The glass edge thermal bridge: the PSIg value

We continue our series of articles on the thermal efficiency of windows, describing the glass edge thermal bridge.

As far as thermal bridges go, this one is inevitable, and it represents the weakest point of a well-designed thermal envelope. It needs to be analyzed carefully, in order to prevent condensation (or ice) to form on the edge of the glass, discomfort, and an overall drop in the performance of the window/door.

From the point of view of thermal comfort, windows and doors have a key role: proper components (passive house suitable) need to be selected depending on the local climate. This allows for good surface temperatures, that avoid asymmetry in radiant temperature, and glass edge condensation. On the opposite, a poor quality window makes feel “cold” in an environment even if the average temperature may be above 20°C (68°F). It can also allow for condensation to form along the edge of the glass.

In our previous articles, we covered thermal transmittance and resistance of glazings, Ug, and of window frames, Uf. Where these two elements meet, glass and frame, you have the glass edge thermal bridge, PSIg.

In a well-designed thermal envelope, the junction between glass and frame remains the thermally weakest node of the entire building. Here you have the lowest temperatures for the interior surface (fRsi factor).

If this junction is not properly designed, you can very easily have condensation forming along the edge of the glass, because the local temperature on the internal surface of the glass drops below the dewpoint. If external conditions are particularly critical, you may even have ice forming along the interior edge of the glass (with the result of the room being very uncomfortable).

You’d better not trust marketing brochures, as they often are the result of pure and simple barbaric marketing, instead of actual analysis on the products sold.

CALCULATING THE GLASS EDGE THERMAL BRIDGE, PSIg

As in the case of other thermal bridges, you need to calculate the PSI value (called “PSIg”, for the glass edge), and the temperature factor, fRsi, with a finite element calculation. To do this, you need the actual section drawing of the frame, the assembly of the glass, and the glass spacers.

The glass spacers are rigid elements used to connect (and to space apart) the glass panes of an insulated glass unit (IG). From the least performing to the most performing one, glass spacers can be made of:

• aluminum
• stainless steel
• polymeric material (with or without stainless steel insert)

From the finite elements simulation point of view, the spacers can be modeled according to the real geometry of the spacer (which is more accurate, but free software like Therm cannot handle it well) or with the 2Box model.

We’re going to compare the two methods in-depth in a dedicated article.

Just like any other transition between two elements having different thermal transmittance, the result of the finite element calculation is a bidimensional heat flow, L2D. This value is different from the sum of the two monodimensional heat flows, calculated for the glass (L1D_g = Ug * bg) and of the frame (L2D_f = Uf * bf). The difference between bidimensional and monodimensional heat flows it the glass edge thermal bridge, PSIg:

PSIg = L2D – (L1D_g + L2D_f)

In the case of the glass edge thermal bridge, the PSIg value referred to the internal measures and the external ones are exactly the same.

WHAT INFLUENCES THE GLASS EDGE THERMAL BRIDGE?

Many factors have an influence on the glass edge thermal bridge, both in terms of heat flow (PSIg) and of surface temperatures (fRsi):

• type of frame (wood, PVC, aluminum, glass fiber, steel, etc.);
• with the same type of frame, the type of profile (bottom, side, central, etc.);
• with the same type of frame and profile, the thermally effective portion of the profile (which varies with the thickness of the IGU);
• type of glass (number of cavities, thickness of cavities, thickness of the glass panes);
• type of spacer (aluminum, stainless steel, hybrid steel-polymer, polymer);
• type of sealant used for the IGU (polysulfide, silicone, polyurethane, hot melt butyl);
• indent depth of the glass inside the frame profile.

The same window has at least two different PSIg values because it also has two different transmittance values for the frame, Uf (one for the bottom profile, the other for the side/top one).

Data provided by the manufacturers are acceptable only if all parameters listed above are exactly the same. In reality, the PSIg value has to be calculated on a by-project basis (for each profile), depending on the exact frame and glass types used.

Can you use “average” values of PSIg?

With the list of variables listed above, it should be clear that the PSIg value does not like to be standardized.

• the correct average PSIg value is calculated with a weighted average of the different PSIg values of one specific window;
• the wrong average PSIg value is any number a designer of the building is ready to sign off for (as in the case of the Uf value for window frames, or the design lambda value for insulation materials).

One value that cannot be made “average” is the temperature factor fRsi, used to verify the absence of condensation on the glass edge. This value has to be calculated via finite elements, using the most realistic geometry of the junction (the 2Box model may not be accurate enough!). The correct fRsi calculation includes the entire junction, with the profile installed in the assembly of the wall/roof, so that the analysis can be as similar as possible to the future conditions of the installed window/door. Also, the acceptability of the fRsi value of the junction depends on the local climate.

CONCLUSIONS

The glass edge thermal bridge, plays a primary role in thermal comfort, absence of condensation (or ice), and energy efficiency of the window and of the entire building.

The values of heat flow (PSIg) and internal temperatures (fRsi), vary significantly depending on the window profile and glass unit (and many other parameters), so that standardized values are most likely wrong.

With higher expectations in terms of construction quality, even in energy retrofits, the glass edge shows how 21st Century designers are required to be more and more prepared. The important nodes of the thermal envelope need to be analyzed in-depth, so that comfort can be guaranteed, and the performance gap avoided.

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