Lukewarm edge? Does not exist!

February 5th, 2016. Just to clarify again: “warm edge” refers to insulating glass edge bond with a thermally improved spacer. There is a clear definition of “thermally improved”. It can be found in the relevant international standard for the thermal transmission coefficient of windows, ISO 10077 part 1, as well as in the standard ISO 12631 for the thermal performance of curtain walling. Both standards contain each a chart with linear thermal transmission coefficients Ψ (Psi-values) for spacers made of aluminium and steel and a second chard with lower values for spacers with thermally improved performance. In cases of coubt, the definition helps to decide which chart is the right to use.

According to this definition, spacers made from steel are definitely NOT thermally improved. “Warm edge” does begin with stainless steel spacers. Stainless steel has a considerably lower thermal conductivity compared to aluminium or steel – it’s not for nothing, that the handles of steel cooking pots are often made from stainless steel.

Therefore, it is definitely not ok to praise steel spacers as somehow thermally improved. And by the way: Just the colour black alone does not provide sufficient evidence for a good thermal quality. Lukewarm edge – something like that just does not exist.


Warm edge – the difference in energy loss is not worth mentioning!??

July 11th, 2014. It is a persisting opinion amongst some window producers, that warm edge would only have marginal influence on the heat loss through a window. Quote from the interview with a window manufacturer, printed in the German glass magazine GLASWELT 5/2014, page 58: “Und so viel Energie geht doch nicht über diesen Kanten-Unterschied verloren.” – And it is not that much energy that is lost through that difference in edge bond.

Good gracious! Maybe this was valid in former times, when thermal transmission coefficients of windows were on rather humble level. For window U-values of more than 2 W/m²K, the improvement by use of warm edge systems was not that much. During the past years though, windows went through an enormous evolution in thermal performance. Nowadays, Uw-values of windows for new buildings in Germany are in the range of approx. 1.1 to 0.8 W/m²K, and this Uw-value is for the whole window, including frame and thermal bridge at the glass edge – not only a center-of-glass value. By rule of thumb, the use of warm edge instead of conventional aluminium or steel spacers improves the Uw-value of wooden- or PVC windows by approx. 0.1 W/m²K – or for metallic windows up to 0.2 W/m²K. Considering that approximately reveals that the effect of warm edge is easily in the range of 10 % – of the overall energy losses through a window, note well!

But for all that, in new windows aluminium spacers still do occur, even if it is triple glazing. What a pity, to pass up the chance for improving the Uw-value. Why? Is it just a lack of knowledge or making false economies? After years of educational work on this area I have no more understanding for that. Compared to other measures for thermal optimization of windows, warm edge is simple to implement and in addition extremely economical. Once the new windows are installed without warm edge, they will stay like that for the next decades. The potential for energy saving is lost for a long time.

1Potential for thermal optimization of windows

This graph shows the energy losses of a single-winged window and how they are shared between the three window components glass, frame and edge bond. Furthermore it illustrates the potential for optimization. Starting point is a window with the dimension 1.23 m x 1.48 m, a frame share in the total window area of 30 %, an Uf-value of 1.2 W/m²K and triple glazing with an Ug-value of 0.7 W/m²K and edge bond with aluminium spacers.

The comparison shows the possible extent of improvement of the Uw-value through

– warm edge instead of conventional aluminium spacers
– improvement of the Ug-value of the glass area
– improvement of the Uf-value of the frame area

If you want to know more about that, you can read my article (in German language) in the
GFF-magazine of march 2014 here .

Psi data sheets for windows updated

May 18th, 2013. The revised data sheets with Psi values for windows have been published just now. You can download them for free on the BF website (German Glass Association Bundesverband Flachglas) here.

The updated data sheets can be recognised  by the release index “April 2013”. Different from the first data sheet versions, where the representative Psi-values for windows have been calculated from individual thermal conductivity values of all the materials composing a spacer, now the Psi-values are determined on the basis of a measured equivalent thermal conductivity value of the complete spacer system including desiccant and butyle. An ift-Rosenheim guideline gives a detailed description of the measurement method – here.

Manufacturers of warm edge systems are allowed to still use old data sheet versions for their products, but they will expire latest 31.12.2013. Coexistence of old and new data sheet version for one and the same spacer system is not allowed. A research project of the BF working group “warm edge” served as a basis for the revision of the data sheets. First information about this research project can be found here (in German language). More will follow in the relevant specialist publications.

As usual, I will keep you informed here in this place.

New ift-guideline Thermally improved spacers

March 18th, 2013. Further to my previous announcement, ift Rosenheim, Germany, has published another ift-guideline about warm edge: WA 17/1 “Thermally improved spacers – Part 2: Determination of the equivalent thermal conductivity by means of measurement”. This guideline is a result from the latest research project of the working group “Warm Edge” of the German association Bundesverband Flachglas (BF). The guideline explains how to determine the so-called equivalent thermal conductivity value λeq,2B of a spacer and how to use it for the simulation of Ψ-values of windows according to EN ISO 10077-2, instead of a detailed geometrical spacer model with a multitude of individual conductivity values of the spacer constituents.

In comparison, this is a simple and pragmatic solution for obtaining of spacer input data for individual simulations. It helps to avoid mistakes with the modelling of rather complex spacer geometries. Furthermore, from now on, the “representative Ψ-values”, published on the BF data sheets “Psi-values for windows” will be determined according to this guideline.

The ift-guideline can be ordered in German language here. An English version will follow soon.

News from the Working Group “Warm Edge”

February 20th, 2013. During yesterday’s meeting in Rosenheim, the Working Group “Warm Edge” of the BF Bundesverband Flachglas agreed to update the data sheets “Psi-values for windows”. As a result of the latest research project of the Working Group, the representative Psi-values stated on the data sheets shall be simulated on the basis of measured equivalent thermal conductivity values of the warm-edge systems, and not as before with individual values of the sometimes complex product components.

The measurement method is described in a new guideline of ift Rosenheim, which will be published soon. Within the next weeks, the new data sheets will be available for free download on the BF websites. Warm-Edge manufacturers may utilize the old data sheets for their products until December 31st, 2013, after that deadline they will expire. You can find a first info (in German) about the research project here. More will follow in the relevant technical press – I will let you know here in my blog and in my literature list.

The Working Group “Warm Edge” consists of well-known manufacturers of warm-edge-systems, representatives from glass industry and members from higher education and research.

Do you have wishes or suggestions for the Working Group? If so, let me know!