A good window is not necessarily the one with the thickest profile, the most chambers or the lowest value displayed in an advert. Comfort and energy use depend on the whole system: the joinery, glazing unit, spacer between the panes, hardware, façade orientation and, just as importantly, how the window is installed in the wall.
In short: ask for the Uw value for the complete window, not only the glass Ug value; choose solar control suited to each façade; and design the installation together with the thermal insulation. A high-performance window installed in an opening treated superficially can lose much of its advantage.
The first criterion: Uw, not just Ug
Window quotations often include several similarly presented values with different meanings. Confusing them can lead to an incorrect comparison between two products.
| Indicator | What it measures | What to remember |
|---|---|---|
| Uw | The thermal transmittance of the complete window: frame, glazing and the edge of the glazing unit. | This is the main figure for comparing windows of the same size. |
| Ug | The thermal transmittance of the glazing unit. | On its own, it does not say how well the installed window will insulate. |
| Uf | The thermal transmittance of the frame profile. | It becomes particularly important for windows with a large proportion of frame or with small panes. |
| Ψg | The influence of the edge of the glazing unit, including the spacer. | A low-thermal-bridge spacer can improve comfort at the edge of the glass. |
For these indicators, a lower value means lower heat transfer. This is why a window with a very good Ug value is not automatically one with an excellent Uw value: the frame, the proportion of frame to glass, sash size and spacer all affect the final result.
When requesting a quotation, explicitly ask for the Uw value for the specific window size in the project, with the exact glazing and profile configuration. A value declared for a large window with a generous proportion of glass does not automatically apply to a small bathroom window, where the frame carries more weight in the calculation.
Triple or double glazing? The answer depends on the project, not fashion
A triple-glazed window may have lower thermal transmittance than a double-glazed one, especially when it includes low-e coatings and inert gas between the panes. However, “triple glazing” is not in itself a sufficient guarantee. Triple-glazed units come with varying levels of performance, and an unsuitable choice can mean additional cost, less natural light or a greater risk of overheating in certain rooms.
When triple glazing is usually a justified choice
- You are building a new home with a good thermal envelope and are aiming for low heating and cooling consumption.
- You are carrying out a deep renovation and replacing the windows, thermal insulation and airtightness systems at the same time.
- You have large windows or glazed areas exposed to wind, cold or outside noise.
- You want more even indoor temperatures near windows, not just lower bills.
When it requires closer analysis
- The window is small, seldom used or located in a wall that will remain poorly insulated.
- The budget does not allow for the installation, sills and insulation around the opening to be properly addressed.
- The façade has large west- or south-west-facing glazed areas without external solar protection.
- The building has moisture or ventilation problems that will not be resolved when the windows are replaced.
In a renovation with a limited budget, it is more rational to choose a balanced product and properly fund installation, solar protection and the correction of thermal bridges than to pay solely for a superior glazing unit that is installed carelessly.
Solar factor: the figure that matters in summer
Thermal insulation matters in winter, but windows must also manage solar radiation. The technical data sheet may list the solar factor, usually denoted by g or SHGC. It shows what proportion of solar energy enters indoors as heat. A lower value limits solar gain; a higher one allows more heat from the sun to enter.
There is no universally “perfect” value. In Romania, the choice depends on orientation, glazed area, local climate, existing shade and how the home is used.
- North: the priority is usually to limit heat loss; direct winter sunlight is limited.
- South: winter sun can be beneficial, but the façade needs external shading designed for summer, such as overhangs, awnings or exterior blinds.
- East: morning light is pleasant, but bedrooms may need effective light-control solutions.
- West and south-west: these are challenging exposures in summer because the low afternoon sun brings heat and visual discomfort. Here, exterior shading is often more effective than an interior curtain.
A common mistake is to order the same glass for all façades without the designer checking orientation. In a home with extensive glazing, the glazing unit can differ by façade, but the decision should be supported by calculations and the actual shading scenario.
Exterior solar protection is part of the window
Blinds, roller shutters, shutters or pergolas are not merely decorative accessories. They can reduce solar gain before radiation passes through the glass and turns into heat in the room. A heavy curtain fitted indoors helps visual comfort, but it does not stop as effectively the energy that has already passed through the glass.
For large west-facing windows, plan solar protection before choosing the window system. Check whether there is room for roller-shutter boxes, guides, electrical supply or automation, and whether the system will unnecessarily interrupt the insulation layer. This detail must be coordinated with the façade, not decided after the finishes are in place.
In a home with automation, light or temperature sensors can operate shading according to a schedule. However, automation does not replace a good physical solution: a poorly sized shading system does not become effective simply because it is connected to the internet. For ideas on integrating smart functions without unnecessary investment, the guide to turning an apartment into a smart home is also useful.
Frame material: PVC, wood, aluminium or combinations
Material should not be chosen on reputation alone. A good window can be made of PVC, wood, aluminium with a thermal break or mixed systems, provided that the complete product has documented performance and suits the project.
PVC
PVC is common because of its balance of price, maintenance and performance. The number of “chambers” claimed in advertising matters less than the performance of the complete window, the reinforcement required for the specified dimensions, the quality of the seals and the execution of the corners.
Wood
Wood can offer a warm appearance and good performance, but it requires maintenance suited to its exposure to sun and rain. Check the finish, water-drainage details and maintenance obligations in the warranty.
Aluminium with a thermal break
Aluminium is suited to large openings, slim profiles and contemporary architecture. It should not be dismissed simply because metal conducts heat: systems intended for efficient windows use thermal breaks. However, for the same performance, the cost may be higher and should be assessed in the context of the glazing dimensions.
Mixed systems
Wood-aluminium combinations or other composite solutions can separate functions: better protection outside and a warm finish inside. They are worth considering when the budget, aesthetics and operating conditions justify the investment.
