[Greenbuilding] roof venting

[Chris Green]

Anja S Kollmuss wrote:
> Hello,
>
> I am regularly asked by people about roof venting (hot roof versus 
> vented roof) I have not been able to find much research on this. Could 
> you recommend some good resources on the topic?
>   
You can start off with this overview:
http://www.roofhelp.com/ventilation_main.htm
If you google for "roof ventilation guidelines" or some variation of 
that, there's lots more links to be found, and other listmembers might 
have other pages to recommend. There's tons of it out there since this 
is one of the more important topics in building, right behind the need 
to build things which won't fall down on peoples' heads.

As a basic rule of thumb, you need about 1 square inch of total net 
ventilation per square foot of floor space. I believe that's the figure 
in the new building code. The above site uses 1/150th as the figure.

Leaving the roof unvented and able to heat up is a bad idea and the 
building inspectors will probably order the work redone. Even if they 
don't, or if you build in some jurisdiction without inspectors, the heat 
will radiate downwards into the living area at the hottest time of the 
year. It will cost money and energy to remove that heat and moisture.
The house I live in had undersized vents and the attic would reach well 
beyond 150F. The main floor required cooling with a swamp cooler for at 
least a month during the hot season (and I live in Canada...). After the 
new vents were installed, the swamp cooler only has to run for a few 
days or a week at the worst of times. The new vents are power vents (my 
landlady's son is an industrial electrician...) run off of a thermal 
sensor. Vents powered by the wind would use less energy, but on the 
other hand, some days there's no air flow to drive the turbine vents...

One of the lesser-known long term effects of subjecting rafters and 
other wooden roof structures to such elevated heat is that in time the 
wood loses strength. On a nano-scale, it is slowly carbonizing. Not 
good, especially if the roof carries a heavy load, say clay or cement 
tiles. If there's a lot of moisture present, the warmer conditions allow 
for various decay mechanisms to work, and metal can corrode. Even more 
Not Good.

Cheers,

Chris Green.