Percentage of heat loss/gain through thermal bridging in a typical home?
jbmoyer
| Posted in Energy Efficiency and Durability on
Any of you energy nerds out there know of quality studies conducted on heat loss/gain through framing (and other thermal bridges) in residential buildings?
Also, if you know of studies conducted on heat loss/gain through air leakage that would be much appreciated.
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It would be fairly easy to calculate the additional heat loss through thermal bridging as long as you know the framing factor and the R-values of the various materials. But I imagine empirical studies are what you are after and not being an energy nerd I can't help you with that. Reliable multi-year properly controlled studies seem to be rare in this business and rarely applicable across different climate zones anyway.
I'm ready to stick my neck out though and hazard a guess that the greater issue in most climates is not so much heat loss/gain itself but the potential for a thermal bridge to bring the dew point toward the interior of the assembly, with condensation, mold and rot as a consequence. I imagine we've all seen extreme examples where thermal bridging results in pattern staining from mold growth along stud lines on the interior of the wall finish. Martin H has posted numerous times on how to avoid both visible and hidden damage by means of correct proportions of interior and exterior insulation.
By the way, the empirical insulation study I would like to see would be a smackdown between SPF and cellulose in wall and roofspace applications. The calculated performance of spray foam insulation as commonly installed at levels below prescriptive code requirements seems to be justified only when measured against inadequate GF installations that do not meet their nominal R-values and have no effective air barriers. This makes no sense.
This isn't actual studies - but it may help with your questioin... If you have access to energy modeling software such as REM/Rate you can compare different senarios and look at losses/gains in a variety of measurements. For example- in my location, an 1800 sf 2 story with R-21, 24" o.c. comes out about $32 each year more than the same home with continuous R-5 on the walls. Different building systems can be modeled. You can also look results in kwh, btu, HERS rating and several other measurements.
The ASHRAE Technical Committee on building envelopes has sponsored research, conducted by Morrison Hershfield, to model thermal bridge effects in commercial building assemblies. Once complete, it is expected to lead to a catalogue of thermal bridge effects for things like floor intersections, corners, roof-wall junctions, balconies, etc. The upcoming Handbook chapters will explain how to use these in a U-factor calculation, namely, take the clear-wall U and add conductances per linear foot for linear bridges and by number for point bridges. Whether this becomes a new standard (all U-factor calculations for purposes of code compliance and building simulation must include bridge calculation) remains to be seen.
Framing factor calculations for wood frame residential are a good step in the right direction.
Thermal bridges are not only paths for heat loss, they can become sites of indoor temperature depression (assuming cold winters) even lower than the indoor temperature in uninsulated assemblies.
In short, bridges form a very important topic, and it is good to see it raised here.
Brett,
I stumbled on this UK example a while back.
Not exactly what you are looking for ...but still interesting
It is based on calculations ... not measured performance
http://www.aecb.net/PDFs/carbonlite/1420_AECB_VOL_2_PM_V8.pdf
Did anyone ever find any estimates?