[Greenbuilding] attic insulation in Seattle house

[JOHN SALMEN]

I have looked at a lot of attics (all vented) and one of the things I have
noticed in pretty well all of the attics is some blackening of the
sheathing. I have also noticed that the moisture evidence is more
predominant on the shaded side of the roof.  This makes sense if you
consider solar exposure heating the roof and generating convective airflow
at the sheathing line drying the sheathing.  I don't think roofs always vent
at the ridge terribly well (lot of turbulence and conductive loss before the
airflow even hits the ridge and backdrafting from the exterior airflow)
which may be why there is such evidence of condensation on the shaded sides.
Just guessing.

 

The other problem though with a roof that is ridge or top vented only is
that it can function as a solar chimney - depressurizing the attic somewhat
and pulling air from the interior - which is why you would need an intake at
the eaves (somewhere) to balance it.  The problem with an older building is
that you also need a slightly higher ratio of insulation at the eave/wall
juncture than is in the wall - so really the only solution is to install
vents (plastic type) in the actual roof a foot or so up from the wall.

 

The other option is to close off all the vents and do a decent job of
sealing the interior at the drywall level. You still have air movement due
to heating of the roof but the energy is dissipated by conductive loss to
the sheathing and framing and with the exception of metal roofing drying
will happen to the exterior. Some people might not like this option and
while the jury is out on that one the codes still require ventilation and
require a ratio of both eave and ridge.

 

That said  - I think the obvious place for roof ventilation is between the
sheathing and the roofing to provide both drying and cooling. If you look
historically at tile roofing (rounded shapes) or even old shake roofs they
worked wonderfully to allow for continuous air flows keeping things dry and
roof temperatures down.

 

 

 

 

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Sacie
Lambertson
Sent: November-14-11 6:56 AM
To: Green Building
Subject: Re: [Greenbuilding] attic insulation in Seattle house

 

Back to the question of insulating an attic in the PNW (Seattle) I have
found nothing from Lstiburek specifically addressing this issue in this
area.  This 1906 house has those small mushroom vents high up the roof but
nothing down low.  No soffit vents.  The attic is currently nicely dry--it
does have some old loose material between some of the 2x4 joists, material
that was probably across the attic floor at that little depth before the
house was updated.  Those folks never got around to insulating the attic
floor except where a new ceiling was put in below, where they put in
fiberglass batts above. (We will take these out, but leave in the loose
stuff).  

Without a clear context, below is what L. does say about ventilating.  I'm
inclined to seal the perimeter to the degree I can and then simply blow in a
foot of cellulose over everything.  Several of you have suggested this,
however I would love to hear from John Straube on this issue.

thanks, Sacie

"Is it better not to vent a roof?" 

Lstiburek's response: "I like to vent whenever I can. But when roof venting
cannot be done so that it will be effective or when it is difficult (e.g.
with hipped roofs or multiple dormers) it is better to have an unvented
assembly."

And from cmhc:

Attic computer models show that attics in damp coastal climates may actually
be drier with less ventilation.





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