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GBlist: Foam sheathing Issues (Minch question)




Polyisocyanurate Foam Sheathing ( Glenn Minch, Redmond, WA  USA)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This type of insulation has an R-factor of 7 per inch (...the old
7.2 per inch# was removed from ASHRAE handbook...) after an aging
period of 180 days.  I generally use R-5 for extruded board stock,
so you can be fairly confident the actual long term resistance to
heat flow is in this range ( R-7 to R-5 per inch ).

One potential caution in your climate would be to ensure good building
tightness so moisture is not seasonally "damed" (after Bill Lotz) in
assemblies clad with low permiance materials.  In the DOE Moisture
Handbook, and in the "Builders Guide" of EEBA (By Joe Lstiburek et.al)
there are passages concering proper use of low-permiance materials.

Many builders using exterior foam sheathing apply construction tape
(NOT "duct" tape) to seal joints and seams, or use an air-barrier
house-wrap "paper" (not really paper but more likely "spun-bonded" 
polymers or perferated plastic films...) over the whole wall and seal
joints in that.  This means a warm, low air-leakage, and potential less
moisture prone wall. (Too bad it often takes 20 to 30 years to find
out how durable various assemblies of materails really are in field)

The bottom line is that there are as many "high performance" wall 
structural and energy systems arrangements as there are energy experts;
so,  much discussion and controversy will remain about which is the
best. Such decisions are climate specific too, and can be influenced
by availability of various building materials, first cost, or opportunities
to use environmentally friendly practices.

Real important -- try to avoid building walls in your climate
that can allow moisture in through mass-transfer < convection of
moisture laden air, driven by pressure differences, via construction
defects in the assembly > that once in the assembly has few opportunities
to dry out.

You might also consider posting this question to the Energy Efficient
Building list.  For more information on that list send a message:

                 info ee-building your e-mail address
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which is the same Net-server that processes the greenbuilding list !

** DOE Moisture Handbook and "Builders Guide" are available both from
  EEBA> http://www.eeba.org  and from  IRIS/Oikos> http://www.oikos.com

--------------- Contact Information------------------
        Mr. Bion D. Howard, Principal          
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     Voice#  410-867-8000  fax#  301-889-0889
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