[Greenbuilding] attic insulation in Seattle house

[RT]

On Mon, 14 Nov 2011 09:55:32 -0500, Sacie Lambertson  
<sacie.lambertson at gmail.com> wrote:

> The attic is currently nicely  dry--it
> does have some old loose material between some of the 2x4 joists,

Without knowing any specifics of the scenario/climate etc., I'd venture  
that the big reason why the attic is "currently nicely dry" is that it was  
likely very air- leaky and the minimal insulation allowed so much heat  
loss to occur that any moisture would easily be dissipated.

Installing a foot of cellulose insulation (presumably in and over the  
ceiling joists ?) will of course reduce the heat loss considerably so that  
the dew point of any moisture (assuming that achieving a good air barrier  
on the older home is difficult) will be in the insulation and there being  
a relatively large attic volume should allow that moisture to cook off  
(assuming that there are some sunny days when the attic would get hot) and  
escape via the mushroom vents (appropriately named) high up on the roof.

I'd be inclined to provide at least a few inlets for ventilation air lower  
down since the new 12 inch depth of insulation will likely be obstructing  
the pre-existing air-leakage points near the soffits.

I'm guessing that the dimension lumber rafters allow only a minimal depth  
of insulation at the rafter attachment points over the walls so I'd be  
inclined to use a better R-value insulation material at that location so  
as to avoid the potential for mould and mildew formation on the ceiling  
corners.


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada

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