[Greenbuilding] R-value --- not the entire story

[Patricia Ewanski]

John,

Thank you! I'm looking into this. I also posted the question somewhere else
and someone said to look at thermal diffusivity. 

Thermal Diffusivity
__________________
Alpha = k/(p*c)
k: thermal conductivity
p: density
c: specific heat capacity

I think that you were referencing the (p*c) part of the equation when you
said " x pounds per square foot of material times y heat capacity for that
material"?? Tell me if I'm wrong. 

So, if I can't find the specific heat capacity, I calculate it using the
European time constants that you referenced? Then I plug that into the
equation, assuming I can find the density and thermal conductivity? Question
marks after all these statements because I want to verify my logic with you.


It does sound a bit scary to me but I guess I'd like to know what the
correct way is to do this. If I then find out that the data isn't out there
to support my method, well, then I'll back down. But it's worth a shot. 

Thanks again!
Patricia 


-----Original Message-----
From: John Straube [mailto:jfstraube at gmail.com] 
Sent: Monday, February 09, 2009 11:45 AM
To: patricia at ewanski.com
Cc: 'GB REPP'
Subject: Re: [Greenbuilding] R-value --- not the entire story

There is no universal mass-enhanced R value. The "enhanced" depends on  
the building design and operation and climate and site.
The best you can do I think is to find out what the time constant of  
the wall is (the number of hours it takes for a wall to respond to a  
temperature drop of the outdoor air).  This is quoted in some European  
publications
You could calculate the "heat capacity inside the insulation" and use  
that. For example, x pounds per square foot of material times y heat  
capacity for that material.  This gets pretty rough.

On 9-Feb-09, at 1:36 PM, Patricia Ewanski wrote:

> Hi,
>
> I'm trying to understand/compare wall technologies. One axis upon  
> which I
> want to compare technologies is 'R-Value'. After doing some reading  
> and
> discovering that there are different kinds of 'R-Values', I decided  
> that it
> would be best to obtain the 'Mass-enhanced R-Value' for each wall type
> because that value takes the buffering effect of thermal mass into
> consideration. The thing was...I could not find reliable 'Mass- 
> enhanced
> R-Values' for all of the wall systems.
>
> So, then I figured it was time to possibly find a second and/or third
> variable that I could use to compare the different wall systems that  
> would
> take this thermal mass effect into account. So, my question  
> is...what second
> or second/third parameter(s) do I use to compare these wall systems?
>
> My end goal is to create a list of 8-10 wall systems, ranked from  
> most to
> least energy-efficient for Southwestern climates (i.e. Tucson, AZ).
>
> Any thoughts on this?
>
> Thanks,
> Patricia
>
>
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Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture
www.buildingscience.com