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<DIV>These two articles referenced by John Straub helped me understand how to
estimate the surface temp behind exterior mounted non pervious insulation (foam)
when in combination with pervious insulation (fiberglass, cellulose, et.).
</DIV>
<DIV> </DIV>
<DIV><FONT size=4>The short version is that the temperature drop through an
assembly is proportional to the R value. However, the air pervious
insulations allow the transport of vapor in air. If half the R value is
pervious and half is non pervious, the temp at that intersection would be half
the delta T. If that happens to be below dew point where the assembly
insulation switches from pervious to non pervious, you have a threat to the
structure. </FONT></DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV><FONT size=4>What occurs to me from understanding this issue is that it
makes sense to have all the thermal barrier on the outside of the
structure. I have found a source of EPS that is the same $/R price as
fiberglass. Therefore, it is simpler to wrap above sheathing in the common
practice framed building walls and roof with thick layer of EPS to the needed R
value. 3/4”x3” plywood furring strips mounted outside the foam provide
support for siding of choice. I used this method on my own retrofit and I
am satisfied with the result. What is new to me is extending the
foam wrap above the roof sheathing. By keeping the thermal barrier
separate from the structure, we achieve the continuity, avoiding thermal
bridges, and we eliminate the conflict with the wiring and plumbing threats to
the thermal barrier when structure is typically insulated in stud and rafter
cavities. </FONT></DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV><FONT size=4>If a finished grade plywood was used for sheathing, studs and
rafters potentially could be left exposed to avoid the cost of drywall or other
paneling. This requires a lifestyle choice. Otherwise, we would
sheath it with GWB to provide what people expect. </FONT></DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV><FONT size=4>First look at this approach seems like the cost would be
comparable to common practice framing, but achieve a much better
performance. </FONT></DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV><FONT size=4>Eli </FONT></DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV><FONT size=4></FONT> </DIV>
<DIV>Do people have experience with locating rigid insulation above the roof
</DIV>
<DIV>sheathing. Where I have cathedral ceilings, I am proposing installing
a </DIV>
<DIV>layer of rigid insulation above the roof deck and m</DIV>
<DIV> </DIV>
<DIV>Ratio of thermal resistance above condensing surface to</DIV>
<DIV>total thermal resistance:</DIV>
<DIV>16 ÷ 46 = 0.348</DIV>
<DIV>Temperature of Condensing Surface = (ΔT x 0.348) + Outdoor
Temperature</DIV>
<DIV> </DIV>
<DIV>-----Original Message----- </DIV>
<DIV>From: Benjamin Pratt</DIV>
<DIV>Sent: Friday, June 15, 2012 12:10 PM</DIV>
<DIV>To: Green Building</DIV>
<DIV>Subject: [Greenbuilding] Basement dehumidifier vs window AC
withdehumidifier</DIV>
<DIV>function</DIV>
<DIV> </DIV>
<DIV>I run a dehumidifier in the basement all summer. However, it does get</DIV>
<DIV>uncomfortably warm down there. I have an office that I use about one
a</DIV>
<DIV>week for several hours, and exercise equipment that use 3 times a
week</DIV>
<DIV>for an hour at a time.</DIV>
<DIV>I was thinking about putting in a widow AC (potentially with a</DIV>
<DIV>dehumidify function). Should i use this only when i need it, or</DIV>
<DIV>replace the dehumidifier. Which would be more efficient? If I am only</DIV>
<DIV>going to use it when i need it, i can will put in an AC that I
already</DIV>
<DIV>have. IF not, I will buy one with an AC dehumidify function. IT would</DIV>
<DIV>obviously be preferable to keep the basement at a comfortable</DIV>
<DIV>temperature.</DIV>
<DIV>-Ben</DIV>
<DIV> </DIV>
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