[Greenbuilding] Designy metal frame windows with good U-value?

[Rob Tom]

Ted Inoue tedinoue at gmail.com wrote:

> On 2/28/07, Rob Tom <ArchiLogic at yahoo.ca> wrote:
>> I suppose it depends upon the climate in which one is building but in
>> mine, simply using backer rod/caulking leaves the space essentially
>> uninsulated... air-tight, yes but uninsulated thereby only addressing  
>> half
>> of the problem.

> I think air/moisture movement is the biggest problem. An uninsulated  
> space,
> 1/4"  around the window, say 3'x 5', is  1/3 sq. ft.. Say it's R2 with  
> the
> foam backer and airspace, so 1/6 BTU/h / F. So in the dead of winter,  
> maybe
> you're losing 10 BTU/h per window.

Ted and all;

I think that what one thinks is important, again, depends upon the climate.

I can tell you that in my climate (~8750 HDD/yr), if you have a component  
that is only R-2 (by your guesstimate), it is essentially a thermal bridge  
through the envelope.

While the amount of heat loss (by your guesstimate) through an uninsulated  
shim space may seem insignificant, condensation on the cold interior  
surface of the thermal bridge will quite likely be very significant.

Given that the thermal bridge is at the sheltered corners deep inside the  
window well, it's highly unlikely that there will be sufficient movement  
of heated air past the cold surface of the thermal bridge to prevent  
moisture in the conditioned interior air from cooling to the dew point and  
once condensed, from drying.

So at the very best, you get wetted building materials that remain wet for  
long periods of time.
That is a sure-fire recipe for microbial activity (bad for humans) and  
subsequent rot (bad for the building materials).

A slightly worse scenario is that the condensation would freeze and over  
time the accumulations could very well be enough to push things  
apart/break things as a result of the expansion and then when the weather  
warms up and the ice melts, in addition to the frost damage, you'd then  
get the wetting problems mentioned above.

If the locale is someplace where the interior humidity is very low and the  
winters don't get very cold, then yes, I would agree that effective  
insulation of the shim space may not be that big of a potential problem.

As the saying goes "Latitude determines attitude".

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply