[Greenbuilding] insulation vs. air sealing

[Norbert Senf]

At 12:09 PM 10/4/2010 -0400, Corwyn wrote:
>(snip)
>>I could do a before and after.
>>My hunch is that the stack effect decreases if 
>>you air seal attic before blowing insulation and increases
>>If you do not air seal the attic before blowing insulation.
>
>Stack effect is the pressure not the flow.  So 
>air sealing doesn't affect the effect itself, just the results of that effect.

The pressure varies with elevation in the house (or chimney).
Relative to outside pressure, it will be negative 
in the basement, and positive at the highest point,
if outside temperature is colder than inside.
Somewhere in between is a neutral pressure plane 
where there is no differential from outside.
The height of the neutral pressure plane varies, 
depending on how the leaks are distributed.
The neutral pressure plane "chases the leaks". If 
you introduce a very large leak, such as opening
a window upstairs, the neutral pressure plane 
will be somewhere around the bottom of the window.
There will be pressure driven flow, limited by 
the size and location of the inlet leaks.

Often, when people tighten a house they'll do a 
better job downstairs, because that's where you notice drafts
because air is flowing in. You don't notice it 
upstairs, where air is flowing out. This results in the
neutral pressure plane moving higher, and 
increasing the negative pressure downstairs. A fairly common
example of this is when fireplace spillage is 
caused by leaky potlights in cathedral ceilings.

A good way to find leaks upstairs is with a blower door and a smoke pencil.

Norbert

-------------------------------------------
Norbert Senf---------- mheat(at)heatkit.com
Masonry Stove Builders
25 Brouse Rd.
RR 5, Shawville------- www.heatkit.com
Québec J0X 2Y0-------- fax:-----819.647.6082
---------------------- voice:---819.647.5092