[Greenbuilding] Exterior insulation retrofit
terrain at shaw.ca
Mon May 23 19:00:13 PDT 2011
Thanks John for the clarification. You threw me for a bit as I thought you
were talking about a 6mm discounting the spacing and area. Thanks for the
graph and the comment about open systems - that is really what I have been
looking for research and clarification on - that an open bottom (drained)
with a dryvit type system is not subject to large airflows. I had allowed
for 20% drate in working out the wall layers for this project because of the
siting and orientation (hillside open to prevailing).
As for the integrity of the waterflows across the envelope?? Is it better to
have a material like ply sheathing that has some integrity for drying or
gypsum which loves to hold onto water?
The drainmat is being used here extensively in residential and rehab.
From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of John
Sent: May-23-11 5:09 PM
To: Green Building
Subject: Re: [Greenbuilding] Exterior insulation retrofit
10 mm grooves is not the same as 10 m continuous. A rough approximation of
the equivalency is the area of the gap. So a 10 x 10 gap every 100 mm would
be equal to 1 mm. In reality, the flow is lower than this.
We measured Dryvit and Sto ribbon systems and did not find gaps of over 3
mm. And when tested for airflow, they were much smaller. Equivalent to 1-2
mm. See attached graph. Wall 1 is the standard dryvit, Wall 4 is grooved
insulation. Test was pressure vs flow and then correlated to equivalent wide
So the two market leaders are testing and and certifying systems of this
type. Could be you are seeing something else, but I deal directly with the
people who are in charge of technical development for these systems and they
dont think they are installing larger gaps.
The drainmat solution I am aware of and this is a bit of problem. This was
all driven by the 10 mm drained and ventilated gap developed for siding and
In short, I would not use insulation over large gaps, like 6 or 10 mm, and
sure would not try to figure out the derating: they would depend too much on
wind gusts, exposure etc.
If the wind hits the building just so, a gap of 10 mm vented top and bottom
can result in massive flow, with derating of insulation of something like
90%. Research results are clear for these types of systems.
It is for the drained systems with small gaps that the question remains, is
the derate 3% or 10%.
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