The relatively simplest and most effective way to judge the thermal insulation performance of a door and window product (structure) is to look at the K value of the door and window (that is, the heat transfer coefficient, the lower the K value, the better the thermal insulation performance) in the performance test report.
The value of the K value of the heat transfer coefficient of doors and windows is obtained by experimental testing or formula calculation. The calculation formula has long been determined in the relevant specifications. Therefore, the heat transfer coefficient of profiles has an impact on the heat insulation performance of doors and windows.
Where does it affect?
Friends who have a strong desire to explore can read this chapter, or choose to skip it, without affecting the following reading:
The heat and energy (that is, thermal insulation) of doors and windows are lost and consumed through three ways: heat convection, heat radiation, and heat conduction:
Heat convection: Gas flow causes heat exchange, such as door and window frames, gaps between fans, and hollow layers of insulating glass. It is mainly solved by improving the sealing of doors and windows (followed by controlling the thickness of the hollow layer, using inert gas, etc.)
Heat radiation: Heat is transferred from one object to another in the form of waves, mainly in the glass with the largest area of doors and windows. It is mainly solved by using three-glass two-cavity, Low-e glass and other lifting glass configurations.
Heat conduction: The heat inside the object will transfer from the high temperature side to the low temperature side, and heat conduction will occur in profiles and glass. By reducing the heat transfer coefficient of profiles and glass, the heat in the indoor and outdoor spaces can be transferred less, thereby improving the overall thermal insulation effect of doors and windows.
Reducing the heat transfer coefficient of the profile can reduce the energy consumption in the secondary heat transfer and temperature difference heat transfer of the door and window frames.
Therefore, in this regard, many researchers in the industry are studying the optimization of the profile end, thereby improving the thermal insulation effect of doors and windows.
Knowing that the heat transfer coefficient of the profile is one of the reasons that affect the thermal insulation performance of doors and windows, we can discuss the extent of this "one of the reasons" and the differences between different profiles:
The proportion of profiles that affect the thermal insulation performance of doors and windows;
The difference in K value of doors and windows brought about by different profile materials and structures (plastic steel vs broken bridge aluminum);
Originally, in order to explore the above two questions, the rigorous Baydee planned to talk about one formula at a time.
But after studying it carefully, I found that not only are the formulas of Sigma, Cousein, and Delta difficult to understand, but also consider the energy ratio of heat convection, heat radiation, and heat conduction, and the different nodes of the window frame K value.
Although the conclusion drawn in this way is rigorous and in line with Baydee's "personality", it is too convoluted and cumbersome to talk about in this way.
So Baydee decided to use the simple method stolen from Dr. "Wan Chenglong" - simplified calculation method and database comparison method to do a brief analysis of two problems:
(1) Proportion of the impact of profiles on the thermal insulation performance of doors and windows
According to the formula for calculating the K value of the heat transfer coefficient of doors and windows mentioned above, it is known that the area of the door and window frame accounts for about 25% - 35% of the area of the whole door and window, and then refer to "Thermal performance of doors, windows and shutters. Calculation of heat transfer coefficient" The appendix of ISO 10077-1:2017 gives a quick calculation table for the K value of doors and windows (▽ extract 30% frame-window ratio).
We can conclude that the impact of profiles on the thermal insulation performance of doors and windows accounts for about 20% - 30%.
(2) Differences in K value of doors and windows caused by different profile materials and structures
Based on the difference in thermal conductivity between different profiles, the phenomenon of heat transfer in door and window frames loses energy.
A database based on energy-saving performance labels of Chinese building doors and windows
The energy-saving database of plastic door and window profiles in China and the door and window evaluation data summarized by Dr. "Wan Chenglong" for many years.
It can be roughly judged that, assuming that except for the window frame, (profile material, structure) are all invariant factors, plastic steel windows are indeed more heat-insulating than similar broken bridge aluminum doors and windows.
And affect the overall K value of doors and windows in the approximate range of 0.2 - 0.6.
Then, the final test question comes, whether the above content can be equated with "plastic steel windows are more worth buying than aluminum doors and windows with broken bridges".
The answer is: no.
Reason 1: In many cases, the quality of plastic-steel windows is not as good as that of broken bridge aluminum.
It is true that the PVC material of plastic-steel doors and windows has low thermal conductivity, and the multi-cavity structure is more suitable for use in green and energy-saving building doors and windows (such as passive houses, etc.). However, there are huge differences in the quality of domestic plastic-steel doors and windows.
I took Baydee to write this article, read the door and window product introductions of Taobao and JD.com for two days, and exchanged experience with my peers in recent years. Among domestic home improvement doors and windows, the quality of low-end plastic steel windows has always been worrying, and their lifespan and safety are not as good as Broken bridge aluminum doors and windows at about the same price;
Based on what Baydee saw and understood on the Internet, there are now more door and window manufacturers making broken bridge aluminum, and more broken bridge aluminum that take the brand line and export door and window products as a system. Manufacturers of plastic steel doors and windows focus more on profile brands.
Reason 2: Doors and windows are a whole, and the purchase should follow the principle of pertinence and cost performance
Baydee’s above analysis is aimed at the profile of doors and windows. As far as the thermal insulation performance of doors and windows is concerned, the influencing factors include glass configuration, product sealing, structure, etc. (more critical than profiles); as far as the final use of the product is concerned, the influencing factors In addition to the above mentioned, there are also product supply chains, manufacturers' workmanship, dealers' installation capabilities, and so on.
To give a specific example, if the plastic steel and broken bridge aluminum windows purchased by consumers are both in the middle and low-end price range, performance considerations are not so important. They should pay more attention to product rubber strips, tops, hardware, Whether the quality of these glass supply chains is guaranteed, etc.
To give an example of cost-effectiveness, the average price of the plastic-steel window below is more than 2,000, and the K value can reach about 1.8 when equipped with insulating glass. At the same price, mid-to-high-end broken bridge system windows are configured with three glasses and two cavities plus Low-e, the K value can also be achieved, and the sound insulation effect may be better.
This is the end of the article, for fear that everyone will say that Baydee "says it means not saying it". Baydee left another point of view on the selection of profiles at the end: under what circumstances are more inclined to plastic steel doors and windows.
Baydee believes that when "you" are a geek and want to decorate your home according to the standards of the "100-year project" and hope to achieve the ultimate in every detail, then you can consider the best-performing plastic-steel doors and windows (relatively).
There are quite a lot of people in this group, just as the concept of "comfortable home" is popular in home improvement now. Through the cooperation of floor heating, fresh air, central air conditioning, doors and windows, more suitable products and more scientific installation, the home is more livable. Energy consumption is more reasonable.
Doors and windows are a closed loop of this series of measures, and the application of lower K value or even reaching the standard doors and windows of passive houses can further promote the realization of this concept.