I have a colonial house in the Boston area. After some measurements, my
attic temperature can be 40F higher than the outside temperature on a
warm day. This is in spite of ridge and soffit vents. There is 10-12"
of attic floor insulation and an air handler for the A/C in the attic.

I have questioned if I have enough venting area. My attic is about 1500
sq. feet, I believe I should have a total of 1500 sq inches of venting
(750sq in. for soffits and 750sq in. for ridge). According to my
calculations, I have about 900 sq inches of ridge vent and 1200+ sq
inches of holes for the soffit vents (many 7/64" holes in the vinyl
soffit). BTW, the attic volume is about 5300 cu. ft. in a ~ 5 pitch
roof.

The soffits appear to be clear (I can see daylight in the attic near
the soffits), but it still gets very hot. Also, in the winter the
hunidity is very high in the attic.

So, I am told it is a bad thing to install a gable vent when you have a
ridge vent.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.
2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?
Any comments?
thanks, Jack

wrote:
...

So, I am told it is a bad thing to install a gable vent when you have
a ridge vent.


True, it can short circuit the air flow. However I don't generally
consider it a real problem, but also I don't consider them a real advantage.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.

No channel needed unless something is blocking the ridge vents.

2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Not likely to do any harm, but also not likely to move anywhere close to
enough air to make any real difference.


Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?

You do want to make sure the soffit vents are clear, not just from
above. Make sure insulation is not blocking them and make sure spider webs
etc are not blocking them from below. Also remember that most soffit vents
are not all free flow. A 4X10 vent has 40 sq inches of exposure, but with
screen lovers etc may only have 20 sq inches of free opening.

Finally I don't find that 40º difference all that high. Attics are
going to get hot and unless you use some really powerful fans they are going
to get really hot on a hot day. Just think of what it would be like without
vents. Also remember that the hotter the temperature difference is the
better passive (gravity) venting works.


Any comments?
thanks, Jack

--
Joseph Meehan

Dia duit

Joseph Meehan wrote:
wrote:
..

So, I am told it is a bad thing to install a gable vent when you have
a ridge vent.


True, it can short circuit the air flow. However I don't generally
consider it a real problem, but also I don't consider them a real advantage.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.

No channel needed unless something is blocking the ridge vents.

2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Not likely to do any harm, but also not likely to move anywhere closeto
enough air to make any real difference.


Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?

You do want to make sure the soffit vents are clear, not just from
above. Make sure insulation is not blocking them and make sure spider webs
etc are not blocking them from below. Also remember that most soffit vents
are not all free flow. A 4X10 vent has 40 sq inches of exposure, but with
screen lovers etc may only have 20 sq inches of free opening.

Finally I don't find that 40º difference all that high. Attics are
going to get hot and unless you use some really powerful fans they are going
to get really hot on a hot day. Just think of what it would be like without
vents. Also remember that the hotter the temperature difference is the
better passive (gravity) venting works.


Any comments?
thanks, Jack

--
Joseph Meehan

Dia duit



The thing I'm especially having trouble with is the high humidity in
the attic in winter. One would think with the amount of venting he
currently has at least moisture would not be an issue. Where do the
bath fan vents go?

wrote:
Joseph Meehan wrote:
wrote:
..

So, I am told it is a bad thing to install a gable vent when you have
a ridge vent.


True, it can short circuit the air flow. However I don't generally
consider it a real problem, but also I don't consider them a real advantage.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.

No channel needed unless something is blocking the ridge vents.

2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Not likely to do any harm, but also not likely to move anywhere close to
enough air to make any real difference.


Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?

You do want to make sure the soffit vents are clear, not just from
above. Make sure insulation is not blocking them and make sure spider webs
etc are not blocking them from below. Also remember that most soffit vents
are not all free flow. A 4X10 vent has 40 sq inches of exposure, but with
screen lovers etc may only have 20 sq inches of free opening.

Finally I don't find that 40º difference all that high. Attics are
going to get hot and unless you use some really powerful fans they are going
to get really hot on a hot day. Just think of what it would be like without
vents. Also remember that the hotter the temperature difference is the
better passive (gravity) venting works.


Any comments?
thanks, Jack

--
Joseph Meehan

Dia duit



The thing I'm especially having trouble with is the high humidity in
the attic in winter. One would think with the amount of venting he
currently has at least moisture would not be an issue. Where do the
bath fan vents go?


They both go outside the attic...

Joseph Meehan wrote:
wrote:
..

So, I am told it is a bad thing to install a gable vent when you have
a ridge vent.


True, it can short circuit the air flow. However I don't generally
consider it a real problem, but also I don't consider them a real advantage.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.

No channel needed unless something is blocking the ridge vents.

2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Not likely to do any harm, but also not likely to move anywhere closeto
enough air to make any real difference.


Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?

You do want to make sure the soffit vents are clear, not just from
above. Make sure insulation is not blocking them and make sure spider webs
etc are not blocking them from below. Also remember that most soffit vents
are not all free flow. A 4X10 vent has 40 sq inches of exposure, but with
screen lovers etc may only have 20 sq inches of free opening.

Finally I don't find that 40º difference all that high. Attics are
going to get hot and unless you use some really powerful fans they are going
to get really hot on a hot day. Just think of what it would be like without
vents. Also remember that the hotter the temperature difference is the
better passive (gravity) venting works.

Yeah, I get at least 40 degrees difference on a sunny 80 degree day (in
New England), and that's after the powered roof vent has been running
all day, as I just happened to post elsewhere today.



Any comments?
thanks, Jack

--
Joseph Meehan

Dia duit

On 6 Jun 2006 07:10:41 -0700, "
wrote:

I have a colonial house in the Boston area. After some measurements, my
attic temperature can be 40F higher than the outside temperature on a
warm day. This is in spite of ridge and soffit vents. There is 10-12"
of attic floor insulation and an air handler for the A/C in the attic.

