[Greenbuilding] Critical Temperature Location (was Re: mounting rigid insulation above sloped framedun-vented roofs.

[RT]

On Fri, 06 Jul 2012 12:30:23 -0400, Eli Talking  
<elitalking at rockbridge.net> wrote:

>
>
> At www.Buildingscience.com article   RR-0108_Unvented_Roof_Systems  
> They use a formula of 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
>
> Any thoughts on this recommendation?

Back in the early 1980's when double-stud walls were popular amongst the  
energy-efficient buildings set, in this part of the world (Ottawa ~8750  
HDD/yr before era of climate change) the rule of thumb was to have at  
least 2/3 of the total R-value of the section (roof or wall) on the "cold  
side" of the vapour-impermeable surface (ie the "continuous air/vapour  
barrier" as it was called up in plans back then).

This meant that you could place/detail the air barrier on the outside of  
the inner 2x4 stud wall (ie nominal R-13) fully insulate the balance of  
the thickness of the wall (ie typically nominal R-26 or better while it  
was still laying horizontal on the floor deck during framing) and after  
the shell was erected, have the sub-trades install all of their stuff into  
that inner 2x4 wall without having to worry about them compromising the  
integrity of the air barrier with penetrations.

In subsequent years having opened up 10 or 20 yr-old double-stud walls  
that I had built back then (ie to do additions & renovations) there was no  
evidence to suggest that that rule of thumb is not valid. ie The framing  
and insulation showed none of the tell-tale signs of moisture-related  
issues.

Of course, it might also suggest that the builders had their anal  
retentiveness quotients "ARQ" (aka "pucker factor") WRT air-sealing set  
very high so that no air-leakage whatsoever occurred. How likely is that,  
eh ?

However if one has a look at the chapter on "Moisture in Building  
Construction" in the ASHRAE Handbook of Fundamentals, in particular the  
section that illustrates how to graphically determine the critical  
temperature location, that  80s rule of thumb is validated.


-- 
=== * ===
Rob Tom					AOD257
Kanata, Ontario, Canada

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