Installation: where the investment is won or lost
A window should not be treated as an item fixed into masonry with foam. The joint between frame and wall must provide three functions: mechanical fixing, continuity of airtightness and controlled drainage of water that may accidentally reach the opening area.
In a home insulated externally, the window position must be coordinated with the thickness and material of the thermal insulation. If the window remains set far back from the insulation layer, the perimeter of the opening can become a cold zone. This is not always immediately visible, but it can mean discomfort, local condensation and a risk of mould where indoor humidity is high.
There is no single universal installation detail. Masonry, lightweight construction, insulation thickness, façade type, exposure to rain and window size all require compatible solutions. What matters is that the designer and installer can show how the window connects to the wall’s protective layers, rather than simply promise a “warm installation”.
What should be included in the technical discussion before ordering
- Position of the frame in the wall: determined in a section detail, not improvised during installation.
- Mechanical fixing: the type, number and position of fixings must be suited to the dimensions, weight and substrate.
- Perimeter joint: filling materials should not be confused with the complete sealing system.
- Interior sealing: it must limit the entry of humid indoor air into the joint.
- Exterior protection: it must manage water and wind without blocking the necessary drainage.
- Sills and thresholds: they must direct water outdoors and avoid preventable thermal bridges.
These decisions are connected to the rest of the building envelope. If you plan under-slab insulation, the façade and window systems at different stages, it is worth checking the continuity of thermal insulation from the outset; the guide to insulation, waterproofing and essential details of an efficient home explains why isolated works can create costly problems later.
Airtightness also requires controlled ventilation
New, correctly adjusted windows reduce uncontrolled air infiltration. This is an energy and acoustic benefit, but it should not be confused with ventilation. Opening windows at random may be sufficient in some homes, but it becomes harder to manage in a very airtight house with many occupants, bathrooms without windows, frequent cooking or clothes dried indoors.
To complement the information above, you can also read Electrical panel for a house or apartment: how to do it correctly, what must not be missing and mistakes to avoid, an article that develops an aspect directly related to this topic.
Condensation on glass does not automatically prove that the window is defective. It may indicate a combination of low temperature at the interior surface, excessively high indoor humidity and poor air circulation around the window. Before blaming the window system, check humidity with a simple hygrometer, avoid blocking radiators or heat sources, and assess your ventilation routine.
For new projects or major renovations, mechanical ventilation should be discussed during the design phase. A well-designed system can control humidity and air quality without permanently cancelling the benefit of an airtight envelope. Guidance on choosing a system is set out in the article on heat-recovery ventilation in a new home.
Documents worth requesting from the supplier
Before paying a deposit, request documents for the proposed product, not merely a general system brochure. For construction products covered by harmonised standards or a European Technical Assessment, the declaration of performance and CE marking help identify the characteristics declared by the manufacturer. CE marking is not a guarantee that the product is automatically suitable for every project; it must be read alongside the declared values and the building’s requirements.
- the quotation, including dimensions, opening type and opening direction for each item;
- the Uw value of the complete window for the quoted configuration;
- the glazing-unit specification: number of panes, coatings, gas, spacer and safety glass where required;
- solar factor and light transmittance, especially for large glazed areas;
- air-permeability class, watertightness and resistance to wind load, where declared;
- the installation detail and clear responsibility for preparing openings, fixing, tapes, sills and adjustment;
- warranty terms and the service procedure.
Handover checklist after installation
Do not wait until the first cold season to inspect the work. At handover, open and close every sash, including tilt-and-turn sashes; check that joints and seals are even; make sure the hardware operates without rubbing and that all accessories are present.
- The windows are plumb, level and in the position specified in the project.
- The frames are not distorted, scratched or stressed by finishes.
- Fixings are properly concealed, and caps and drainage accessories are fitted.
- Perimeter sealing is continuous and compatible with the planned finishes.
- Exterior sills slope outwards and do not obstruct water drainage.
- The hardware is adjusted, and the sashes do not touch the frame or threshold.
- The glass, profile and roller shutters, where present, are recorded in the handover report, along with any observations.
Conclusion: buy system performance, not a simplified label
Windows should be chosen as part of the home’s building envelope. The most useful comparison starts with the Uw of the complete window, continues with a solar factor suited to the orientation and ends with a verifiable installation detail. Frame material and the number of glass panes matter, but they cannot compensate for a poorly prepared opening, a thermal bridge around the perimeter or lack of solar protection on an exposed façade.
For a new home, include window systems, shading and ventilation in the same set of plans. For a renovation, first assess the wall, moisture and the way the home is used. This way, the investment in windows will bring not only a new appearance, but also genuine comfort in every season.
Frequently asked questions
What is the difference between Uw and Ug for windows?
Ug measures heat transfer through the glazing unit, while Uw measures the performance of the complete window, including the frame and the edge of the glazing. For comparing two windows, Uw is the more relevant indicator.
Is triple glazing mandatory for an efficient home?
There is no universal rule. Triple glazing is often justified for new homes and deep renovations, but the choice must be assessed against the building envelope, orientation, glazed area, budget and installation quality.
Why does condensation appear on the inside of a new window?
Condensation can occur when indoor humidity is high, the glass surface is cold or air does not circulate well around the window. A new, airtight window reduces infiltration, making home ventilation even more important.
What does the g solar factor of glass mean?
The solar factor shows the proportion of solar energy that enters through the window and contributes to heating the room. A lower value better limits overheating, especially on west- and south-west-facing façades.
Is polyurethane foam sufficient for window installation?
No. Foam can fill and insulate the joint, but durable installation also requires mechanical fixing, continuity of airtightness and a solution that manages water on the exterior.
