Attic Cooling and Ridge Venting

[jfolivieri]

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

[Joseph Meehan]

[e-mail] wrote: ..

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.

No channel needed unless something is blocking the ridge vents.

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

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.

[trader4]

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?

[jfolivieri]

They both go outside the attic...

[Tom The Great]

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 @

[trader4]

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

[nicksanspam]

No.

No.

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