From: sasbge@ant.unx.sas.com (Gantt Edmiston)
Newsgroups: misc.consumers.house
Subject: Re: urethane spray foam insulation
Date: 13 Apr 92 17:07:40 GMT
    <1992Apr9.204204.3063@unx.sas.com> <1992Apr10.192600.26380@phx.mcd.mot.com>
Reply-To: sasbge@ant.unx.sas.com
Organization: SAS Institute Inc.
Originator: sasbge@ant.unx.sas.com
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In article <1992Apr8>, fredch@phx.mcd.mot.com wrote:
>>>I've been thinking 2x6 walls with R-19 batts plus 1" white styrofoam (R3.6?)
>>>with stucco over ext sheathing.  Might be able to cut out some of the up-front
>>>costs by going to 2x4 walls with 3" sprayed foam and stucco directly over
>>>the sheathing (+ felt paper).


In article <1992Apr9> sasbge@ant.unx.sas.com wrote:
>>
>>If you go with 2x6 studs on the perimeter, you can space them 24" on center
>>instead of the 2x4/16" on center method.  Your local building codes may
>>vary.  The wider gap decreases the number of gaps and thus the total
>>"uninsulated" space.

In article <1992Apr10>, fredch@phx.mcd.mot.com wrote:
>
>Yes, I know about 24" OC when using 2x6.  Apart from what kind of insulation
>you can put in, using 2x6 24" OC consumes 11" of wood, while using 2x4 16" OC
>consumes 10.5" of wood.  Thus, including window and door frames and top plates,
>you consume more wood using 2x6.  20% more seems typical.  Add to that the need
>to use thicker wallboard.  The cost for this part of framing is thus about
>20% more.  We argue that it's a good thing to do because you can put in
>more insulation (R-11 to R-19) and have fewer thermal gaps (few studs)


Then I attempted to clear up the "math" that Fred uses to state his point:
Fred Christiansen writes:
> 
> 
> One of the criticisms of 2x6 construction has been that it uses more wood
> (a "green" issue, I guess, altho, IHMO, a red herring, since construction
> use of wood represents only a modest part of lumber harvesting).  Let's
> take a 4' section of frame.  Ignoring the "end" piece, which we shall
> say is actually part of the next 4' section, 2x4 studs are spaced
>   |    |    |   .
> while 2x6's are spaced
>   |      |      .
> 
> 2*5.5" = 11", 3*3.5" = 10.5".  Hence, more wood is consumed, if you're
> concerned about green issues.  (I've not been persuaded of that because
> of the loss due to milling (maybe more loss in cutting 2x4's).)



I *agree* that more wood (per cubic inch) is used by incorporating 2x6 studs.

OK. Now that I know the basis of your calculations, let's take another look.
I think your math is a bit skewwed.  You are not comparing apples with apples.
Here's how I see it:
GIVEN:   A "2x4" is actually  1.5"x3.5"
     and a "2x6" is actually  1.5"x5.5".  

In your math, you multiply, 2 studs * 5.5" (a 2x6) which = 11".  But you
really should be multipling by 2.5 studs.  In your graphic illustration, you
incorrectly show how 3 "2x4" studs equate to 2 "2x6" studs, NOT 2.5 studs.
The math then comes out MORE in favor of the use of 2x4's from an
environmental aspect.  Let's examine the  volume method of calculating the
actual difference in cost.

Volume method:
  Given a section of wall 32" wide, 8 feet high.  Assume 16" OC construction.
  32" / 16" per stud = 2 studs 
  Using 2x4s:  2 studs * 3.5" * 1.5" * (8 feet * 12"/foot) = 1008 cubic inches
  Top/Bottom sections= (32" top + 32" bottom) * 3.5" * 1.5 =  336 cubic inches
                                                            -----
                                                             1344 cubic inches

  Given a section of wall 32" wide, 8 feet high.  Assume 24" OC construction.
  32" / 24" per stud = 1.333 studs
  Using 2x6s: 1.3 studs * 5.5" * 1.5" * (8 feet * 12"/foot) = 1029.6 cubic inch
  Top/Bottom sections= (32" top + 32" bottom) * 5.5" * 1.5" =  528 cubic inches
                                                             -----
                                                              1557.6 cubic inch
                                                   rounded to 1558 cubic inches

So 2x4 construction  uses 1344 cubic inches of material versus 1558 cubic
inches from 2x6 contruction.  1558 - 1344 = 214.   214 / 1344 = ~.16 * 100 = 16%

Therefore 2x6 construction takes 16% more resources than 2x4 construction
for any *equal* section of wall, assuming that manufacturing costs are equal
and wastage differentials are negligible.  Of course the height of the wall is
not of concern, as the percentage will not vary.  I included it in the figures
only for "completeness".

Having spoken with several builders, they say that there is no difference
in "labor" cost for using 2x6 construction as the carpenters build the
same way with either boards.  My recommendations were to only use 2x6 on the
perimeter of the building and 2x4 on the interior walls.  Comparing the 16%
calculated value with the 20% net.wisdom values, they look pretty close.

Assuming the 2x6 construction *does* give LESS gaps in insulation, your house
will be *MORE* energy efficient.  The longer the house stands and burns up
fossil fuels for HVAC (heating, ventilation and Air Conditioning), the 2x6
house will be preferable from the "green factor" you mention above.  I called
my local utility company about the differences in "gaps".  They say it *is*
significant.  Unfortunantly, they also said that many municipalities, require
16" OC construction for both 2x4 and 2x6 perimeters.  Local building codes
will vary. 

Does this clear things up some?  Or does it raise other concerns? =-)

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