How to choose the best glass for doors and windows while saving money?

Until now, there was no text that in a simple and concise way explains which glass to choose for doors and windows. Here you will get a direct answer and explanation, written to be understandable to everyone. This text could save you quite a bit of money, but also remove the misconceptions that 95% of people have.

Glass makes up the largest surface of the window (opening). As important as it is for wood parts to have a good seal, it is also important that glass does not transfer heat in the winter and the cold in the summer.

Glass performance

figure 1 – Ug and g factors

This is why there are two factors that define these values:

Ug factor represents the loss of heat (energy) from a room. If there is a room that is heated in winter, this factor will show how much heat is lost through the glass.

       Technically: Ug – heat transfer coefficient, defined by the EN-673 standard, with the unit of W/m2K. The lower the U-value, the higher the heat insulation property.

G factor is the transfer of heat into the room. Contrary to the first case, when there is a cooled room in summer, this factor will show how much heat enters the room through the glass.

       Technically: g – total transfer of solar energy through the glass. Defined by the EN-410 standard. The lower the g-value, the lower the heating of the room.

Let's take it one step at a time.

Single layered glass.

In the summer when temperatures outside reach 35-40°C, glass that is heated transfers the stored heat into the room. If there is one layer of glass (3mm thick for example) it will transfer 100% of the stored heat. We emphasize the stored heat and we say that if the outside temperature is, for example, 40°C, the  inside temperature won't reach 40°C, but only the temperature that the glass itself reaches (figure 2), because the glass emits the stored heat (100% of it in case of single layer glass). It is like having a heater the size of the glass in the room. This is the case of heat transfer from outside to inside (g factor)

 Double layer or threelayer glass

Figure 2 – Glass temperature at 0°C outside and 20°C inside temperatures

When dealing with energy transfer from inside to outside (Ug factor), in the case of single layer glass, it is around 83%. That is, 83% of the cold (in summer) is lost.

Now we get to what actually the Ug coefficient is. This type of glass is said to have a Ug factor of 5.3 to 5.8. This is how we list glass energy transfer coefficients.

This type of glass is present on old houses. Considering that these energy losses are too high, double glass windows were made (two sets of windows, one behind the other) and these are the windows we still have on many houses and buildings (figure 3). Then the next step was taken and that was...

old version of double windows

Figure 3 – old version of double windows

Double layer glass

Glass that is actually two layers of glass with a spacer of, for example, 16mm between them (4+16+4).

two layers of glass

figure 4 – two layers of glass, 4mm thick, separated by a spacer of 16mm

First and last values of “4” stand for glass thickness of glass and the middle value of “16” marks the thickness of the spacer between them. The spacer can be 6-24mm, but the best performing distance is 12-16mm. Just as 6, 8 and 9mm spacers are not good, neither are those of 22 or 24mm (as we would say, they are “equally worse”).

Now that we understand the symbols, it's the right time to provide a table that we can follow.

Ug and g factor values

Table 1 – Ug and g factor values, with visible transmittance

We can see that, after adding another layer of glass, the Ug value of 5.3 has gone down to 2.8. That is, instead of 100% heat leaving the room in winter, now we lose 56% and transfer 77% of heat inside in summer. Therefore, we have a serious improvement by just adding another layer of glass.

The following conclusion seems justified: Then we can add another layer of glass (triple glass) and get even better results! However, this does not work.

There are some optimal values regarding characteristics of heat/cold, sunlight etc., and we will see that from this point forward, adding another layer of glass to already double layer glass, is merely a marketing scheme, paid dearly by the customer.

Apart from the third layer of glass, Argon, heavily advertised, enters the mix at this point. We agree that there are significant advantages, but they are not nearly as extreme, nor as accurate, as mentioned. The table shows that regular double layer glass with a Ug value of 2.8, when combined with Argon, gets a Ug value of 2.6. This minor improvement of only 0.2 is not proportionate to its very high price. This improvement can be considered significant in only some cases, which we will discuss later.

Of course, this refers to double layer glass, because it has many different iterations. For example, if you visit a glass store:

You: Good morning.

Glass salesman: Good morning.

You: I would like insulated glass, low-emission and filled with Argon.

Glass salesman: You got it.

