[Greenbuilding] Quick Question re Thermal Mass!

[Mary Bull - Greenwood Earth Alliance]

Thanks so much, Corwyn--the room is actually circular and the walls are
almost 11 ft high. I'm not sure how much direct sun the walls versus the
floor will get. The windows face S/SE and cover about 36 SF (3' wide x 5'4"
high), directly opposite them is the kitchen strip--wood cabinets preventing
direct contact with the opposite wall, so you see... It sounds from your
calculation, the slate will add significant thermal mass. Since we will have
no other heat than passive solar and a wood-burning stove in this room, I
think we should stick with the slate, don't you?! Thanking you ever so
kindly! --Mary


Mary Bull, Co-director
Greenwood Earth Alliance, Save the Redwoods - Boycott the Gap Campaign
252 Frederick, San Francisco, CA 94117 http://www.gapsucks.org
Chalice Farm and Sustainable Living Center, 748 Montgomery Rd, Sebastopol CA
95472
415-731-7924 - 415-509-1188 chalicenew at earthlink.net


----- Original Message ----- 
From: "Corwyn" <corwyn at midcoast.com>
To: "Mary Bull - Greenwood Earth Alliance" <chalicenew at earthlink.net>
Cc: "Greenbuilding" <GREENBUILDING at LISTSERV.REPP.ORG>
Sent: Monday, October 15, 2007 2:15 PM
Subject: Re: [Greenbuilding] Quick Question re Thermal Mass!



On Oct 15, 2007, at 16:05, Mary Bull - Greenwood Earth Alliance wrote:

> Greetings, Greenbuilders!
>
> I hope this email finds you well!
>
> We are having to make a decision about flooring for the "great room"
> (420 SF--not so great, really ;-) where all the south light hits in
> winter. I have assumed for a while now that it must be of some sort of
> masonry (we are thinking slate tiles now). However, my new natural
> builder says that these will not provide the mass needed for adequate
> thermal absorption because they will not be thick enough (about an
> inch or so including the mortar they are set in). However, he says
> that the interior walls themselves coated with a thick layer (2 inches
> or so) of earth-clay will provide enough thermal mass. I am not
> convinced, because not as much wall will be receiving direct sunlight
> as floor.

Slate:
using a specific heat value of .2 (for general stone) and a density of
167 lb/ft^3 for a 1 inch thick over an area of 420 ft^2 we get: 33.4
BTU/ft^3 and 35ft^3 of material = 1169 BTU / ºF.

Clay:
using a specific heat value of .22 (presumably wet clay) and a density
of 63 lb/ft^3, for a 2 inch thick layer over 656 ft^ (sqrt(420)
(assuming a square room) * 4 walls * 8 feet high) we get: 13.86
BTU/ft^3 and 109 ft^3 of material = 1515 BTU / ºF.

So you would get 30% more thermal mass from the walls as from the floor
(All Else Being Equal).  However, the general rule of thumb is that
thermal mass in direct sunlight performs about as well as 6 times as
much out of direct sunlight (since it can reach a temperature above
room temperature).

So what percentages of floor and wall are getting direct sun, for how
long?

Thank You Kindly,

Corwyn

-- 
Corwyn
Kermit didn't know the half of it...
http://www.greenfret.com/
corwyn at greenfret.com