Common mistakes when installing windows

Even premium windows costing €1,000/m² can become a source of problems if installed incorrectly. Drafts, sweat, cold, mold—and these aren’t defects of the window. They’re the result of installation errors, which are often unnoticeable at first, but become apparent six months to a year later.

In Spain, about 40% of windows are installed improperly, especially by cheap crews. Owners then blame the manufacturer, although the cause lies with the installer. Let’s talk honestly about what can happen and why you can rest assured with a professional team.

Problems start with measurements

When measuring “by eye”

Unprofessional measurers often measure the opening only at one point, in the center, without taking into account that walls are rarely perfectly straight. The result? The window arrives the wrong size. If it’s too large, it doesn’t fit, forcing you to cut the walls or rebuild the window. If the window is too small, it will “wander” and become impossible to securely fasten, creating huge gaps that even foam won’t fix.

Correct measuring technique:

• Measure each opening at three points: top, middle, and bottom—both width and height.
• Take the smallest measurement and subtract the correct installation gaps (15-25 mm on the sides and top, 30-40 mm on the bottom).
• Check the opening geometry with a level—how vertical and parallel are the walls?
• Take photographs of each opening for accuracy.

Insufficient opening preparation

If you install windows directly in a dirty, dusty, or damp opening, after a year or two, the foam will begin to peel off the walls—it simply doesn’t have the proper adhesion. Anchors rust faster in a dusty opening. And if the walls are uneven, the frame will be impossible to align.

How ​​to properly prepare the opening:

• Clean off dust, dirt, old paint, and peeling plaster
• Smooth out any significant unevenness
• Apply primer—this significantly improves the adhesion of the foam to the wall
• Let everything dry before installation

When the frame is attached incorrectly

“The foam holds everything” is the most dangerous mistake

Many cheap crews simply insert the frame into the opening, apply the foam, and that’s it—without a single anchor. “The foam holds,” they say. It really does seem fine for the first year. But after 1-3 years, the foam begins to deteriorate from sun, moisture, and temperature fluctuations. The frame begins to move in the opening, the sashes warp and stick, and gaps appear. The window’s lifespan drops from 30 years to 5-7.

Proper fastening:

The frame must be secured to the wall with anchor bolts. For a standard 120x140 cm window, there are at least 3 attachment points per side of the frame. For larger windows, there are 4-6 attachment points per side. The distance between attachment points is 40-70 cm. The foam acts as insulation, while the metal anchors hold the window in place.

When there are too few attachment points

Sometimes the frame appears to be secure, but there are only a few attachment points—for example, only 4 anchors for a large 150x200 cm window. The frame sags between the attachment points, becomes deformed, the sashes become warped, and the foam is damaged by the uneven load.

Mounting standards depend on the window size:

• Windows up to 1.5 m² — minimum 8-10 mounting points
• Windows 1.5-3 m² — minimum 12-16 mounting points
• Windows larger than 3 m² — 18-24 mounting points

When fixing to weak walls

In Spain, many houses are made of porous ceramic tiles or old brick. If you use regular anchors in such walls, they simply tear out. The frame becomes loose, and in strong winds, the window can even fall out.

How ​​to do it right:

For soft materials, use special dowels of increased diameter with spacer wings or chemical anchors. They hold securely even in porous materials.

When the frame is installed crooked

Even a slight deviation of the frame from the vertical or horizontal—just 2-3 mm—is a problem. The sash begins to open or close spontaneously. It sags after a year. Hinges and fittings wear unevenly. Seals fail faster.

Installation accuracy requirements:

• The frame must be strictly vertical (check with a plumb line or laser level)
• The frame horizontally must be level
• The frame diagonals must be equal
• The permissible deviation is no more than 1-1.5 mm per meter

When Anchors Are Overtightened

If the anchor bolts are overtightened, the frame profile bends inward. The sash won’t close or closes with difficulty. Cracks may appear in the profile.

Correct Technique:

Tighten the anchors until they stop, but without deforming the frame. After each tightening, check that the profile remains level.

This is part of our installers’ professional skills—they know the correct tightening torque for different window types.

Installation Joint: A Fine Line Between Comfort and Problems

When Foam Is Left Unprotected

This is the most common mistake made by cheap installation crews—they just apply the foam and that’s it. They say, “The foam will harden on its own, and nothing will happen.” They save 30-40 minutes and 5-10 euros in materials per window. They don’t warn the client that unprotected foam quickly deteriorates.