The salesman didn't lie to you. You get low-emission, insulated glass, filled with Argon, but they didn't tell you it comes with a hard coat. This glass has a 1.8 Ug, or when filled with Argon, 1.6. Can you see the Ug values for soft coat low-emission (4+16+4 LOW-E SOFT COAT). This value, even without Argon, is 1.3, which is better than hard coat with Argon, and the price is lower. If we further add Argon to this glass, we get a Ug value of 1.1. This is the situation where Argon is a significant advantage. The Ug value of 1.1 is the value specified by most laws in western Europe that deal with this area, where glass is taken to the limits of allowed values, and as such, can be used in construction. Therefore, Argon is not used primarily because of insulation, as most people think, but because it is an inert gas, not as aggressive as air. The coat that is placed on the inside of the glass (between two layers) is based on Aluminium-oxide and if this space is not filled with Argon, this coat gets damaged in contact with air and, in time, disappears, which causes its insulating properties to vanish as well.

There are proponents of the argument that Argon leaks out of the glass over time, that is disappears and that it is useless to pay for it anyway. This is true, but only to an extent. The loss of Argon depends on the glass manufacturer, which is why we should choose companies that have certificates that the loss of Argon in their products is no more than 3% per 10 years. That means that in 10 years we will not lose more than 3% of the Argon, which is an insignificant loss, and in realistic situations causes Argon to last longer than the glass itself.

There are also examples where glass is combined with Krypton, done to achieve better insulation with thinner glass, but we will not focus on those here.

We will pay attention to this glass combination. 4+16+4 LOW-E, SOFT COAT. It's Ug value is 1.1. That means that in winter, we lose 22% of heat, instead of 56% but in summer, we receive 63% of heat from outside. A serious improvement compared to regular insulated glass but a small advancement regarding the g factor.

Local laws are mostly focused on Ug values, that refer to heat loss in winter, but people also desire advantages during summer, to prevent heat from entering inside. Therefore, SOLAR 4+16+4 FLOT, where the characteristics are identical to 4+16+4 LOW-E SOFT COAT, but the transfer of heat from outside has been decreased to 42%. This is the final limit when it comes to transparent glass. All other reductions and improvements with Ug and g factors influence the transparency of glass (also known as unpainted glass), while keeping a double layer of glass.

Triple layer glass

We are slowly getting closer to a conclusion, and it will not justify the statement of the rich neighbour who says “I put the most expensive triple layer glass windows on my home. It's the best there is.”

triple layer glass window

Figure 5 – triple layer glass window sample

Let's go back to table 1 and look at the values of triple layer glass. These are Ug 1.8 and g 1.7, and double layer glass allows us to get to 1.3 and 1.1 with significantly lower costs, because this type of glass is cheaper than triple layer glass.

What do we get with triple layer glass? We make the window/door heavier and raise the price... resulting in a lower quality product.

Wait, can't we also add a soft coat, Argon etc.?

Yes. If we take 4+16 Argon+4+16+4 LOW-E, we get a Ug factor of 0.8 and g factor also around 60% (nothing spectacular).

The maximum improvement with triple layer glass is the combination SOLAR 4+16Argon+4+16+4 LOW-E. Then we get a Ug of 0.6, and a percentage of heat transfer from outside is under 40%.

We will go straight to...

The conclusion:

If you have read the entire text so far carefully and followed the table of factors, the conclusion regarding the entire story about glass has already formed inside your mind.

the best selection for glass

Double layer, low-emission, soft coat, Argon filled glass is a combination that is better, and significantly cheaper, than triple layer glass. An added benefit is a lighter window that lengthens the life of the window.

This initially arouses suspicion in people, and whoever is not knowledgable about glass production technologies (and who among us knows everything), can easily become a victim of sweet talkers whose main task is to empty the warehouse of their store.

Yes, but there are certain combinations of triple layer glass that offer better characteristics than 4+16arg+4 LOW-E SOFT COAT!?

Yes, there are. However, their price is extremely disproportionate to the improvement they offer and that is beyond discussion. We would say that the price of glass (per m2) grows exponentially relative to the increase in quality.


Go with 4+16arg+4 LOW-E SOFT COAT!



Perhaps you will be surprised by the following:

You won't be able to test yourselves the glass that you buy. There is a detector that can determine if you have regular double layer glass or premium double layer glass with a coat, but it will cost you a few hundred/thousand euros. There is a laboratory method that can be carried out, but who among us will take their window to a lab, pay for the testing, and then install windows?

We will protect ourselves by choosing reliable and well known companies, because losing trust from clients is simply not worth it for such companies. Austrian building inspections ordering the removal of their windows or being forced to change their production and installation systems are nightmare scenarios for these companies.

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