[Greenbuilding] thermal bridging--letting it go

[Keith Winston]

It occurs to me, finally, that we don't have to do the "thought 
experiment" I mentioned before. Just consider this:

According to California Energy Commission calculations, metal-framed 
wall assemblies with R-30 insulation have an effective R-value of less 
than 12.  
(http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/940707.html)

Now, all that heat loss is occurring through a metal stud, who's 
thickness in cross-section is perhaps 22 gage (damn thin!), though with 
very good conduction! We've got all that insulation-edge-effect we're 
talking about still going on, and yet a huge reduction in effective R-value.

Of course, (sigh) those are probably batts, so there's still probably 
convection and gaps. If only things could be simple...

Keith


Corwyn wrote:
>
> On Aug 1, 2006, at 00:34, Keith Winston wrote:
>> Heat conduction is from hotter to cooler points. You could perhaps more
>> accurately imagine that heat (like water) is "trying" to go in all
>> directions, but can only actually succeed going in the direction of
>> cooler areas. The stud is the coolest point because the heat is
>> transfered through and out of the stud fastest (into the cold outside
>> air), compared to the Icynene. Heat will try to transfer through the
>> Icynene, but due to the high R-value, will not move well through the
>> foam, and the foam will, in essence, not be cooler.
>
> Looking from the outside, the studs will be the warmest part.   Thus, 
> for any given slice of wall (crossways to the heat flow, parallel with 
> wall surface) the stud will be the warmest section and heat will move 
> from it into the cooler insulation.  This is why thermal bridging 
> isn't as bad a simple computation by R-values would indicate.
>
> Thank You Kindly,
>
> Corwyn
>

-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com