A Learning Curve Worth Navigating

For most people in the building industry, thermal bridge calculations are a far-distant concern—if they even know what they are. But for Passive House practitioners, calculating psi values is often a painful fact of life. While high-quality instruction is increasingly available, the commonly available software, THERM, seems stuck in the mid ’90s, with hard-to-use drawing interfaces, finicky geometry imports, and severely limited calculation functions that require using additional spreadsheets.

Wouldn’t it be nice if you could simply identify a detail in a computer-aided design (CAD) drawing, and with a few clicks of a button, the psi value would automatically pop up, complete with report and documentation? While nothing like that exists currently, there is software that comes surprisingly close.

Europe has seen a raft of two-dimensional thermal-modeling software developed in recent years. One of those programs—Flixo—is relatively unknown in North America, but offers considerable promise for Passive House practitioners. Let’s look at some of the features—good and bad—of this software.

Pre-formatted Flixo report page showing a rim joist condition with a psi value calculation on the left (with model showing materials, boundary conditions, and iso-therm lines) and an fRsi calculation on the right (with model showing heat flux). Credit: CSBR (Center for Sustainable Building Research, a University of Minnesota research center).

The first thing people generally notice about Flixo is how easy it is to draw geometry. It has a thoroughly modern drawing interface, with a host of intuitive, smart snap settings, copy-and-paste options, and transformations. For experienced THERM users, Flixo is refreshingly forgiving when geometry doesn’t line up perfectly. In fact, one of the coolest features of Flixo is the ability to overlap and layer polygons. If a beam needs to be thickened, it can simply be stretched, without the need to redraw or reposition every piece of geometry that touches it. This makes testing different versions of the same model with slightly different insulation thicknesses a snap, for example.

Another nice feature of Flixo is the customizable component database. Particular pieces of geometry that can be time intensive to draw, such as I-joists, can be created once and saved to the component database. When incorporated into a project template, they become available for every modeling project with a few clicks of a button.

Drawing angled geometry is a pain, however. Would you like to draw a line segment exactly 6 inches along a sloped roof? Prepare to get creative. Flixo expects horizontal and vertical line segments. Its intuitive parallel and perpendicular drawing guides and direct-value entries don’t help much when the geometry is canted. This means that a sloped roof, for example, is best drawn with horizontal and vertical geometry and then rotated.

One way around this limitation is to import complicated geometry from a CAD program. Here Flixo really shines. When importing a Drawing Exchange Format (DXF) drawing, the program automatically opens an import wizard. Before dropping the geometry into your model, the wizard zooms you around the object, automatically identifying extra line segments, unclosed polygons, and other little errors that usually cause fits. You can delete or fix these, one after another, until the wizard finds no more issues and drops the cleaned-up geometry into the Flixo drawing. The ease of this import process is one reason why Passive House window manufacturers, such as Zola, are switching to Flixo for their thermal- modeling needs.

Once you’ve drawn an object, Flixo provides a vast database of materials and associated thermal properties and boundary conditions to complete the model. Currently, all of these databases use European standards such as ISO and DIN, which makes compliance with PHI Passive House standards straightforward. There is currently no ASHRAE-based material database.

While we’re on the topic of American versus European standards, Flixo’s method of dealing with unit conversions is more nuanced than that of most programs. Rather than offering an either/or proposition, Flixo lets users select IP or SI units for specific properties and calculation results. For example, you can draw in inches and feet, but view results in metric units. One sorely missed feature is a pop-up converter for quickly converting R-value per inch to metric units.

Flixo is a tab-based program. Once you complete a drawing on one tab, called a Model tab, you calculate heat flow with a button click. Results pop up on an associated but separate Report tab. These two pages are now linked, so changes to the model on one page transfer over to the Results page, where both geometry and calculations are automatically updated. You can configure a Report page to display a host of different results and calculations. Depending on the Flixo version, a Report page can include different types of heat flow visualizations, psi values, U-values, heat flux values, and even fRsi factors.

Report calculations can be displayed alongside both dimensioned views of the model and graphic keys, such as material properties. In this way, all the important assumptions and values going into the calculation appear together. You can set up these pages as minireports, with page numbers, labels, company logos, and other text. A Master Report page adds these features to every Report page, vastly simplifying both the calculation and reporting procedures that are so time consuming for THERM users. Each of these pages can be exported separately or combined in a multipage PDF report. If you only want a snapshot of the heat flow through a detail, you can copy that picture onto the clipboard and paste it into other word-processing or spreadsheet documents.

Perhaps the best feature of Flixo is its ability to model complex windows. All of the Flixo versions (with the exception of the most basic version, Flixo Energy) come with a DXF import feature. Two versions, Flixo Frame and the top-of-the-line Flixo Pro, provide several tools that can cut at least in half the time spent modeling windows. With Flixo Frame, you can create glass unit objects, eliminating the need to create separate glazing models that must be imported. Flixo Frame also comes with an air cavity wizard. Simply click your mouse around a group of air cavities, and the program automatically identifies and fills them with the appropriate thermal conductivity and emissivity, calculated for each, depending on their specific size, geometry, and surrounding material. This feature doesn’t help window manufacturers calculate National Fenestration Rating Council (NFRC) values, which are done according to a different set of rules, but it does come in handy when calculating window frame U-values and psi-install values for Passive House certification.

Flixo Frame provides a special window frame boundary condition that automatically adjusts the frame’s surface film coefficient at different points along its interior surface, according to ISO 10077-2. When it comes time to perform the thermal simulation, experience shows that Flixo has a much more stable calculation environment than THERM. Crashes are rare even for complex window frames. Taken together, these features make Flixo Frame a hands-down favorite for window manufacturers.

Unfortunately, not all of these cool features are available in every Flixo version. Flixo Frame and the expensive Flixo Pro are the only versions that come with the specialized window-modeling features. None of the other Flixo versions comes with the air cavity wizard, glass unit object, or special frame boundary condition, meaning that they cannot calculate official window frame U-values. And if a window manufacturer opts for the cheaper Flixo Frame, there is no psi value calculation tool. This confusing and frustrating functionality split is probably the worst feature of Flixo.

If you’re using Flixo Energy Plus, the psi value calculation tool is standard, but there are no specialized window-modeling features. While you can create and evaluate window models, psi values incorporating a window may or may not be accurate—it depends on the type of window frame. Frames such as North American vinyl and fiberglass windows are most affected, because they have a large number of air cavities. Solid-frame wood windows with a small number of cavities are least affected.

Where does that leave Passive House folks? In essence, Flixo Frame is for window manufacturers that are modeling windows and calculating certified window frame U-values. Flixo Energy is for Passive House consultants calculating heat loss via psi values. If you need both sets of values, some creativity (or extra money) may be required, depending on the desired accuracy. Note that both PHI and PHIUS allow estimates of psi-install values for windows—as long as the values are on the “conservative side.”

For most Passive House consultants occasionally tasked with calculating thermal bridge coefficients, an upgrade to Flixo makes great sense. Users say the experience is akin to going from the horse and buggy to the automobile. The time saved on the first project alone typically more than covers the cost of the software. If new users get stuck, there are a number of high-quality animated tutorials available and good product support from the manufacturer. In Flixo 8.1, coming soon, users will be able to import old THERM files into Flixo projects to get fRSi, psi value, and other calculations directly in the software. Welcome to 2018.

—Rolf Jacobson

Rolf Jacobson is a research fellow at the Center for Sustainable Building Research at the University of Minnesota. He also consults for CertiPHIers, which sells Flixo.


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