On Jul 27, 11:33*am, rangerssuck wrote:
On Jul 27, 11:25*am, rangerssuck wrote:

You've got a bunch of apples and oranges mixed up in your fruit salad.
In your attic, you have a roof which is being heated by the sun. Then
you have convection, which mixes up the attic air and brings it in
contact with the lower floor's ceiling. The object of insulation
material is to keep the air still, to (virtually) eliminate the
convection.

The object of some insulation, such as fiberglass batt, does for a
large part utilize trapped air to achieve it's overall R value. Rigid
insulation...not so much. But I agree with your point as far as
trapped air adding to the insulation value. Basically it's taking
another step down on the heat transfer hierarchy of efficiency -
taking the step down from convection to radiation through the trapped
air.

a properly vented attic space should have a considerably lower
temperature. Air is drawn into the soffit vents and expelled through
the ridge vent. Even better is a powered ventilator or, the push-pull
fans you mentioned. In the ventilated space, the sun-heated air is
replaced with cooler ambient outside air. This has nothing to do with
the insulation value of the air, just its temperature.

Besides the house cooling issues, it is generally a good thing for the
longevity of the roofing materials to keep the inside attic temperature
as close as possible to the outside temperature.

A reflective roof would make a huge difference. I don't understand the
fascination with black (or othr dark color) roofing shingles.

I think the dark roof thing has a lot to do with tradition and what
people are used to seeing. It also has to do with minimizing the
objectionable appearance of roof discoloration. A darker roof
usually 'wears' better.

Now, having said all that, I have a related question:

My attic floor is insulated, but rather poorly. Adding more insulation
in the attic would not be easy. I am getting ready to repair the
ceilings in two bedrooms. The plaster is badly cracked and there are
definitely some broken keys (plaster / wood lath). The standard
parctice is to install furring strips and sheetrock over the plaster.
What I'm thinking about is adding a couple of inches of ridgid foam
insulation between the plaster and sheetrock.

I'd use screws and plaster washers to secure the existing ceiling to
the joists. Then use screws with fender washers to secure the
insulation, and then screw the sheetrock through the wholew mess to
the joists with, say, 3 1/2" sheetrock screws. I CAN afford to give up
a couple of inches of ceiling height.

Is this plan at all sane?

Perfectly. Normally I'd be tempted to pull the old ceiling plaster
and lath, and start fresh, but with the insulation up there, and the
fact that it's the middle of the summer...I like your plan
better. Exactly how poorly insulated is your attic floor and
where is the house?

The rigid insulation at the ceiling level is a superior solution
overall. If your ceiling joists or rafters are on 16" centers, that's
getting near 15% of the overall ceiling area being taken up by
joists. Depending on your insulation, those joists might be a thermal
short circuit.

3 1/2" drywall screws won't work with the existing plaster and 2" of
rigid insulation and the new drywall ceiling. Even 4" screws are a
little short for my tastes in that situation.

R