increasing attic eave ventilation - round holes

[richy127]

3600 sq ft attic 12/8 pitch with 9.8 sq ft of gable vents but only 1.7 sq ft of eave ventilation made up of 2x4x22 end caps with either 2" diameter or 1.7" round holes. There are 94 holes. With only every third or fourth end cap having holes in it. Either four 1.7" or three 2" holes on the end caps that have holes. Below picture is a three holer. House is in Danville, California and was built in 1989.

What is the best way to increase eave ventilation? If I put holes in the balance of end caps I would only gain 5.6 sq ft and am still out of balance. Also this would not meet the 1/300 rule. Also my calculations above does not take into account the impact of the metal fabric screen on all the ventilation openings.

Thanks,

...Rich

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[John Kogel]

Best way? Knock out all the 2X4 blocking and install normal perforated soffit covers.

[Jim Katen]

The 1:300 thing is a complete and total wild-ass guess about what proper ventilation should be. There is absolutely no evidence that it provides proper or adequate ventilation to attic spaces. In reality, the need for attic ventilation and the amount and configuration of attic ventilation are complex issues that simply can't be boiled down to such a simple rule.

Are you experiencing any problems associated with this supposed imbalance of ventilation? If not, you don't need to do anything.

If you are, then you need to address the problem specifically.

[Charles46]

Based upon the information provided and using the 1/300 rule as a guide you have a need for 12 sq ft of vent space. You currently have 11.5 sq ft of vent space, so as Jim said, unless you are having an issue that needs to be addressed specifically, leave things as they are. If you want to put holes in the balance of the end caps, and doing so would not affect things structurally, then adding an additional 5.6 sq ft of vent space your ratio of vent space to attic space would become something like 1/210.

Again, as Jim put it "Are you experiencing any problems associated with this supposed imbalance of ventilation? If not, you don't need to do anything. If you are, then you need to address the problem specifically."

If you have a problem, to address the problem, you should find the source of the problem and treat the source of the problem and not the symptoms of the problem.

[richy127]

"Are you experiencing any problems associated with this supposed imbalance of ventilation?"

High attic temperatures that eventually breaks through forcing me to use the HVAC. Roof is flat tile and the sun creates an oven effect in our cloudless summers. Once the outside temperature goes below 84 I am able to use my whole house fans to cool the house.

!32 degrees when the outside is less than 100. Radiant barrier in the garage only reduces the temperature by four degrees. Gable fans has little impact on the attic temperature. See enclosed temperature chart. Thanks. ...Rich

Download Attachment: Young temps 7-2013.PDF - Adobe Acrobat Pro.pdf

318.36 KB

[Jim Katen]

"Are you experiencing any problems associated with this supposed imbalance of ventilation?"

High attic temperatures that eventually breaks through forcing me to use the HVAC. Roof is flat tile and the sun creates an oven effect in our cloudless summers. Once the outside temperature goes below 84 I am able to use my whole house fans to cool the house.

!32 degrees when the outside is less than 100. Radiant barrier in the garage only reduces the temperature by four degrees. Gable fans has little impact on the attic temperature. See enclosed temperature chart. Thanks. ...Rich

Download Attachment: Young temps 7-2013.PDF - Adobe Acrobat Pro.pdf

318.36?KB

Ventilation doesn't do much to reduce heat gain. You could double your ventilation openings and not see any difference because most of the temperature rise is caused by radiant heat gain.

Think of it this way: If you have a steak under a broiler, will "ventilating" the space above the steak make it cooler?

You can reduce radiant heat gain by installing light colored tiles. You can reduce heat transfer into the house by installing better insulation. If you really want to be a rebel, you can move the insulation from the ceiling plane to the roof plane. (That's the solution that lies in the future for most new houses.)

[kurt]

I think Joe L provided the math and physics showing how ventilation does nothing to reduce attic temps in his last Insight column.

Roof color is the single largest determining factor in roof temperatures. Try a reflective UV resistant roof paint; it makes a huge difference on a flat black roof....not sure how much in a tile roof.

[hausdok]

All good answers,

Attic ventilation isn't required in order to reduce temps; it's required to help disperse moisture-laden air that moves up into the attic from the house so that it doesn't condense within the attic or roof plane and cause a bunch of rot. If the roof isn't rotting, the ventilation you have is doing its job just fine.

Painting the roof is a great suggestion but some folks don't want to do that and some neighborhood rules might not allow it. If you want to really reduce solar gain, increase the depth of the insulation in the attic and use a type that will greatly reduce air flow through the insulation such as cellulose. You'll appreciate it in the winter too.

ONE TEAM - ONE FIGHT!!!

Mike

[richy127]

Do appreciate everyones feedback.

Like John Kogel's response but are there structural considerations that need to be considered? "Best way? Knock out all the 2X4 blocking and install normal perforated soffit covers."

Please point me to Joe L's post. I could not find it.

Also I started installing perforated radiant barrier under the roof on the truss rafters giving 3.5 inches of air space. Is that effective?

When I added blown in insulation ten years ago I saw significant improvements. It sounds like adding more the way to go. Again, thanks. ...Rich

[kurt]

Please point me to Joe L's post. I could not find it.

I have too many links; I can't find it either. He said it; that's fine with me, I'm not on the stand. It was within the last year, and more like 6 months...he was extemporizing about all these things in either an Insight, or one of his periodic email subscription deliveries.

What Mr. O said.

[Marc]

William Rose said something about that in a 2007 issue of JLC. Scroll down to 'next page' then 'Building Codes'.

Marc

[richy127]

Very good article. Think I am done making more holes. Thanks.

[allseason]

You can reduce radiant heat gain by installing light colored tiles. You can reduce heat transfer into the house by installing better insulation. If you really want to be a rebel, you can move the insulation from the ceiling plane to the roof plane. (That's the solution that lies in the future for most new houses.)

I agree. Bring the attic into the conditioned space.