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Re: Attic ventilation from fans

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> From: Drew Morris <dmorris(--nospam--at)>

> attic space is insufficiently vented.  After the original construction,
> there was an attempt to add increased attic ventilation with the 
> installation of  the vertical spherical fans found on top of buildings.
> The turbines don't appear to be powered and get more active as the day
> gets longer and the sun heats up the attic space.  Does anyone have an
> idea on how to calculate affects of fans with respect to IBC 2000 
> section 1203 on ventilation?  There is a vapor retarder between the 
> ceiling and the insulation, so the net free ventilating area is 1/300 of
> the area of the space ventilated.  I can calculate the eave and gable
> end venting areas, but am stuck on the turbines.

Manufacturer's data? website?

I have these on my house installed by the previous owner. The throat
size of the vent is limiting and the momentum of air passing through
must also give up energy to spin the turbine. I do not believe that they
provide additional ventilation but they look like they are doing
something. Gives buyers a level of comfort to see activity.

> From: Christopher Wright <chrisw(--nospam--at)>

> On May 15, 2006, at 7:23 PM, Drew Morris wrote:
> in Minnesota you used to see a lot of these things installed to 
> improved attic venting due to heat leaks. You don't see them much any
> more, and the CW here is that they don't work all that well. They look
> like they're don't all sorts of stuff, but they aren't. I'm told that
> they really can't keep up with moist air leaks through attic bypasses.
> We ventilated the living hell out of our attic to try to deal with ice
> dams caused by warm air leaking through open walls, but it never really
> worked. Once we got the heat leaks shut down the ice dams weren't a 
> problem.

As long as the air in the attic space is more humid due to vapour leaks
(not "perfectly" ventilated) in the winter, there will be frost
accumulation at the cold spots.

R. Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)> <>

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