[Greenbuilding] attic insulation in Seattle house

[JOHN SALMEN]

The best study I have seen on attic ventilation was a canadian cmhc study
that set up trial houses in bc wet zones (similar to Washington) and in our
Midwest climate. The study sealed attics and ventilated attics looking at
moisture control. The end result was basically that exterior ventilation was
outweighed by interior moisture migration to the attic area.  There was
virtually no difference between vented or unvented assemblies in both
climates. The best answer I have ever gotten about attic ventilation is that
‘it is not science – it is history’.

 

So yes I think  the general consensus is that the major concern in terms of
building science is air sealing between the conditioned space and the attic
as well as adequate ventilation of the conditioned space. It still leaves
the question as to attic ventilation.

 

In the PNW ventilation airflow in an attic space consists of drawing pretty
moist winter air through the cavity between insulation and the roof
sheathing.  It defeats common sense but that is still the state of the art.
In an older house the soffit venting would be entering the roof space at the
wall junction through a 4” cavity (2x4 truss or roof framing). To allow for
the ventilation that means the insulation is tapered to 2.5-3” which is not
adequate so typically there is a lot of condensation and molding of
sheathing at the eaves.  That is what I have seen in numerous buildings. The
other concern for me with airflow is reducing the effectiveness of the
insulation – but supposedly with cellulose that is less of a consideration
than it would be with fibreglass

 

Codes still require ventilation here but as mentioned I think that is
primarily in consideration of how difficult if not impossible it is to
achieve an airtight ceiling in a living space. I am surprised to hear that
Listiburek is adamant about airflow for the PNW. I would still argue that
drawing moisture laden air into a potentially leaky attic space is an
invitation for trouble. . I think Listiburek  is considering new
construction and the fact that builders can’t create good air seals.

 

I have had to comply with soffit ventilation in an older renovation at
ceiling level so as a result I compromised and I specified a foam board that
was ½” shy of the roof sheathing along the perimeter (3x6 over the top plate
so it ran a few inches in) cellulose was blown in and roughly tapered to the
foam edge. Inspectors argued that they wanted 1 ½” clearance but I argued
that the ½” met the roof soffit/ridge ventilation ratio and they accepted
that (at the time). Normally I would  look at insulating at the roofline if
allowed or create a vent gap at the roofline and insulate.  This allows for
conditioning the attic space as it is good storage and I like the building
roofline to be part of the envelope.

 

Roofs are complicated in our older buildings and can turn into a mess. In
some ways ignorance is a blessing. Just blow in 12” of cellulose or more and
mostly it creates a decent air seal and blocks the soffit ventilation and
the building will be fine.

 

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Sacie
Lambertson
Sent: November-10-11 6:14 PM
To: Greenbuilding
Cc: Dan
Subject: [Greenbuilding] attic insulation in Seattle house

 

All, I would appreciate advice.  I am going to Seattle to help our son
install insulation in an unconditioned attic in a 1906 house that has been
completely upgraded with insulation everywhere except the attic.  (As an
aside, I was amazed to find in Seattle NO pre-50s houses on the market had
attic insulation and most had none in the walls either--pretty amazing).

We plan to carefully seal any openings between the conditioned space below
and the attic, then blow in however much cellulose we need.

J. Listiburek is quite adamant about the need for a wash of air via
continuous soffit ventilation when insulating this way.  But no one in the
Seattle marine climate has continuous soffits vents.  What is our
alternative?  What else should we be concerned about?  Ice dams btw are not
a problem in Seattle.

L. doesn't like one to store anything in the attic either, but if one has
walk-in space up there, why not?  What are the considerations here?

Thanks,  Sacie

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