Cob: Metal Roofing

[Darel Henman]

Shannon,
   Thanks for the explanation about the air channels under the roof's
upper skin you were talking about. I thought you were talking about an
attic vent or two.  But, I see from your description below a channel for
air to flow under the upper roofing material and above the surface of
the second solid roofing top? 

"Shannon C. Dealy" wrote:
........... snipped ....
> ...  requires ventilation UNDER the outer skin, so much of what would be gained
> by the shingles on top is lost due to air flow underneath, and in
> addition, the relative insulation value of a layer of shingles is dwarfed
> by the insulation value of the actual insulation layer below. 

........................ snipped

> Perhaps you will see my point if you examined it from this perspective,
> imagine a roof that sits in the air 10 feet above the insulation and
> vapor barrier, with nothing in between them, no walls, just air flowing
> through, the roof keeps the rain off but doesn't do much else.  In this
> case, clearly the roof material makes absolutely no difference in the
> insulation value to the house regardless of what it is.  Now if you put
> this roof down onto the house, but put in vents to allow air to continue
> to flow through between the roof and the insulation, you will continue to
> get much (though not all) of the same effect, which will continue to kill
> much of the insulation value, and while you will gain some (though more is
> likely to come just from the layer of air), what you get is going to be
> very small relative to what the insulation below provides.

Yes you will get the addition of two air film resistance for each
interior surface, which value depends on the air flow speed through the
channels.

> 
> This simply isn't true, you statement reads like the material the
> outer layer of your roof is made out of is the primary source of
> insulation in the roof, when frankly, you get far more insulation value
> out of the plywood underneath the singles than you get out of the singles
> themselves, and when the roof vents are taken into account, the effective
> insulation value of even the plywood is dwarfed by the insulation layer
> installed below (and there is nothing that says you can't use plywood
> under the metal roofing material).

I was only trying to indicate that every little bit helps.  Not that it
is the primary insulation element.

Darel