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Re: Heat loss question



Robert Vesalga <rpv@compumedia.com> wrote:

>Is there a simple answer to this question?

Yes :-)

>If I were designing a house in a 32 degree climate, how much insulation 
>(in R-value) would it take to have no heat loss through the walls, while 
>maintaining a 72 degree interior? (Assume this to be like a walk-in freezer;
>doors, windows, infiltration, etc., notwithstanding. Just pure wall).

R-oo should do the trick, ie infinity. But that is expensive.

If the house were two stories, 16' x 32' x 32', ie about 3000 square feet
of surface area exposed to the weather, and it had no windows or air
infiltration, and there were two 100-watt people living inside, using
500 kWh of electricity per month, there would be a continuous internal
heat generation of (2x100 + 500x1000/(30x24)) = 894 watts, which is
equivalent to 894 x 3.41 Btu/hr/watt = 3000 Btu/hr. Since the temperature
rise from one side of a wall to another is equal to the heatflow times
the thermal resistance ("Ohm's law for heatflow"), the R-value of the walls
would have to be such that 72F-32F = 40 = 3000 x R/3000, so R40 should do.

If you had a 50 watt dog, you could use R38.

>Would the floor and ceiling be the same ?

The floor would have less heat loss than the ceiling, if the insulation were
the same, because the temperature under the floor is higher in wintertime. 
I usually figure the ground temperature as 55F and the thermal resistance
of the earth as R10, so you might want to put more insulation in the ceiling.

Or you might keep cows in the basement, as do the Swiss. Figure 11 on page 9.7
of my 1993 American Society of Heating, Refrigeration and Air-Conditioning
Engineers Handbook of Fundamentals (ASHRAE HOF), inch-pound edition, indicates
that at 72F, an average Dairy cow, say a 1000 pound Jersey, stantioned in an
enclosed stable, produces about 2000 Btu/hr of latent heat, so with 5 cows
in the basement, the above house would only need R = (72F-32F) x 3,000ft^2/
13,000 Btu/hr = R9 walls. This could probably be less with free-range basement
cows. In either case, you might want a vapor barrier under the first floor.

Hope this helps.

Nick

In article <3sa82u$eo7@news.compumedia.com>,
Robert Vesalga <rpv@compumedia.com> wrote:
>Is there a simple answer to this question?
>
>If I was designing a house in a 32 degree climate, how much insulation 
>(in R- value) would it take to have no heat loss through the walls, while 
>maintaining a 72 degree interior ? ( assume this to be like a walk-in 
>freezer; doors, windows, infiltration, etc., notwithstanding. Just pure 
>wall).
>
>Would the floor and ceiling be the same ?
>
>I guess that's 2 questions.
>
>Bob V
>--
>