, rot or deformations not visible from outside. An old aluminium frame may have corroded anchors or lost its joint seal integrity.
The professional recommendation: if the frame is over 15-20 years old, replace it. Check options and prices on our window calculator.
Bonus mistake: Not verifying with a thermal camera
Thermal camera verification (infrared thermography) should be the final step in every window installation.
A post-installation thermographic scan reveals in seconds whether there are thermal bridges, air infiltrations or unsealed areas. Its cost is minimal (€50-100 per home) compared to the cost of repairing a poor installation.
At Instal Tancaments we include thermographic verification in our premium installations. See more details in our article on professional installation techniques.
How to avoid these mistakes: verification checklist
Before hiring a window installer, make sure they meet these requirements:
- Uses low-expansion foam or self-expanding tape, never conventional construction foam.
- Applies triple sealing to the joint: interior airtightness, thermal insulation in the core, exterior rain impermeability.
- Levels and plumbs each window with a spirit level or laser before fixing.
- Checks the old frame before reusing it. If it's over 15 years old, replace it.
- Offers post-installation thermographic verification.
If you're looking for a professional installer, request a no-obligation quote or calculate your window price with our online calculator. You can also read our article on what differentiates a manufacturer, fabricator and installer.
Frequently asked questions about installation mistakes
Does poor installation void the window warranty?
Yes. Most manufacturers like Veka or Kömmerling require installation by an approved professional. If the window is poorly installed, the manufacturer may reject the claim.
How do I know if my window is poorly installed?
The most common symptoms are: draughts with the window closed, condensation or mould around the frame, the sash rubbing when opening/closing, or the handle not closing smoothly. A thermal camera check is the definitive test.
Can a poor installation be fixed without replacing the window?
It depends on the problem. If it's a sealing defect, the trim can be removed, tape or foam correctly applied and refinished. If the frame is out of level, it must be dismounted and reinstalled.
How much does a thermal camera check cost?
A thermographic check of a complete home costs between €50 and €100. It's a minimal investment that can save you thousands.
Is polyurethane foam or self-expanding tape better?
Self-expanding tape is superior in all aspects (airtightness, durability, vapour permeability), but costs more. For standard homes, low-expansion foam is sufficient. For Passive House, tape is mandatory.
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