Advice needed on attic ventilation

acdriverd15

I need advice. I have read many of your posts but have not come across enough information that relates to my particular situation.

*Location=eastern Carolina. In other words, it gets hot here in the Summer.
*About 3000 sq. ft. attic space.
*About a 5/12 roof pitch
*House is rectangular in shape with 1-hip and 1-gable


Current situation:

1-1500 CFM power ventilator (on hip end of house)
1-15" round gable vent
11-8"*16" soffit vents

*Attic temperature on hottest days WITH power ventilator running = 130-140 degrees.
*4 soffit vents on hip end (36 ft.), 7 along the back of the house (90 ft.), 0 along front of house (90 ft.) which also gets 60 % of the sun.


Other information:
*68-72 ft. of ridge running from gable to hip.


OK, so here is where I need the help... I know it is waaay to hot in the attic in the summer. Once the attic heats up and starts pushing the hot air into my living space. Once that happens, my HVAC units can't keep up with the overbearing heat and end up running all day long. They only "catch up" once my power ventilator has cooled the attic off to about 100 degrees around 10 PM. Needless to say, my Summer power bill is nutz!

Can someone please look this over and give me some advice?

Thanks!!

B.

Ed the Roofer

What is your insulation R-Value in the attic?

Also, does the attic floor have a vapor barrier installed under the insulation?

Here is a study and results discussing Balanced Intake and Exhaust Ventilation.

http://www.airvent.com/pdf/literatur...estResults.pdf

Ed

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acdriverd15

Got about Between R-30 and R-40 in the attic. I blew it in myself last march after I had gotten jipped by an insulation contractor.

No vapor barrier at all. House was built in 1975. Even today, the new houses here almost never have a vapor barrier unless it is the facing on batt insulation... But most attics are blown in with no VB.

dmc@RCR

I would suggest, in addition to what you have, installing as much soffit venting as possible, especially the front, and full ridge vent.

Can you post pics of the house?

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acdriverd15

Here we go. I hope this helps. Sorry, but I had to cut the pics down to get them to upload...

Oh, BTW! I was wrong on the count of soffit vents...13 total, not 11.

4 on the hip end and 9 on the back side. Of coarse, none (0) at all on the front which gets the majority of all the direct sunlight.

Attached Images

      

handy man88

If I were you, I would definitely add more soffit vents, but ensure the existing ones are not blocked since you have a low pitch roof. Did you add baffles when you blew your insulation?

Also, you may want to consider upgrading the attic fan (or adding another one) while closing off the gable vent and maybe even the ridge vent. That way, the power vent pulls directly from the soffits for circulation, and eliminates any chance that air may be pulled inside the attic from the ridge vent or gable vent.

Or, as many have mentioned, you could close up the gable vent and the power vent, and just upgrade the ridge vent.

acdriverd15

I do not have baffles but...I made sure that I left a gap so air could flow over the top of the insulation. In short, the current soffit vents are not blocked. That being said, I have considered replacing all of the old ones as we are going to change our paint to white anyway......

I do not currently have a ridge vent. It is something I am considering but I do not have one yet. I just don't know if a ridge vent alone will give the ventilation I need....or if we will need to add hip vents to that as well????if I go that route???

I had a friend suggest adding a second fan...but I'm not sure how many soffit vents to install to ensure proper ventilation if I go that route. Too little and the fan starts to pull air out of the living space is what I have been told....

seeyou

How did you make sure there was a gap? Just curious. What type insulation did you add?

Are the old vents paint clogged?

Don't add a second fan yet. I suspect part of the a/c problem is that the fan is already pulling some conditioned air out.

Here's what I'd do:

Phase I- Add 8x16 soffit vents at about every 4' all the way around. Push a baffle in through the hole before you install the grille. Also, take a stab at air sealing the attic. Foam around pipe and wire penetrations in the ceiling. Work on the attic trap, etc.

Phase II- Remove the power fan and add either ridge vent (you probably don't have enough ridge) or slant back vents near the ridge on the least visible side. Leave the gable vent as is.

Here's info to get the system sized:
http://www.airvent.com/professional/...shooting.shtml

acdriverd15

Ahhh, that is the type of link I was looking for but couldn't find.

So it is 1/150 and not 1/300. And each 8*16 is 56 sq. in.

Looks like I need 25 soffit vents and 74 feet of ridge vent. Looks like I'm 6 feet short for ridge vent. Even being a little short, that should be a HUGE improvement over what I currently have..... Any thoughts on that though?

seeyou

If you didn't figure in the existing gable vent that will likely act as exhaust if the soffits are properly vented. But your thinking is sound, IMHO.

Bear in mind that those are MINIMUM recommendations. I always strive for more if possible, but there's usually a trade off between function and cosmetics. More venting won't hurt and will likely help.

I'd guess that with the existing situation (inlet starved), the power vent is pulling some conditioned air from inside and using a fair amount of electricity to do it. Switching to all passive venting will cut down on the draw from inside the house, but air sealing the attic will reap you big benefits if you haven't done it already.

dmc@RCR

Ad the extra soffit vents and the ridge vent. The hip end would be a good place for an extra large slant back vent. As above make sure the soffit vents are clear.

For the hot days in summer, I would recommend leaving the power vent. You will need approximately 5 square ft. of intake for that. The soffit vents will be plenty.

Here is a short video of the effectiveness of ridge vent on a hot calm day. A smoke test in a large attic in summer.



Not to impressive.

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seeyou

Dennis -

Did you test further? 10 degrees warmer/colder? Wind 5mph faster? Any sort of comparison in the same attic?

What would you suggest to get the air moving in that scenario? Or in the OP's scenario?

My house is similar to the OP's and I know the results of what I did to it. I'm a firm believer in air sealing and insulating VS trying to cool the inside of a house by fooling with the roof. But we need to keep the moisture out.

mem

You would be better served with continuous soffit vents instead of the individual ones. They afford a more uniform flow of intake air. Cut in the ridge vent and make up for the shortage with hip vents, ideally, or a large roof vent on the hip roof. Dedicate slightly more than half of your vent area to the intake. Along with the air sealing mentioned, this will help prevent conditioned air from being sucked up from the living space below.

Keep in mind the blown in insulation can move over time and fall into the soffits, blocking the vents. A well designed baffle will prevent this from happening.

acdriverd15

So what is up with the 1/300 that I have see on some of the ventilation product packaging? Is it an old standard? Is it a government standard? Is it a standard for northern regions? What is up with that?

Don't get me wrong, I meet that already and it obviously isn't adequate....

mem

The 1/300 rule is minimum code and applies to construction that is air/vapor sealed very well between the attic and living spaces. This is difficult to accomplish as a retrofit. Homes with less than perfect air/vapor sealing should use the 1/150 recommendation for ventilation.

As excerpted from: http://www.checkthishouse.com/18/att...oof-needs.html

Industry standard for proper attic ventilation recommends (for no vapor retarder type of attic insulation – no paper, plastic or aluminum layer between the attic floor and insulation layer) 1 sq. foot of ventilation for every 150 sq. feet of attic space divided 50 / 50 between the inlets and outlets.

For vapor retarder equipped attic insulation (for example fiberglass blankets/bats with paper facing), you should have 1 sq. foot for every 300 sq. feet of attic space – assuming that everything else is perfect…