[Greenbuilding] roof venting

[Chris Green]

Nick Pine wrote:
> Chris Green writes
>
>   
>> You can start off with this overview:
>>     
> http://www.roofhelp.com/ventilation_main.htm 
>
> But more IS better, no? 
Seems like it, but there's probably a point at which there's no return 
for increasing the area of venting.
> Adding vents between soffit and ridge vents 
> can only raise the total attic airflow and lower the temperature.  
>   
I've never actually seen any ridge venting on any house in my region. It 
doesn't seem to be done around here.
>> As a basic rule of thumb, you need about 1 square inch of total net 
>> ventilation per square foot of floor space.
>>     
>
> Sounds good in summertime. Or maybe not... 10 years ago I replaced 
> my 1820 stone farmhouse 
Wow, that's old. The oldest building in all of B.C. is a tower that 
dates from 1843  or so. The oldest house probably dates from 1860- 
something.
> <snip>
>> ... 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.
>>     
>
> You might exhaust the swamp cooler into the attic ("up ducts")
> to keep it cooler and move the cooler indoors 
We use one of those portable things the Aussies came up with, and it's 
in the living room upstairs. It gets put back in the store room after 
things cool down. I could probably  eliminate the need for that simply 
by building a vine arbour on the long wall facing the south-west. Then 
the stucco wouldn't bank as much heat during this period. The grapes 
would do well.
At least until a bear shows up for his snack... :-)
> <snip>
>> 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... slowly carbonizing.
>>     
>
> Wisconsin code officials forced Soldier's Grove solar attic owners 
> to line them with drywall to avoid fires, until the town pharmacist 
> pointed out that their carbon vs time/temp graph was in degrees C 
> with a log scale, and they were interpreting it as if it were in 
> degrees F with a linear scale and wrongly predicting fires every 
> 20 years, vs every 20 centuries :-)
Admittedly, this factor is not much of a potential problem by itself, 
unless there is also moisture trapped in the unvented roof because of 
leaks or unsealed ceilings.  But I have seen photos of rafters and roof 
decking that had been scorched by localized excess heat near 
uninsulated  metal chimneys. Again, that's not likely to happen with 
modern heating systems. You don't see many black-pipe chimneys these 
days. Except the one in my woodworking shop...

Cheers,

Chris Green.