[Greenbuilding] insulating on sheathed roof

[LarenCorie]

I wrote:

> That is dense-pack cellulose.
> It has an Rvalue of 3.8/in and requires no vent space,
> so the increase in Rvalue will be very impressive.

From: Tom Gorman <tom at honeychrome.com>

> Why do you say blown cellulose requires no vent space?

Hi Tom;

  I did not say that.  I said that "dense-pack" cellulose does
not require a vent space in "cathedral ceilings"  You may be
reading in something that I did not mean.  Loose fill cellulose,
and uninsulated attic spaces require venting, just like with
fiberglass insulation.  Foams and dense pack cellulose, that
will not leave an attic space below the roof deck, do not
require venting.  The reason for this, is that dense packed
cellulose stops almost all air leakage, where as fiberglass
needs venting because it is basically a furnace filter, with
no seal around its edges, and lets air easily pass, both
through and around it.  The irony of the situation is that
the venting pulls even more air through the fiberglass (or
any bat insulation) than without the necessary venting. But,
the air carries moisture, and fiberglass unlike cellulose has
no means of dealing with much moisture.  As a result, for
the same RValue rating, cellulose generally insulates about
35% better than fiberglass (26% less heat loss, according
to University of Colorado testing).  So, in most depths of
cathedral ceiling roofs, with the vent space filled, that means
you will get nearly double the effective insulation, as with
fiberglass, and its necessary vent space.

http://www.nuwool.com/products/technical/cathedral.html
http://nationalfiber.com/uploads/universe/docs/wysiwyg/documents/Unvented_Cathedral_Ceilings_and_Flat_Roof_Assemblies.pdf
http://www.applegateinsulation.com/conundrums.aspx

-Laren Corie-
Natural Solar Building Designs, Since 1975
www.LarenCorie.com
www.ThermalAttic.com

My home base: -LittleHouses-
http://groups.yahoo.com/group/LittleHouses

Read my Solar design articles in
-Energy Self-Sufficiency Newsletter-
Free at  www.rebelwolf.com

-WoodGas- Power from Wood. http://groups.yahoo.com/group/WoodGas

-Refrigerator Alternatives-
http://groups.yahoo.com/group/RefrigeratorAlternatives