[Greenbuilding] thermal bridging--letting it go

[Keith Winston]

No, I can't imagine this is exactly correct. What I was getting at is, 
the insulation will conduct heat slower. In effect,  that means there'll 
be a hot layer at the edge of the insulation, where the heat is "piled 
up". Yes, the center of the insulation is cool, but that's because the 
heat doesn't have access to that (except slowly, through the 
low-conduction foam). The stud will seem warmer on measurement, from the 
outside, only because heat is moving through it, because there's 
conduction happening. Consider a thought experiment: solid silver stud, 
and vacuum cavity. The cavity is cold, or cool, but no heat moves into 
it (ignoring radiation for a moment!). The stud is warm, BECAUSE heat is 
moving through it. Heat flow in this situation is still warmer to 
cooler, but the actual condition is more subtle than at first may 
appear. Conductivity/R-Value is still key.

When trying to understand why a "simple computation by R-value" doesn't 
quite work, we don't have to get complicated: heat doesn't move in a 
straight line. It moves from hotter to cooler. As it moves through the 
stud, some of it will "bleed-off" into the insulation. But little of it 
will be able to do that, assuming there's good insulation there: we 
could say that those peripheral areas will get thermally saturated, and 
no more heat will move that direction.

I can't immediately find a comparison between simple R-value 
calculations and more detailed modeling or measurements. I suspect the 
difference is less than 10%. I see an article on Buildingscience.com 
that mentions that a brick veneer can negate the effect of exterior 
foundation insulation through a narrow and circuitous path of heat loss 
(check page 4 of 
http://www.buildingscience.com/resources/foundations/Understanding_Foundations.pdf). 
So in this case, a 4" deep masonry bridge is nearly negating perhaps 5' 
of exterior insulation. Bridging is important, moreso in relation to 
more extreme exterior temperature conditions, and a higher degree of 
difference between stud and cavity R-values.

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