[Greenbuilding] [BULK] Re: vented roofs

[George J. Nesbitt]

    Too little attic venting is a problem, I have that problem at my 
house. I would not recommend powered attic vents, there is little/no 
evidence that they work, although people that have them swear by them. I 
don't recommend "whirly-birds" either (its crewel to spin the poor birds 
around like that), they have a tendency to leak water, and can be noisy. 
Ridge and soffit vents is the best attic venting strategy, eyebrow vents 
work too, but stick out more, although it is possible to do venting that 
blends into shingle roofs.
    The real issue with attics is radiant heat transfer, the roofing 
(usually black) gets 75 deg F above ambient air temperature, and it 
transfers it's heat through the attic air to the insulation (or ceiling 
if there is no insulation) and heats it up, which heats up the ceiling, 
and re-radiates it into the house, increasing cooling load and reducing 
comfort.
    There are 2 solutions, radiant barriers under the roof sheathing to 
reflect the heat back up and out, this does increase the temperatures of 
the roofing materials and will shorten life some. The better solution is 
a cool roof, light color is best, but they have developed pigments that 
reflect even in darker colors (although not as well). I painted my roof 
white and my house went from uncomfortably hot, to comfortably warm, I 
have almost no venting, and have some walls that have no insulation (or 
radiant barrier) up against the attic (Oakland CA isn't exactly a hot 
cooling climate).Cool roofs are good at helping keep the ambient air 
temperature cooler (which is a problem in denser areas) and is a partial 
solution to Global Warming by reflecting more of the suns energy back 
out the atmosphere.
    Keep Cool!
      


Chris Green wrote:

>Lawrence Lile wrote:
>  
>
>>Despite some strong arguments on this list, I
>>could not be convinced to install a "hot roof" nonventilated attic.  I
>>still don't know if I made the right choice, but that's how we are
>>building it.  
>>  
>>    
>>
>The 1960's era house I live in had screened vents that were way too 
>small, and in the hottest part of the year the heat in the attic would 
>build up past the 150*F range and radiate into the top/ main floor. When 
>my landlady's son put in power vents, set to turn on at a certain 
>temperature,  the attic temps dropped and the upstairs apartment became 
>much cooler compared to previous years. Good quality non-powered 
>'whirly-bird' or turbine vents would be nearly as effective and use no 
>electricity.
>
>Chris Green.
>
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