[Greenbuilding] Exterior insulation retrofit

John Straube jfstraube at gmail.com
Mon May 23 14:52:21 PDT 2011

The article says "The flow of air through a 20 mm cavity is not great, but any airflow reduces the insulating effect of the outer-most layer."
No one would put a 10 mm gap behind the insulation. Or rather, no one should.
The 10 mm gap is an OK minimum for ventilation, or fine if you want to use it as capillary break, but systems that drain behind the insulation do not need or want to be ventilated and use some sort of material to act as the capillary break.
That 10 mm thing is such a major confusion, I wish they had never put it in there.

The research we did, together with ORNL, stopped at cavities of 6 mm since this was thought to be bigger than anything sensible.  Building science researchers often dont imagine that people might try to do something they shouldnt (like using a 10 mm cavity behind insulation!). This is the work we did to show that a 10 mm gap was not needed for drainage: alas, the IRC/NRCC tested a 10 mm gap (nothing smaller) and that is what got into the code.

On 2011-05-23, at 2:02 PM, JOHN SALMEN wrote:

> Which cavity are we talking about? 
> The NZ requirement I referenced was for any drainage/ventilation cavity - I
> don't think a size was specified. Our code requires 10mm which is an
> arbitrary number except that it is a nice round one (and close to 3/8" -
> though it is surprising how picky building inspectors can be on .0187 of an
> inch. Our building code requires bottom drainage of a 10mm gap but does not
> require top venting. 
> So what I was originally looking for was any research relevant to d-rating
> insulation over a 10mm (or so) drainage gap and it seems there really isn't
> any. 

Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture

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