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How OpenStudio works with U-factor, R-value, thermal bridges?

asked 2022-12-17 07:16:41 -0600

OlivierC37's avatar

Hi everyone,

I am trying to understand how OpenStudio works. I don't understand why I can't use a U-factor rather than an R-value for a door or even a wall? I don't know if thermal bridges are automatically taken into account around a door or a window ? i.e. a door or something else is not airtight. If not, does anyone know a trick to take this into account? Is there a document where I can find out how openstudio works, with this kind of questions?

Thanks in advance,

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answered 2022-12-17 15:37:56 -0600

updated 2022-12-17 15:52:14 -0600

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. The requirements are similar for multilayered constructions (e.g. of walls or roofs), although it's preferable to disregard massless materials 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 that autodetects such major thermal bridging from OpenStudio geometry and auto-derates linked surfaces, before OpenStudio launches an E+ simulation (see initial UMH post, or the online Guide ... where we dive a bit deeper on what I describe above). One can 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+.

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Thank you, your comment and the links are very helpful.

Regarding U vs R, I would like to know if OpenStudio uses Rsi and Rso to get Rtotal to find the U- value (1/Rtotal)? I think it works that way but I'm not sure because I've learned that this type of calculation is usually used in steady state and not in transient. As I said, I have not yet found any documentation that explains which calculations are used even in E+.

I have another question about your comment, what is a massless material, I mean even air has mass, so when is it important to use it?


OlivierC37's avatar OlivierC37  ( 2022-12-18 07:29:56 -0600 )edit

Rsi usually means SI units (vs IP units). You're asking about inside/outside film resistances? The SimpleGlazingSystem U-factor does indeed include them - it's expecting product NFRC-rated values. Otherwise, ALL other E+ constructions (e.g. other windows, walls) do not consider air films in their definitions - they are applied at run-time. If one has a rated product U-factor for an opaque door, one must subtract standard film resistances to isolate material resistance(s), e.g. 1-layer door construction. Look up "U-factor no Film" in a generated eplustbl.htm.

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-12-18 09:37:11 -0600 )edit

Massless? As stated in the IO Reference Manual: "Use this definition when only the thermal resistance (R value) of the material is known." I see it mainly used in vintage models. I avoid it, as it's incompatible with certain E+ features (e.g. ground exchange models like KIVA).

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-12-18 09:41:54 -0600 )edit

Yes, I asked about inside/outside film resistances and with your explanation it's very clear. Earlier you said that "E+ needs more than that to describe a product's behaviour with respect to solar loads, long-wave radiative and convective gains/losses". I saw in OpenStudio that I have to indicate the Thermal Absorptance, Solar Absorptance and Visible Absorptance for all the materiels because it seems important to describe the behaviour. Where I can find these informations? Is there a reliable list?

OlivierC37's avatar OlivierC37  ( 2022-12-18 13:13:21 -0600 )edit

Absorptances (key for exterior/interior surfaces only - not all layers): I usually resort to default E+ values (0.9 for thermal, 0.7 for solar/visible). I may switch to low-e/reflective treatments/finishes (e.g. E < 0.2), although one should be cautious and expect long-term degradation for exterior surfaces ... they oxidize, get dirty, etc. If one doesn't have rated properties (including specific heat, conductivity, etc.) from manufacturers, recommended sources include engineering handbooks, Wikipedia, research reports, standards like 90.1 (Appendix A) and applicable energy codes.

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-12-18 14:29:06 -0600 )edit

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Asked: 2022-12-17 07:16:41 -0600

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Last updated: Dec 17 '22