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General attic venting

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Posted by dold on July 17, 2006, 10:20 pm
My soffit venting consists of three 2 inch diameter holes in every third
rafter block, yielding 9.5 square inches of vent for every six lineal feet
of eave run.  I have a total of 48 sets of three, 452 square inches total.

These are similar to this photo, which shows two holes:

Near the peak of the roof, I have vents like these:
http://www.awardmetals.com/product_vent.htm  True 38
That would seem to be 152 inches.

This seems phenomenally under-sized to me, if I apply information from
general purpose pages on the web, and even some pages that seem to
reference building codes.  For my 2458 square foot house, I think I need
800 square inches of venting at the soffit and at the eave.  With the
suggestions being 150-300:1 ratio of attic square feet to vent area,
divided between eave and soffit.

I looked through other posts in this group that speak of a formula for
total air-flow based on the height of the peak, and the temperature
differential between soffit and peak.  I did see one comment about "until
the air stops flowing", as the temps balanced out.

The posts seem to be interested in recovering heat for other use.
I'm more interested in discarding the heat.  It's 109F ambient here today.

Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

Posted by Robert Scott on July 18, 2006, 3:33 pm
On Mon, 17 Jul 2006 22:20:23 +0000 (UTC), dold@XGeneralXa.usenet.us.com wrote:

I don't know about precise calculations, but I designed my overhangs to be 2
feet long completely open with screening.  The attic is hardly much warmer than
the outside air.  I have been living in this house for 20 years and it is very
cool in the summer.

Robert Scott
Ypsilanti, Michigan

Posted by dold on July 18, 2006, 6:02 pm

Do you have vents at the peak as well?
Maybe you don't need that.  Your roof sounds almost like a tent fly.

I think I need a lot more venting, but I don't know how much retrofitting I
can afford.

Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

Posted by News on July 18, 2006, 7:19 pm

Sounds like the UK.  Most attics are cold with vents at the eves and no
ridge tiles.  The roof is only to keep the rain and snow off.  The
insulation on the attic floor insulates the house.

They are cool enough in the summer if the vents are left unblocked by
Rockwool.  It is horrendously cold up there in winter - that is when you
appreciate how well insulation works.

I know a few who have made a cold roof/warm attic. Ridge tiles are fitted
about 3 or 4.the rafters are boarded up with drywall and insulation and the
air enter at the eves, up a 2" gaps behind the tiles and out the ridge

The house tends to be much warmer in winter and goods stored are then
protected against extremes of temperatures - many do this to protect
valuable stored goods.

Posted by Robert Scott on July 19, 2006, 1:13 pm
 On Tue, 18 Jul 2006 18:02:54 +0000 (UTC), dold@XReXXGener.usenet.us.com wrote:

It is an unconventional roof shape.  The lower overhang is as I have described
it.  But the top of the roof (4:12 pitch) is another overhang with a little less
area (18 inches out) and also screened.  That is where it meets and overhangs
another roof, which is very steep.  I get so much ventilation that sometimes the
wind will blow the edges of the fibreglass insulation batts up and fold them
over.  I had to add some light strapping to keep it in place.  In retrospect I
think I could have done just as well with a lot less venting.  Another
advantage:  during the daytime, I don't need electric lights to see my way
around up there.

Robert Scott
Ypsilanti, Michigan

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