RE: [GBlist] Passive Solar/Thermal Mass Comparison

[Speireag Alden <Joshua.M.Alden.91@alum.dartmouth.org>]

Sgrìobh Ray Zorz:

>I exchanged e-mails with a member of the Oak Ridge staff.  In a 
>nutshell, thermal mass is beneficial in predominantly warm clients, 
>and he also said you'd be better off not heating up the mass to 
>begin with.  In other words, my money would be better spent on less 
>mass and more R-value.  But that's primarily because I live in AZ I 
>think.

     Unless this is an oversimplification, I disagree.  If it's an 
oversimplification, then it's too simple to be of use.  Let me 
explain.  (Gavin just knew I wouldn't let this one rest, didn't he?)

     First, anything is thermal mass.  Even insulation has mass, and 
stores a bit of heat.  The sheetrock in a normal house provides some 
mass, as do all the furnishings, and the joists, and so on.  When 
people speak of *thermal* mass, they generally mean lots of mass in a 
small volume, which means either (a) water or (b) really heavy 
objects, like rocks, cement, and so on.  Water has more mass for a 
given volume than rock or cement, but it has an annoying tendency to 
go places it's not invited, or to culture small organisms and get 
green and cloudy.  Rocks just sit there, so generally they're more 
practically valuable as thermal mass.  Wood is also thermal mass, 
especially dense, heavy woods, but not as massive per unit volume as 
rock.

     There are, broadly, two ways to use thermal mass:  with and 
without insulation around the outside of it.  If you use it without, 
then you are relying strictly on mass effect to moderate internal 
temperatures.  That only works if the daily fluctuation runs both 
above and below the desired interior temperature, at least most of 
the time.

     It doesn't work so hot up here in the frozen north, where the 
exterior temperature stays below the comfort zone for months at a 
time.  Then your thermal mass becomes a sort of delayed-action 
conduit for heat, away from your body and into the house's 
surroundings.

     However, you can also put some thermal mass on the inside of some 
insulation.  Now you are greatly retarding heat loss to the outside, 
but you are permitting heat exchange between the thermal mass inside, 
and the air inside, and the people inside.  The thermal mass acts to 
cushion, or moderate, any temperature swings from opening and closing 
doors and windows, or from slow heat loss through the insulation 
layer to the outside.

     If you aren't going out of your way to pack vast amounts of 
thermal mass into the structure, but simply stick with putting in 
what's easy to put in, then this system works in all climates, with 
one caveat:  it's only an advantage in continuously occupied 
structures.  If you're going away for a week at a time, then you end 
up heating up the mass when you get in, and leaving that nice, warm 
mass behind you when you leave.  If it weren't there, the house would 
heat up faster when you occupied it, and with less fuel.  It would 
also cool faster after you turn the heat off.

     Now, if thermal mass and insulation were equally costly, you 
might find that you wanted to buy lots and lots of insulation. 
However, generally speaking thermal mass is pretty cheap compared to 
insulation.  Sometimes it's even dirt cheap, literally.  So they 
aren't equal, and if you have a good layer of insulation all around 
the house, you will find that additional insulation has a diminishing 
return, while a small investment in some thermal mass can make a big 
difference in stabilizing temperatures in the house, providing 
greater comfort and energy efficiency.

     Hope that helps.

-Speireag.
-- 
Speireag Alden, aka Joshua Macdonald Alden

...the last word in ignorance is the man who says of an animal or 
plant: 'What good is it?' --Aldo Leopold, _A Sand County Almanac_ 

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