[Greenbuilding] insulation vs. air sealing

[Brian Uher]

 Hi David-

It seems that the time to ventilate is when you need to evacuate
moisture. You need to evacuate moisture when it builds up or when it's
new equilibrium is above the holding capacity of the materials - when
the settling point supports mold growth.

Winter air is dry, so ventilation is not needed in winter-as it warms,
it gets even drier, relatively speaking.
But keeping interior vapor out of the attic is critical.

Summer air is humid, but if the attic matches the temperature of the
exterior, the humidity matches as well.  Condensation does not occur, so
accumulation of moisture in the wood should not occur.
But keeping the interior cooling (both via exfiltration and conduction)
out of the attic is critical.

So, maybe you really *never* need ventilation but *always* need airflow
and thermal flow control. 

The one exception: rain penetration will wet the wood.  But that
situation is catastrophic and a repair since it means your design sucks
or your roof is leaking.

Thoughts?
-Brian


On 10/3/10 11:09 PM, Futureship wrote:
> Good question ? What are the conditions in the attic that would 
> Merit ventilation after air sealing 
> And insulation ?
>
> On Oct 3, 2010, at 1:27 PM, 
> David Wentling 
>
>> "With the airseal measures in place, and the moisture contribution from 
>> ex-filtration to the attic eliminated, the question is do you need to ventilate?"
>>
>> David Wentling
>>
>> From: Futureship <futureship0000 at hotmail.com>
>>
>>
>> I have a question with regard to 
>> Your results:
>>
>> In the first case you described that "Results were 15% higher energy usage with 
>> 10% greater leakage."
>>
>> How does a blower door measurement
>> Tell you 10% greater leakage? The 
>> Blower door measures holes in 
>> The building envelope . Assuming no
>> Air sealing was done the blower door 
>> Would not measure a 10% greater leakage. I would argue the increase in stack 
>> effect In this case caused an increase in pressure thru the same holes therefore 
>> causing more airflow thru the same holes. 
>>
>>
>>
>>
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