[Greenbuilding] skepticism about mass wall values

[Topher]

On 4/23/2013 3:44 PM, Alan Abrams wrote:
> f'r'instance, a claim was made that a 12" AAC wall has a functional 
> R-value of over 20, much greater than its "steady state" value of 
> something like R-12...
>

In the high mass building I did energy analysis for, I used the 
Minnesota data for a Maine climate, and the final results differed from 
my predictions by about 15% (with large error bars on a number of 
things, of course).  Certainly even in my climate there is a benefit 
from high mass walls, IMO.   The results would have been far more 
divergent if no benefit was evident.  The occupants are very happy with 
the house, if that is any indication.

I would view with /profound/ skepticism any claim such as the one you 
quote, which lacks climate reference.

The trouble with peak load is that it is generally used to size HVAC 
equipment.  As such, I would NOT recommend ignoring the affect of high 
mass.  The 'time to heat' a high mass house can be an order of magnitude 
longer than a light house, and its return to ambient is similarly long.  
At this point, I want to know the owner's occupancy patterns, and 
tolerance for temperature swings. If they are present for much of the 
day and prefer a stable temperature, one can size HVAC equipment based 
on an average heat loss (i.e. no considerations of mass).  If occupancy 
is varied, or temperature is expected or desired to vary, HVAC equipment 
will need to be reassessed and probably increased in size.

Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
topher at greenfret.com

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