Cob: Re: venting attics

[Darel Henman]

Interesting letter from Ray, which I include below.

So, in considering the differences between an open roof or open beamed
roof, meaning that the ceiling and roof are the same, metal roofing
would unquestionably have an adverse effect on the thermal performance
of the structure, due to its high conductivity and also radiative heat
transfer.

Conventional roof systems with attics between the roof and ceiling, in
where the upper ceiling is to be insulated and the attic vented the lose
due to the metal shingles would be less, but you would still have some
in my opinion, both radiative and conductive.  This could be reduced, by
installing a radiant barrier with the sunny side down you can mitigate
heat loss.

For summer time you would want the radiant barrier on top of the
insulation to keep the radiant heat from entering the insulation.

Interesting article for hot area locals or hot summer locals is:
    http://www.ci.austin.tx.us/greenbuilder/glfs_radiantbarrier.htm

Since modern metal roofs can be coated you can avoid the "Cat On A Hot
Tin Roof" type of heat that can build up in an unfinished metal roof.
Also the following 10 page document shows that metal roofs-- in this
case industrial roofs-- due to heat raditation out radiation at night
can cause the roof to be 21 degrees fahrenheit BELOW outside ambient
temperatures.  Condensation can form in the inner face of the outer
metal cladding easier, and there it can freeze or corrode fixings.
     http://www.epic.uk.com/thermal.pdf

From:  
http://www.consumerenergycenter.org/homeandwork/homes/construction/roofing.html

"Research by the Florida Solar Energy Center in 1985 showed
         that metal absorbed 34 percent less heat than asphalt
         shingles, and homeowners switching to metal roofing
         reported saving up to 20 percent on their energy bills. "

But the problem with this is it says "absorbed" and not conducted or
radiated.  The article then goes on to say that metal roofing is made
from between 60 percent to 65 percent recyclable material.  But energy
is required to recycle it.


For those who wish to now a little more about metal items, see the U.S.
Department of Energy's following page of FAQ about metal components and
a free software to help make comparisions.
   http://www.energycodes.gov/support/metal_faq.stm

So consider all the issues before choosing any particular type of
roof.   Ease of maintenace, fireproofness, etc are also important, as is
sensitivity to weak magnetic fields and other considerations.

A note on Ray's mail below, he shares his experience about snowey and
freezing climates.  He mentiones about keeping the attic cold, to
prevent melting snow build up to prevent ice dams.   Anybody else have
any ice dam stories to tell.

Darel
 

Ray Luechtefeld wrote:
> 
> Hi Darel,
> 
> Thanks for your informative comments on the cob list.  I'm writing back
> personally since my email changed and I don't think I can reply to the
> list.
> 
> In cold climates it is very important that the attic be kept cold.  If
> heat gets to the roofs upper surface it will melt any snow that has
> accumulated on the roof.  The water will then run down to the eaves
> (which are generally colder) and then freeze.  After time you get what
> are called "ice dams" along the edge of your roof.  After the ice dams
> form runoff will back up underneath your shingles (or whatever you are
> using) and leak into the house.
> 
> If your attic is not well ventilated the only option you have is to
> climb up on your roof and shovel it every so often during the winter.
> This is not pleasant (as I know from 8 years of personal experience in
> New Hampshire).
> 
> Feel free to post this to the list if you would like.
> 
> Ray