"Wes Stewart" wrote in message
...
On 16 Oct 2005 19:13:29 -0700, wrote:

I have a 5 year old house. I can't remember the R-value, but I know
I've got the yellow insulation batts in my attic. Its what is required
for code, no more I'm sure
The insulation has paper on 1 side (down), and nothing on the exposed
side (facing up) in the attic. I was wondering if I buy the white
insulation from H.Depot (higher R-value) and put it down ON TOP of the
yellow insulation. I don't think it will compress it much - the bats
are light. Perhaps that will provide me with $$ savings?


Here's a (really) simple little thought experiment. Pretend you are
in a cooling situation so that you are trying to minimize the internal
heat gain.

Let's start with walls and roof with no added insulation and say that
the construction materials give an R value = 1. The heat gain comes
from the following sources:

1. Internal (appliances, hot bodies, etc.) 30%

2. Roof 40%

3. South wall (lots of overhang) 5%

4. North wall 5%

5. East wall 10%

6. West wall 10%

Assume the total daily heat gain for a typical day is 100,000 BTU, so
the distribution is as follows:

Internal = 30,000 BTU

Roof = 40,000 BTU

South wall = 5,000 BTU

North wall = 5,000 BTU

East wall = 10,000 BTU

West wall = 10,000 BTU.

Now, let's stuff some R-10 (U = 0.1) insulation under the roof. This
reduces the transfer to 10% of the starting point or 4,000 BTU.

The total heat gain drops to 64,000 BTU, or a 36% reduction. Hey,
this is progress, let's double the amount and really save some dough.

Adding another R-10 layer for R-20 total reduces the roof heat gain to
2000 BTU, for a total of 62,000 BTU, or a 38% reduction from the
starting point.

Wait a minute! We doubled the insulation (and its cost) and saved 2%.
Clearly, the other sources should be addressed before going overboard
with ceiling insulation. You can argue about the beginning
distributions, but the trend is clear. Unfortunately, building codes
sometimes take the position that if some's good, more's better, which
is complete nonsense.

I also find it amusing that some builders, as I've seen here in
Tucson, offer hype such has "R-40 ceilings, and R-30 walls" when 80%
of an unshaded west facing wall is sliding glass doors. The 20% shear
wall is R-30 alright, but the 80% is R-1.5 and might as well be a hole
in the wall through which you pour money.


Consider the gross simplification where all 4 walls and ceiling had the same
area and R factor (ignoring the floor). Doubling the amount of insulation
would decrease the heat lost by 50% over 20% of the area or a total of 10%
overall.

Since ceilings are generally better insulated than walls (which have doors
and windows) and (especially in a 2 storey house) have less area than the
total wall area, the actual improvement from doubling the ceiling insulation
I would expect to be much less than 10%.