You've spent weeks in showrooms. You compared thermal transmittance values (Uw), chose the top energy-rated windows on the market and feel confident that next winter will find you warm. But there's a hard truth: The best window in the world performs like a cheap 1990s unit if the installation is done wrong.
Installation accounts for 50% of success. Unfortunately, due to haste, ignorance or cost-cutting, many crews in Greece repeat specific construction errors that void your investment. If you're planning a renovation or new build, these are the 5 mistakes you must absolutely forbid your installers from making!
This is perhaps the #1 construction "crime" in Greece. The marble installer lays a single piece of marble that starts on the balcony and continues straight into the living room. The window fitter then screws the brand-new thermally broken window right on top of it.
Marble has a thermal conductivity of 2.5-3.5 W/(m·K) - it is an excellent heat conductor. The cold from outside travels through the stone, passes underneath the window frame and reaches your living room. The floor freezes, indoor humidity condenses on the inner marble surface, and your hardwood flooring begins to rot. Even worse, this thermal bridge completely nullifies the aluminium's thermal break - like running an uninsulated electrical cable.
The marble must be cut in the middle (under the window) and the gap filled with insulating material - extruded polystyrene (XPS) or Purenit - to break the thermal bridge. The indoor and outdoor marble sections then operate independently, without "talking" to each other thermally.
You watch the installer place the window in the opening, fill the gap all around with yellow PU foam and consider the job done, perhaps driving just one or two screws "for good measure". This is the second most serious mistake after the continuous marble.
Foam is an insulating material, NOT a structural support. A modern window with triple glazing weighs over 100-150 kg. Under this enormous weight (and with summer's thermal expansion-contraction cycles), the foam gradually compresses. Within months, the window "sags", drags on the floor and won't lock properly - a scenario we describe in detail in our next article.
The window rests on specialised, hard plastic load-bearing shims at the bottom corners, is laser-levelled in 3 axes and anchored around the perimeter with concrete Turbo screws every 50-70 cm. Elastic closed-cell foam is applied only at the end, solely for thermal insulation of the joint - never for support. This method ensures even a 200 kg sliding door won't shift by a single millimetre.
The window is installed and the fitter grabs the silicone gun, squeezing a thick bead inside and outside "to seal it". Many homeowners even consider this a sign of "good work" - "Look, he sealed everything!" Unfortunately, this is one of the worst practices.
Standard silicone traps moisture. Warm, humid indoor air from bathrooms and kitchens finds invisible gaps, enters the window-to-wall joint and gets trapped behind the external silicone. Moisture condenses at the dew point (~12.6 °C), resulting in severe black mould around the frame. Mould isn't just cosmetic - it releases spores that cause allergies and respiratory problems.
Demand installation to the RAL standard - three separate sealing levels. Interior: airtightness membranes (EPDM or Fleece) that block vapour. Middle: elastic closed-cell foam for thermal insulation. Exterior: self-expanding Compriband tapes that keep rain out but allow the wall to "breathe" - the exact opposite of silicone.
The bricklayer, trying to avoid finishing work, leaves an opening exactly the size of the window. The aluminium goes in "wedge-tight", almost touching the brickwork on all sides. It looks "perfect" - but physics disagrees.
Materials "live and breathe". In summer, especially if dark-coloured, aluminium expands by 2-3 mm on large frames. With no room to expand, it pushes against the wall, warps the frame, cracks the plaster and the sashes jam. At the same time, with no gap there's no space for any insulation material - no foam, no Compriband, no membranes. The joint is left completely open to air and rain.
A construction joint (air gap) of 10-20 mm must always be left on every side. The exact dimension depends on the window size, profile colour (dark = greater expansion) and the sealing method. The gap is filled with elastic insulating materials that absorb movement without cracking or splitting - flex foam, Compriband and membranes.
Before the aluminium goes in, the contractor "plants" a square steel frame (galvanised sheet metal) in the wall to level the plaster and screw the window onto. The problem? Sheet metal has a thermal conductivity of 50 W/(m·K) - 1,400 times higher than an insulating material. You are screwing a super-insulating window onto a freezing piece of steel! The sheet metal voids the aluminium's thermal break, transfers cold indoors and over the years rusts, leaving brown stains on your plaster.
For new builds, demand insulating subframes - made of Purenit (recycled high-density polyurethane, λ = 0.06-0.08 W/(m·K), compressive strength >30 N/mm²) or high-density EPS. These provide top structural support without conducting cold. During renovations, if the sheet metal cannot be removed, it can be externally "wrapped" with insulation to reduce the thermal bridge.
Proper installation isn't just about "good hands" - it's about know-how and modern materials. A perfectly installed mid-range window outperforms a premium window with shoddy installation. Next time you receive a quote, don't just look at the brand - ask the installer exactly how they plan to fit the windows. Demand RAL-standard installation, insulating subframes, proper levelling with Turbo screws, and elastic sealing materials.
Before paying the installer, check: (1) Is the marble sill cut in two? (2) Are there load-bearing shims and Turbo screws every 50-70 cm? (3) Does the joint have three sealing levels (membrane inside, foam middle, Compriband outside)? (4) Is there a 10-20 mm gap all around? (5) Is the subframe insulating? A serious professional will be happy to explain their work!
This article also has a video version with the same topic in a more direct walkthrough format.
Short-form video covering five common installation mistakes that hurt airtightness, insulation and durability.
TikTok version with the most common installation mistakes seen on renovation jobs.
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