RE: GBlist: no roof ventilation

[John Straube <jfstraube@golden.net>]

On Tuesday, January 26, 1999 11:02 AM, Caaradrbl@aol.com 
[SMTP:Caaradrbl@aol.com] wrote:
> My (totally non scientific) observations on roofs with a large snow load 
at
> temperatures approaching melting lead me to believe that the amount of
> _insulation_ and not  ventilation is the key factor in preventing of ice
> damming.
Rick, you are correct, but this does not change the amount of ventilation 
needed.
If a house has a large amount of insulation right up to the outside line of 
the  walls (often not the case), ventilation is not that important, BUT 
will still help significantly reduce ice damming if snow accumulates.  As 
usual, ventilation wont solve the problem is you do other things wrong ( 
like not enough roof insulation) but then by the same token lots of 
insulation likely wont help if you dont ventilate.
How important each factor  is depends on the climate.  If you have 12" of 
snow on the roof, it has an Rvalue of about 8.  If the temp outside is 
-10C, inside is 21, the attic is insulated to R16, the temp at the 
uninsulated roof surface would be just above zero = melting and icedamming. 
 If the R-value of the roof was 32 and 2 feet of snow, the result would be 
the same.  Ventilation would help bypass the thermal resistance of the snow 
and reduce the potential for problems.
Once you get to R40 to 50 it takes warm temps (which dont allow refreezing) 
or huge amounts of snow (3 feet or more) to have a problem, and so 
ventilation becomes less critical.
Does that gibberish make sense?

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John Straube
Building Envelope Engineering 	Building Engineering Group
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