Sunlight turns the foam yellow and brittle within 1-2 years. Rain and condensation penetrate, causing the foam to become damp and lose 50-70% of its insulating properties. Wind erodes it, creating voids. After 2-5 years, the joints deteriorate, leading to the same problems: drafts, freezing, condensation leaks, and mold growth. Heat loss increases by 30-50%.

Our Approach:

We offer two options and explain the differences:

Option 1: Basic Installation

• We professionally install windows using foam and properly secure them with anchors.
• The foam remains exposed for subsequent finishing.
• Suitable if you have your own finishing specialist or are planning a renovation.
• Important: The installation joint must be sealed within 2-3 months to prevent the foam from deteriorating.

Option 2: Installation with Full Finishing (additional service)

• We install the windows.
• We seal the installation joint with three layers:
• Inner layer: vapor barrier tape (protects against moisture from the interior).
• Middle layer: foam (thermal insulation).
• Outer layer: waterproofing tape or polyurethane foam (protects against rain and sun from the exterior).
• We install the slopes: PVC, plaster, or drywall—your choice.
• The installation joint is fully protected and will last 20-30 years. Years

We don’t push a full finish. That’s your choice. But we do explain frankly that the mounting seam requires protection—this isn’t marketing hype, it’s a technological necessity for the longevity of the windows.

When cheap foam is used

Poor-quality mounting foam or foam with an expired shelf life is a disaster. Excessive expansion deforms the frame. Weak expansion leaves voids. Poor foam quickly crumbles and crumbles, doesn’t retain heat, and lasts 1-3 years instead of 15-20.

Requirements for mounting foam:

• Low expansion coefficient (no more than 30-40%)
• Low thermal conductivity
• Moisture resistance
• Current shelf life

We only work with professional foam from reputable manufacturers. We monitor expiration dates and store materials under the right conditions—temperature and humidity are important.

When there are voids in the foam

If the foam was applied unevenly, unfilled areas remain—these are direct cold bridges. These will cause drafts, localized freezing in winter, and condensation and mold will form. The frame will not hold up well.

How ​​to work with foam:

Apply it evenly around the entire perimeter of the frame, filling the entire gap without leaving any voids. Monitor the foam’s expansion process to ensure there are no voids or excess pressure.

When using silicone instead of specialized materials

Non-professionals often cover the foam with regular silicone sealant and think they’ve “sealed it.” Silicone is vapor-permeable—it doesn’t protect the foam from moisture. It degrades in sunlight. After a year or two, the silicone peels off, leaving the foam exposed. This creates the illusion of protection, but the foam still deteriorates.

The correct approach to sealing:

If the client chooses a basic installation, the foam remains exposed for subsequent finishing by their contractor. If a full finish is ordered, special vapor barrier tapes are used for the inner layer and waterproofing tapes or PSUL for the outer layer. Silicone is used only for the final frame-to-slope joint—this is purely aesthetic, not to protect the installation joint.

Window Sill: Little Things That Matter

When a Window Sill Is Incorrectly Installed

A window sill installed strictly horizontally, or worse, at an angle toward the window, is a guarantee of problems. Condensation from the window drips onto the sill and accumulates near the frame instead of draining. The mounting foam becomes wet, deteriorates, and mold forms on the frame.

Correct installation of a window sill:

The window sill should have a slight slope away from the window—1-2°, approximately 2-3 mm for a 25 cm window sill width. Water flows into the room, but away from the frame, not toward it.

With us, window sills are always installed with the correct slope—this is checked with a level.

When there’s a void under the window sill

A window sill resting on several support points, with large voids underneath, will sag under load, rattle when touched, and may crack over time. Voids under the window sill also create thermal bridges.

How ​​to do it right:

The space under the window sill is completely filled with foam over the entire surface. The window sill becomes monolithic, does not sag, and does not rattle.

When a window sill blocks heat

A window sill that is too wide (more than 30 cm), completely covering the radiator under the window, prevents warm air from rising to the window. The glass becomes colder, there is more condensation on it, and heating efficiency decreases.

A reasonable approach:

If the radiator is under the window, the window sill should extend no more than 5-7 cm above it. Alternatively, vents with grilles can be installed in the window sill to allow warm air to circulate.

Drip mould: quiet and airtight

When the drip mould joint is not sealed

The drip mould is screwed in, but the joint with the frame is open or sealed only at the top, leaving gaps on the sides. Rainwater seeps between the drip mould and the frame, dampening the mounting joint underneath, and can even leak into the room. Mold and rot can develop.

