Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
U vs R: It depends on the construction (e.g. door vs wall). For a glazed door or window (glazed subsurface), one can rely on the SimpleGlazingSystem option (product U, SHGC, VT). For an unglazed subsurface, a construction with a single composite (massless) layer may be adequate (product R, roughness, absorptances, etc.). Sometimes, all one has is a lab-tested U-factor from the manufacturer. But E+ needs more than that to describe a product's behaviour with respect to solar loads, long-wave radiative and convective gains/losses, etc. The links offer default values to consider for parameters other than U or R. For multilayered constructions (e.g. wall, roof), the requirements are similar, although it's preferable to disregard massless materials and rely instead on standard opaque materials. This allows E+ to calculate the transient response of each construction.
Thermal bridging: It is generally expected that users describe a multilayered construction (e.g. of a wall) while considering the effects of repetitive (or minor) framing, e.g. studs, Z-bars ... that thermally bridge (or derate) insulating layer(s). The resulting thermal resistance of the construction is often labelled as a clear-field R-factor - clear of major thermal bridging (e.g. columns, corners, additional framing around a subsurface). We've been maintaining an OpenStudio measure that autodetects such major thermal bridging from OpenStudio geometry and auto-derates linked surfaces before OpenStudio launches an E+ simulation. The links points to the online Guide (where we dive a bit deeper on what I describe above), yet one can simply access the measure from BCL.
I set aside any notion that thermal bridging is strongly linked to airtightness. Both concepts are treated separately in OpenStudio/E+.
| | 2 | No.2 Revision |
U vs R: It depends on the construction (e.g. door vs wall). For a glazed door or window (glazed subsurface), one can rely on the SimpleGlazingSystem option (product U, SHGC, VT). For an unglazed subsurface, a construction with a single composite (massless) layer may be adequate (product R, roughness, absorptances, etc.). Sometimes, all one has is a lab-tested U-factor from the manufacturer. But E+ needs more than that to describe a product's behaviour with respect to solar loads, long-wave radiative and convective gains/losses, etc. The links offer default values to consider for parameters other than U or R. For The requirements are similar for multilayered constructions (e.g. wall, roof), the requirements are similar, of walls or roofs), although it's preferable to disregard massless materials and rely instead on in favour of standard opaque materials. This allows E+ to calculate the transient response of each construction.
Thermal bridging: It is generally expected that users describe a multilayered construction (e.g. of a wall) while considering the effects of repetitive (or minor) framing, e.g. studs, Z-bars ... that thermally bridge (or derate) insulating layer(s). The resulting thermal resistance of the construction is often labelled as a clear-field R-factor - clear of major thermal bridging (e.g. columns, corners, additional framing around a subsurface). We've been maintaining an OpenStudio measure measure that autodetects such major thermal bridging from OpenStudio geometry and auto-derates linked surfaces surfaces, before OpenStudio launches an E+ simulation. The links points to simulation (see initial UMH post, or the online Guide (where Guide ... where we dive a bit deeper on what I describe above), yet one above). One can simply also access the measure from BCL. (look for "tbd").
I set aside any notion that thermal bridging is strongly linked to airtightness. Both concepts are treated separately in OpenStudio/E+.
