[Greenbuilding] Above Roof Sheathing Rigid Insul

Eli Talking elitalking at rockbridge.net
Wed Sep 5 11:22:43 PDT 2012

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.). 

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.  

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.  

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.  

First look at this approach seems like the cost would be comparable to common practice framing, but achieve a much better performance.  


Do people have experience with locating rigid insulation above the roof 
sheathing.  Where I have cathedral ceilings, I am proposing installing a 
layer of rigid insulation above the roof deck and m

Ratio of thermal resistance above condensing surface to
total thermal resistance:
16 ÷ 46 = 0.348
Temperature of Condensing Surface = (ΔT x 0.348) + Outdoor Temperature

-----Original Message----- 
From: Benjamin Pratt
Sent: Friday, June 15, 2012 12:10 PM
To: Green Building
Subject: [Greenbuilding] Basement dehumidifier vs window AC withdehumidifier

I run a dehumidifier in the basement all summer. However, it does get
uncomfortably warm down there. I have an office that I use about one a
week for several hours, and exercise equipment that use 3 times a week
for an hour at a time.
I was thinking about putting in a widow AC (potentially with a
dehumidify function). Should i use this only when i need it, or
replace the dehumidifier. Which would be more efficient? If I am only
going to use it when i need it, i can will put in an AC that I already
have. IF not, I will buy one with an AC dehumidify function. IT would
obviously be preferable to keep the basement at a comfortable

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