[Greenbuilding] thermal bridging--letting it go

[David Delaney]

At 01:20 PM 31/07/2006, Alan Abrams wrote:
>But in the case of tight fitting insulation--in particular, sprayed in place
>foam--why doesn't at least some of the heat want to take a shortcut to the
>left or right, a max distance of 3/4", into insulated void? 

It does (well except for the max distance part.)

> To a certain
>extent, the insulation is buffering this tendency, but the effect would be
>progressively less as the heat travels farther and farther away from the
>interior.  

Correct.


>The point being, that there must be a certain point where the bridging
>effect is substantially nullified by lateral dissipation of heat.  

This is exactly wrong.  The lateral heat transfer increases the bridging effect. 
You would get the minimum heat loss through the stud if the 
insulation were perfect (infinite thermal resistance) In 
this case all of the heat loss would be through the studs, 
but its absolute value would be smaller than for any level 
of insulation less than perfect. 

As the thermal resistance of the insulation decreases from infinite,
the heat flow through the internal face of the stud increases as the 
flows into the insulation lowers the thermal resistance seen by heat flowing 
into the face of the stud. The loss of heat from the stud
into the insulation actually increases the apparent resistance 
of the insulation near the stud to heat flow into interior face of the insulation.

The crossover point occurs when the 
thermal resistivity of the insulation material is the same as the thermal 
resistivity of the stud material. At that point there is no bridging, i.e. 
the heat flow through the wall is the same everywhere, as the 
the heat sees a thermally uniform wall.  As the thermal resisitivity of the
insulation becomes lesss than that of the studs, the situation reverses 
and the insulation becomes the "bridge", i.e. the lower resistance path.

>And I
>suspect that with the use of icynene or similar insulation, it happens a lot
>closer to 5 1/2" away from the interior than the common wisdom has it.

You can calculate a *lower bound* on heat transfer through a stud by assuming 
that its sides perpendicular to the walls (usually its wide sides) are perfectly insulated,
i.e. insulated by insulation of infinite resistivity.  Less than perfect insulation material,
or air gaps, will produce greater heat transfer outward through the interior face of the stud

David Delaney, Ottawa.