Thermal Bridging & Derating

Download .zip Download .tar.gz


A specialized library that accurately - and very efficiently - handles foundation heat transfer within EnergyPlus. For designers, it offers several practical features that make it a one-stop solution for most projects:

  • multiple insulation configurations
  • custom thermal breaks, e.g. around slab edges
  • sloped terrain & walk-out basements

Only a few hard limitations prevent its systematic use by designers, notably KIVA’s current inability to integrate radiant slab heating or cooling. The softer challenges of using KIVA have more to do with managing inputs (especially with highly-articulate building configurations) - this is where TBD comes into play.


The OpenStudio SDK indeed offers access to KIVA inputs, yet unfortunately this is not widely supported by third-party apps based on the SDK - including the OpenStudio Application itself. In a nutshell, TBD supports OpenStudio users interested in using KIVA by automatically generating valid KIVA inputs behind the scenes. For most cases, relying on the Apply Measures Now mode is recommended - it’s likely that TBD’s default insulation configurations (see below) will need some manual tweaking before launching an EnergyPlus simulation. To tweak TBD-generated KIVA inputs, it’s important to CHECK the Alter OpenStudio model option under Apply Measures Now, while selecting the “(non thermal bridging)” option (see Menu Options below).

Settings: The very first thing TBD does is populate an OpenStudio model with a template for KIVA settings - like soil conductivity. This is usually not required (the default, autogenerated KIVA settings are fine), but their explicit inclusion in an OpenStudio model offers easy access to further tweak them by hand - if required.

Foundations: KIVA requires that OpenStudio surfaces - tagged with “Foundation” as a boundary condition (slabs, basement walls) - reference new KIVA foundation objects in the OpenStudio model. This is not necessarily a one-to-one match - basement walls need to point to the same foundation object as the basement floor they’re connected to. So the second step TBD carries out is to autogenerate unique KIVA foundation objects per foundation floor surface, and then link up any foundation (basement) walls each floor surface is connected to. For fully-enclosed geometries, TBD keeps track of which individual wall is connected to which floor, and along which unique edge. This topological consistency - offered behind the scenes by Topolys - translates into time savings (as geometry take-offs are not required) and minimizes risk through automation.

Exposed perimeters: Not done yet. KIVA may also require users to link aforementioned KIVA foundation objects to new KIVA exposed perimeter objects. Imagine a project built on a sloped terrain, with its front entrance at street level, yet the back service entrance a full story below. The lower-level spaces may have basement floors, (front) basement walls, (side) interzone partitions, and finally (back) walls facing the outdoor air. KIVA allows for such complexity, yet requires from users the total length of slab edges along outdoor-facing walls. Just as TBD automatically links basement walls to the right KIVA foundation objects, TBD automatically generates new KIVA exposed perimeter objects and correctly sets the right perimeter lengths - for fully-enclosed geometries. Again, this is a straightforward, behind-the-scenes operation as TBD already keeps track of each unique edge linking each outdoor-facing wall to each slab - again, saving time and reducing risk through automation.

Insulation configurations: TBD must make some initial guesswork when it comes to full or partial insulation coverage. For instance, TBD assumes that foundation (basement) walls already reference full-height, insulated constructions in OpenStudio. This may not be the case, as many (past) energy codes only required partial-height insulation of basement walls. For floors, TBD assumes that referenced constructions are not insulated at all. Yet for slab-on-grade cases, TBD does generate 2-ft (600 mm) of perimeter insulation (1-inch or 25.4 mm of XPS) for each corresponding foundation object. This is a typical circa-1980 slab-on-grade setup, which may also not reflect design intent. After autogenerating KIVA inputs (via TBD in Apply Measures Now mode), users may need to tweak an OpenStudio model (.osm file) using a text editor and modify autogenerated KIVA foundation objects (including insulation configurations) to correctly reflect design intent.

TBD offers 2 checkboxes to autogenerate KIVA inputs (see Settings), again ideally under Apply Measures Now mode:

  • for surfaces only with “Foundation” boundary conditions
  • for surfaces with either “Foundation” or “Ground” boundary conditions

The latter is recommended for most cases - OpenStudio often sets “Ground” as outside boundary conditions for lower floor surfaces by default. TBD also provides a Purge existing KIVA inputs checkbox to ensure existing KIVA entries are first removed before adding new ones - EnergyPlus raises severe errors if duplicate KIVA entries are found.

Once KIVA inputs are permanently added to an OpenStudio model, and maybe subsequently altered by hand if needed, users can then safely run OpenStudio/EnergyPlus simulations - hopefully again with TBD handling major, above-grade thermal bridges.

One last (yet very important) point: when autogenerating KIVA inputs (to subsequently edit by hand) in Apply Measures Now mode with the Alter OpenStudio model option, users should ideally select the Default thermal bridge set “(non thermal bridging)”, which prevents TBD from derating any above-grade surfaces. It is usually not desirable to permanently derate opaque surfaces this way.


KIVA support offered by TBD is loosely adapted from a published NREL simulation test, yet is generalized here to accommodate arbitrary model geometries.