### Uprating

By default, TBD presumes an OpenStudio model’s opaque surfaces (maybe in the hundreds) refer to one (or very few) common multilayered construction(s) that reflect *clear-field* design intent. TBD subsequently *derates* these multilayered constructions on a surface-per-surface basis for energy simulation purposes. So while each construction has a unique *clear-field* **Uo** factor, each surface has a unique *derated* **Ut** factor (once TBD is done).

Depending on the *extent* of thermal bridging (due to each surface’s edge lengths and *psi* factors), surface-specific *derating* can range from *barely noticeable* to *extensive* (e.g. > 50%). One may start off with a single, common *clear-field* construction for all exterior wall surfaces, the latter will end up with sometimes radically different **Ut** factors.

```
Ut = Uo + ( ∑psi • L )/A + ( ∑khi • n )/A
```

This presumption is consistent with typical building energy simulation workflows. Yet determining what should be the initial (common) construction **Uo** factor may not be straightforward. What happens when designers are unsure of what initial *clear-field* **Uo** factor they should start off with, given façade layouts and thermal bridging design choices? Is it economically wise to aim for much lower **Uo** factors, as a means to compensate for *major* thermal bridging?

In some cases, building professionals may even choose (or are required) to achieve a maximum, area-weighted average **Ut** for all exterior wall surfaces. It’s the case for prescriptive requirements of the Canadian NECB 2017 and 2020 editions, e.g. a final wall area-weighted average **Ut** of 0.210 W/m2.K (R27) for climate zone 7 (NECB 2017). Depending on the *extent* of thermal bridging, the initial *clear-field* **Uo** factor for that single, common construction may need to be 0.160 (or much, much lower).

So in addition to *derating* construction **Uo** factors (to final surface **Ut** factors) for energy simulation purposes, TBD offers designers the option of first autogenerating required *clear-field* **Uo** factors (a process called *uprating*) to meet a given target, by reordering the above equation.

```
Uo = Ut - ( ∑psi • L )/A - ( ∑khi • n )/A
```

*Uprating* menu options (see Settings), are paired together for *walls*, *roofs* and/or exposed *floors* (let’s make things easy here by limiting the discussion to walls). The default value assigned to the “Wall construction(s) to ‘uprate’” pull-down menu option is “NONE”, disabling any *uprating* calculations for walls. TBD nonetheless pre-scans an OpenStudio model to retrieve referenced wall constructions in order of prevalence - referenced constructions covering a larger area are listed higher up in the pull-down list. Users can either limit *uprating* calculations to one (1x) such referenced wall construction, or to “ALL wall constructions” in a building model. The latter is an all-encompassing solution, overriding previously set construction assignments (the most prevalent wall construction is nonetheless retained as the basis for subsequent *uprating* - and then *derating* - calculations). Users can also set the desired, area-weighted **Ut** factor for selected walls (default factors are those of the NECB 2017 for climate zone 7).

TBD will log (and flash on screen if using the OpenStudio Application) the calculated *clear-field* **Uo** factor required to achieve the desired area-weighted **Ut** for walls (see Reporting).

```
An initial wall Uo of 0.162 W/m2•K is required to achieve an overall Ut of 0.210 W/m2•K for ALL wall constructions.
```

The *uprating* calculations are similar to UA’ assessments, yet in reverse order. In any *UA*-type exercise (like these *uprating* calculations), a significantly weaker component will have a disproportionate effect vs its area (as summarized in the very first paragraph of this guide, “In a nutshell …”). The degree of *uprating* may be quite reasonable for very efficient envelope designs, e.g.:

- continuous outboard insulation
- thermally-broken cladding clips
- minimal fenestration
- favourable façade aspect ratios

Yet it may be very challenging (and onerous) to meet such ambitious **Ut** targets when factoring-in weaker components, e.g.:

- spandrels
- poor detailing
- lots of fenestration

We strongly recommend to first investigate this feature while relying on Apply Measures Now feedback (ideally UNCHECKing the **Alter OpenStudio model** option), to get a sense of how significant the *uprating* calculations may end up altering your initial designs. In some cases, EnergyPlus’ CTF convergence calculations may even fail with very thick (*uprated*) insulation layers !