Drip-sill installation technology:

The joint between the drip-sill and the frame is sealed with silicone or polyurethane sealant around the entire perimeter—top and sides. The drip-sill should extend 10-15 mm under the frame, ensuring a completely airtight joint.

When drip-sills rattle

A metal drip-sill, screwed directly to the frame without a backing, rattles loudly in rain and makes an annoying noise in windy weather.

How ​​to avoid noise:

A vibration-damping tape—either bitoplast or special damping tape for drip-sills—is applied under the drip-sill. It dampens vibrations from rain, hail, and wind. Drip-sills operate quietly.

Naturally, with us, you won’t encounter rattling drip-sills—the vibration-damping tape is included as standard.

When water stands on the ebb tide

If the ebb tide has no slope or only a minimal outward slope, water sits on it in puddles and doesn’t drain. In winter, it freezes, and ice can deform the ebb tide. Rain makes more noise.

Correct installation:

The ebb tide is installed with an outward slope of at least 5-10° to ensure rapid water drainage.

Hardware: Smooth operation from day one

When the sashes are not adjusted

The sashes are hung but not adjusted—the pressure is uneven, the sash touches the frame, and the hardware is difficult to operate. Uneven pressure causes drafts, the sash wears out the seal, and the hardware quickly fails.

What does proper adjustment include?

• Sash pressure—uniform along the entire perimeter
• Geometry—the sash should not touch the frame anywhere
• Smooth opening and closing
• All hardware modes (pivot, tilt, micro-ventilation)

When the hardware isn’t lubricated

Hardware that runs dry from day one wears out 2-3 times faster. After 6-12 months, creaking and jamming begin.

How ​​it should be:

Before hanging the sashes (or immediately after), lubricate all moving parts of the hardware with a special lubricant. The hardware operates smoothly and quietly from day one and lasts longer.

Slopes: Beauty with Insulation

When Slopes Are Cold

Slopes made of plasterboard or PVC, installed without insulation behind them, are called cold slopes. In winter, condensation forms on them, mold grows in the corners, and thermal bridges and additional heat loss occur.

Correct technique (if full finishing is ordered):

The space between the wall of the opening and the slope is filled with insulation—mineral wool or foam. The slopes are warm, there is no condensation, and there is no mold.

If you ordered full finishing from us, the slopes will be installed with insulation—this is standard.

When the joint between the slope and the frame is not sealed

Gaps between the slope and the frame allow drafts in, dust, and are unsightly.

How ​​to do it:

The joint is carefully filled around the entire perimeter with acrylic or silicone sealant, without any gaps. The result is a smooth and airtight seal.

When ordering a full finish, all joints are sealed thoroughly—this is a matter of not only functionality but also aesthetics.

How to recognize professionals: a few simple signs

When taking measurements, we:

• Measure each opening at three points
• Check the geometry with a level
• Take photographs of the openings
• Discuss finishing options—basic or full
• Provide a detailed estimate with a breakdown, not just a single figure

During installation, our teams:

• Prime the openings before installation
• Secure the frame with anchors (6-10+ points for a standard window)
• Check the level and plumb line multiple times during installation
• If a full finishing is ordered, use a three-layer seam protector
• Apply foam evenly, without any voids
• Install a sloped window sill
• Apply vibration-damping tape under the drip edge
• Adjust each sash
• Clean up completely after use

Upon acceptance:

• Inspect each window with you
• Explain how to properly use and care for the windows Behind them
• We offer a 3-5-year installation warranty (not just six months!)
• We provide a full package of documents

Why you can rest assured with us

We don’t try to be the cheapest. We honestly do our job the way it should be done. We explain options, don’t push unnecessary things, but we also don’t hide the important stuff. We use the right materials, adhere to the technology, and control quality at every stage.

We have different crews, but they all follow our installation standards. These aren’t just recommendations—they’re mandatory requirements that we verify.

Windows last 30-40 years. Don’t skimp on the installer. The price difference between professional installation and DIY work is 10-15%. But this percentage means your peace of mind, comfort, and problem-free installation for decades to come.

The first winter after installation is the ultimate test. If there are no drafts, no freezing, no condensation on the slopes, and no mold in the corners, the installation is high-quality. With our windows, you will live through your first winter (and all the following ones) in warmth and comfort.