DIYbanter - View Single Post

wrote:

Choreboy is about to add...

...ridge vents because they are the most cost-effective method.

Moreso than a gable vent with the same free area?

That's what I've read. If you have rige vents and soffit vents, you
have several feet of height for convection, and your air movement is
spread out under the roof.

My question was whether it's less expensive to install (say) 2 2'x2' gable
vents than a 2"x48' ridge vent, on an existing house, with soffit vents
in either case. Also, it seems to me that 2 2'x2' soffit vents may be more
cost-effective than continuous soffit vents, with equivalent performance.

On an existing house, you remove the cap shingles, saw a slot along the
ridge, and nail on the vent, which costs about $2 a foot.

I understand one reason for venting is to make the roof last longer, by
reducing heat gain from the sun and removing moisture from tiny leaks.
Ridge vents look ideal to me because air would tend to move under the
entire surface.

A heating system may have one big air inlet because as you say, that's
the most cost-effective way to move a given amount of air. It will have
a lot of small outlets because one big outlet wouldn't distribute the
heated air properly. Similarly, it seems to me that several small
soffit vents would be the way to make sure ventilation was distributed
throughout an attic.

If the soffit vents are in the center, air will spread out under the roof.
If not, convection currents through the large free "vent areas" inside
the attic will tend to equalize the air temperature.

You might keep your attic 100 F on an 80 F day with a 1000 cfm fan,
moving about 1000x20 = 20K Btu/h out of the attic, OR use soffit
and gable vents with area A and an 8' height difference, where 20K
= 16.6Asqrt(8)20^1.5, ie A = 4.8 ft^2, with no fan.

With A = 8 ft^2 (the 2 2'x2' vents), 20K = 16.6x8sqrt(8)dT^1.5, so
dT = 14 F, and the air coming out of the gable vents would be about
80+14 = 94 F. With soffit vents near the gables, the air in the middle
of the attic might be dT warmer than the air near the gables, where
10K = 16.6x48ft^2sqrt(8)dT^1.5, so dT = 2.7 F, for a 4/12 roof.

My 100' greenhouses ventilate well with roll-up sides and gable vents.

Nick

Roll-up sides and gable vents sound like a good way to regulate the air
temperature at plant level. If the purpose were to use a small amount
of ventilation to keep condensation from forming, I'd go with a ridge vent.