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<font face="Helvetica, Arial, sans-serif">Eli is correct re: the BSC
RR-108 document when he says "</font>The assumption, I guess is
that air will penetrate the rest of the assembly and show up at the
bottom of the foam."<br>
Based on extensive experience, air WILL penetrate drywall and air
permeable insulations, including dense pack cellulose.<br>
Exceptional workmanship and simple shapes may be made sufficiently
airtight, but cannot be practically relied upon. We have seen or
heard of endless failures of denspack cellulose and fibreglass
unvented roofs. Either the interior air must be kept very dry
during cold weather or condensation must be controlled.<br>
There are two solutions to controlling condensation by using
insulation:<br>
Either <br>
1. place enough foam outside the air permeable insulation<br>
or<br>
2. use sufficent spray foam between the rafters<br>
to warm the surface above the expected average winter dewpoint of
the interior air.<br>
Allow drying to the inside for any moisture that might be in the
studspace by avoiding very low interior vapor barriers (eg poly, alu
foil) or finishes.<br>
<br>
The old rule of 1/3:2/3 R-value on the interior:exterior of a vapor
barrier mentioned by Rob Tom has now been superseded by experiments
and calculations and does not apply to most walls or roofs (but was
often very conservative)<br>
<br>
BSD-163 provides an enhanced explanation of how to calculate the
appropriate ratio of R-values to avoid condensation and some of the
basic underlying physics.<br>
<br>
<br>
<br>
<div class="moz-signature">Dr John Straube, P.Eng. <br>
<a class="moz-txt-link-abbreviated" href="http://www.BuildingScience.com">www.BuildingScience.com</a></div>
<br>
On 12-07-06 12:30 PM, Eli Talking wrote:
<blockquote cite="mid:33A93518B8FC4269A2BC9212DC1A4011@NewCentury"
type="cite">
<br>
<br>
At <a class="moz-txt-link-abbreviated" href="http://www.Buildingscience.com">www.Buildingscience.com</a> article RR-0108_Unvented_Roof_Systems
describing unvented roofs they reference their concern that the
foam layer be of sufficient thickness to insure that when air
reaches the bottom of the foam, the surface temp will be above dew
point. The assumption, I guess is that air will penetrate the
rest of the assembly and show up at the bottom of the foam.
However, if the framed rafter was filled with dense pack cellulose
or fully stuffed fiberglass or roxul, no wires or plumbing are
installed, and the drywall is managed to be an air barrier, where
would the air come from? With no holes in the foam layer, there
will be no driving pressure to deliver moisture laden air to the
bottom of foam. However, more typical rafters with wires,
recessed electric boxes, unsealed plumbing vents, this would
provide a path for air to get to the bottom of foam, particularly
if cold.
<br>
<br>
If this is a problem, this would be a good reason for all the
insulation to be rigid above the roof sheathing, one of the
scenarios I described for use with an attic truss. See below
<br>
<br>
They use a formula of Ratio of thermal resistance above condensing
surface to
<br>
total thermal resistance:
<br>
16 ÷ 46 = 0.348
<br>
Temperature of Condensing Surface = (ΔT x 0.348) + Outdoor
Temperature
<br>
<br>
Any thoughts on this recommendation?
<br>
<br>
</blockquote>
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