I have questioned if I have enough venting area. My attic is about 1500
sq. feet, I believe I should have a total of 1500 sq inches of venting
(750sq in. for soffits and 750sq in. for ridge). According to my
calculations, I have about 900 sq inches of ridge vent and 1200+ sq
inches of holes for the soffit vents (many 7/64" holes in the vinyl
soffit). BTW, the attic volume is about 5300 cu. ft. in a ~ 5 pitch
roof.

The soffits appear to be clear (I can see daylight in the attic near
the soffits), but it still gets very hot. Also, in the winter the
hunidity is very high in the attic.

So, I am told it is a bad thing to install a gable vent when you have a
ridge vent.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.
2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?
Any comments?
thanks, Jack


Good questions, I have a ridge/soffit vent system, and measured the
hotest temp in my attic at the attic door level. 140F on a 90+ degree
day. Hot.

Before I tackled taking on a new diy project, I had to find out was it
really a problem. I found high attic temps only do two bad things,
cost more cooling, and prematurely wear roof shingles. I previously
added R-30 on top of blown in orginal R-30 insulation(it compressed so
might have been only R-20). I found with a R-50-R60, not much heat is
being transfered into the house, but I was concerned about the
roofing.

I talked to a contractor, and found I did anything other than improve
the natural circ, I would have to block the ridge vents. If I put in
gabled end fans, I could draw cool air in and out through the vents,
dead airing the hot space. Also, increase my electric costs, and add
a new project intial costs. I was told that my high temps in
Pennsylvania was nothing compared to Florida, and shingle last there
too, so stop worrying, and start another DIY project.

So, I sound out that in the priorities of things, if you attic is
insulated and the ventulation isn't blocked, there are other places to
spend your money.

Good luck,

tom @ www.MedJobSite.com

Tom The Great wrote:
On 6 Jun 2006 07:10:41 -0700, "
wrote:

I have a colonial house in the Boston area. After some measurements, my
attic temperature can be 40F higher than the outside temperature on a
warm day. This is in spite of ridge and soffit vents. There is 10-12"
of attic floor insulation and an air handler for the A/C in the attic.

I have questioned if I have enough venting area. My attic is about 1500
sq. feet, I believe I should have a total of 1500 sq inches of venting
(750sq in. for soffits and 750sq in. for ridge). According to my
calculations, I have about 900 sq inches of ridge vent and 1200+ sq
inches of holes for the soffit vents (many 7/64" holes in the vinyl
soffit). BTW, the attic volume is about 5300 cu. ft. in a ~ 5 pitch
roof.

The soffits appear to be clear (I can see daylight in the attic near
the soffits), but it still gets very hot. Also, in the winter the
hunidity is very high in the attic.

So, I am told it is a bad thing to install a gable vent when you have a
ridge vent.

Some suggestions I have had:
1) One contractor suggested stapling heavy aluminum foil across the
rafters to within 18" of the soffits and ridge to create a channel for
the rising air.
2) Install a solar powered roof fan near the center of the ridge (down
12-18")

Is there any way to measure the draft (maybe smoke, etc)? Has anyone
had this problems?
Any comments?
thanks, Jack


Good questions, I have a ridge/soffit vent system, and measured the
hotest temp in my attic at the attic door level. 140F on a 90+ degree
day. Hot.

Before I tackled taking on a new diy project, I had to find out was it
really a problem. I found high attic temps only do two bad things,
cost more cooling, and prematurely wear roof shingles. I previously
added R-30 on top of blown in orginal R-30 insulation(it compressed so
might have been only R-20). I found with a R-50-R60, not much heat is
being transfered into the house, but I was concerned about the
roofing.

I talked to a contractor, and found I did anything other than improve
the natural circ, I would have to block the ridge vents. If I put in
gabled end fans, I could draw cool air in and out through the vents,
dead airing the hot space.

A lot of people claim this to be true, but I wonder if anyone has any
real data. It would seem to me that if you put a gable fan in with a
ridge vent, some of the air might short circuit via by coming in the
ridge vent and out the gable. But I would think most of the air that
gets sucked out is going to be hot attic air. So, I can see it might
make it say 10-20% less efficient, but I tend to doubt the gable fan
doesn't still move a lot of hot air out.






Also, increase my electric costs, and add
a new project intial costs. I was told that my high temps in
Pennsylvania was nothing compared to Florida, and shingle last there
too, so stop worrying, and start another DIY project.

So, I sound out that in the priorities of things, if you attic is
insulated and the ventulation isn't blocked, there are other places to
spend your money.

Good luck,

tom @ www.MedJobSite.com

wrote:

Tom The Great wrote:

wrote:

... I am told it is a bad thing to install a gable vent when you have a
ridge vent.

No.

I talked to a contractor, and found I did anything other than improve
the natural circ, I would have to block the ridge vents.

No.

A lot of people claim this to be true, but I wonder if anyone has any
real data...

Page 23.9 of the 1993 ASHRAE HOF says

Greatest flow per unit area of openings is obtained when inlet and
outlet areas are equal; Equations (17) and (18)

Q = 14.8Asqrt(HdT) cfm, with A in ft^2 (18)

are based on this equality. Increasing the outlet area over inlet area,
or vice versa, increases airflow but not in proportion to the added area.
When openings are unequal, use the smaller area in the equations and
add the increase, as determined from Figure 5.

Figure 5 says 15% more air flows when one opening is 50% larger than the
other, 27% more flows with a 2:1 ratio, 34% more flows with a 3:1 ratio,
and 37% more flows with a 6:1 ratio.

So it's like this: more vent area makes more airflow, with dimininishing
returns for unequal areas...

Nick