Re: steel and straw

[Robert W Tom <archilogic@juno.com>]

on 20 Feb 2001  <Ralph.Davis@state.nm.us> wrote:

[snip]
> As to over insulation, I can't figure out why someone would put 
> up a  R-50 wall and then cap it with R-36.  The injected foam is a two 
> stage polyurethane   To insulate  to a lesser amount doesn't save
> any significant dollars.

I ran some numbers on this scenario (2400 sf ceiling area,
polyurethane insulation (R-6/inch), Abq. NM and assuming
an outdoor design temperature of 40 degF) for the
months Sept -April... and no appreciable heat loss through
the ceiling was found until the thickness of the insulation was 
reduced to 3.75" thickness (R-22.5)... the heat loss occurring 
in January which according to the data I have, experiences 
on average,  970 heating degree-days per month.

In the above, the net monthly heating load for January was
104,000 Btus (30.4 kWh)  which in my area where the hydro 
is billed at  about 8 cents per kWh , would cost $ 2 . 43 

So, using the data above (of which only the outdoor design temperature
of 40 degF might be incorrect) it would appear that Bill Hunt was
correct, 
that insulating a ceiling to R-80 in Abq. NM would not make a whole lot 
of sense in terms of payback.

However, steel trusses would alter the above scenario appreciably.
Some of you may recall the studies which showed that the effective 
R-value of steel stud-framed walls was reduced by something like 
40-50% due to thermal bridging so insulating steel trusses with 3.75" of 
polyurethane may in reality, yield an effective R-value of 14 or less.

I think that rather than attempt to dispose of a mouse with bazooka by
insulating to R-80 with sprayed PU,  it might be more effective to
insulate 
to 1/3 that level, on top of the roof deck , in  a configuration such
that 
thermal bridging is minimised or eliminated.Doing so would also get the 
polyurethane outside so that if a fire did occur, the nasty chemical
smoke
would less-likely get at the occupants.

                 ===    *     ===
Robert  W. Tom          Kanata, Ontario, Canada       
Rob_Tom@ErehwonDesignGroup.intranets.com 

please visit:  http://www.theHungerSite.com daily

________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
http://dl.www.juno.com/get/tagj.