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Old 03-20-2013, 03:46 PM   #1
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How to plan for attic ventilation for central air when modifying gable vents?


We bought a 1950s Ranch which came with a 3'x3' whole house fan and gable vents. We're going to installing central air in about a month, and I want to get the attic ventilation in a reasonable order before they do this. The whole house fan is going and all we have is some gable vents, no soffit vents. We plan on doing better attic ventilation long term, but for right now I know the ventilation is bad, and looking for some suggestions short term.

We don't plan on replacing the roof for a few years, so a ridge vent isn't an option at the moment. The roof is 15 years old and I don't want to tear into it for lack of matching shingles and damaging the water protection that is there now.There is a large 2x3' gable vent on the west side of the house which is louvered to open when the whole house fan turns on. I'd assume I just replace that with a permanently open version? Then there are 2 smaller (2x3' and 2x4') gable vents on the west and south (front) side of the house. This seems insufficient to me. As far as I can tell there aren't any soffit vents, so it appears the air is supposed to move through the house east to west?

Short term, how can I best vent the house without modifying the actual roof shingles? Near term I plan on doing a lot of airsealling and installing soffit baffles (to keep future cellulose from blocking the soffit air vents). Long term I plan on removing all of the boards where the soffits would be and install a soffit the length of the house. Would putting round soffits every 2-3 feet be a good addition for the short term?

Thanks, Bill







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Old 03-20-2013, 04:18 PM   #2
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How to plan for attic ventilation for central air when modifying gable vents?


#1 you have about 1/2 the amount of needed insulation right now.
http://www.energystar.gov/index.cfm?...sulation_table
#2 Big mistake to not add a ridge vent. Right now you have no were close to the needed venting.
Your do not have to distory any of the shingles, just the ridge gets removed.
A ridge vent would vent that whole roof not just one small area like you have now.
Once the ridge vent gets installed the gable vent gets covered up.
Just adding the ridge vent will add many years to the life of your foor and help keep the house cooler.
I would not get rid of the whole house fan, just fix it. It cost a whole lot less to run that then central air and in your zone that's all you would need about 50% of the time.
Got to get some gable vents!
None of your roof vents work without it.

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Old 03-20-2013, 07:05 PM   #3
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How to plan for attic ventilation for central air when modifying gable vents?


Add insulation in the floor, and put a fan in one gable end with a thermo/humidistat in the attic. Get some cross ventilation going up there to remove some of the radiant heat from the roof.

Nice House, By the way
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Old 03-20-2013, 07:22 PM   #4
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How to plan for attic ventilation for central air when modifying gable vents?


Thanks, I meant to say we plan on adding a lot of blown in cellulose in the future, but before that happens I'm going to air seal everything very well (have a foam gun + cannisters waiting). No sense in making it harder to move around to seal things. I took another look, and the east side where I thought there was a reasonable gable vent, basically there is none (the siding people mostly covered that up before we bought the house). So I'll have to look at improving that cross flow for now. Long term we have to improve ventilation, as there basically isn't any, but we haven't started "outdoor work" and probably won't for a while as we're bogged down on interior work.

I'm not yet sure about the whole house fan, besides a big eye sore, louvers that don't stay open, and a big gap in the air seal, I've yet to see it in action, so it might actually be of some use. We're set on having central air, it's just a matter of whether the house fan stays.

I'll probably make some walk ways as the joists are only 2x6, and that would make the current walk ways buried under the insulation. I just am not going to get a ridge vent anytime soon, so I wanted to supplement the bad ventilation we currently have.
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Old 03-20-2013, 08:32 PM   #5
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How to plan for attic ventilation for central air when modifying gable vents?


Make sure you work with your HVAC people to be sure the system will be designed to work properly with the insulation. If you change the insulation and the envelope around the ducting you run the risk of causing problems with it. Be sure they plan for each other. As in, if you have a unit put in that expects to be in uninsulated space, and you later insulate the roof you'd run the risk of the unit cycling on/off too frequently.

Do you have clear soffit vents under the outside edges? They work to let normal convection pull air in under the eaves and then up and out the top. Be sure you don't block them when you add insulation. They make foam baffles you can staple to the underside to help keep a clear channel open from the eaves up and over any insulation.
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Old 03-20-2013, 09:04 PM   #6
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How to plan for attic ventilation for central air when modifying gable vents?


You can install a ridge vent without disturbing anything. All you need to do is make a cut through the existing shingles and plywood sheathing. Those should come right off the rafters after cutting. You should be then able to install a ridge vent.
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Old 03-21-2013, 07:30 AM   #7
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How to plan for attic ventilation for central air when modifying gable vents?


@wkearney99

The hvac people we're working with are very good and did a full manual J assuming I'm putting in insulation and airsealing. Other quotes put the a/c unit at 4 tons, these guys used 3 tons (plus multispeed blower for longer run times). They also sized the boiler from the 1953 size of 160k btu down to 100k. So we should be good on that account.

I've looked and there aren't any soffits that I can see under the roof's ledge. Now we "think" we remember being able to see light from outside when we're in the attic, so maybe there's some hidden openings, but nothing obvious from below.


@huesman

I didn't realize you could install a ridge without severely affecting the shingles. Don't you need a shingle for above the ridge vent to blend it in? If we can reuse the cap shingles, I would be all for it when we get to that point. But we still have no soffit vents, so we'd have to take the circular saw and cut all the way down the length of the house and I assume install the plastic replacements. Or I'd really rather take that entire soffit ledge as it's rather worn, and I know a big part of air sealing is making sure the top caps of the outside stud walls are sealed. So that would give me access to it at that point.

Thanks all for the reccomendations, now we're wavering on getting rid of the whole house fan. I'm going to have to do some more research on whether to keep it.
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Old 03-21-2013, 07:45 AM   #8
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How to plan for attic ventilation for central air when modifying gable vents?


Your not going to be able to reuse the old caps.
A bundle of hip and ridge caps will cover about 25'. (they claim 30' but 1/2 the time some are stuck together and get trashed)
The whole thing would take about 1/2 day with one person.
It's a really simple DIY job to do.
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Old 03-21-2013, 07:53 AM   #9
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How to plan for attic ventilation for central air when modifying gable vents?


Right, with decent air circulation under your roof, from soffit vents up to the ridge vent you keep the whole space cooler. This helps extend the life of the shingles some and would also help the performance of your AC system. As you point out, if you putting in the soffit vents you'll also have a access to the top outside edge of the inside stud wall to perhaps better insulate the area. But you'd likely also get that by using the foam baffles and blown insulation. Replacing the soffit board would probably make it a simpler and better looking job.

Sure, your roofer just runs the saw along the top edge, cutting through the existing shingles and the sheathing, but not down into the tops of the roof rafters. Then they put down a filtering mesh foam piece and cap it with ridge shingles. You can probably get shingles 'close enough' to match the existing ones.
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Old 03-21-2013, 08:26 AM   #10
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How to plan for attic ventilation for central air when modifying gable vents?


Thanks it sounds like it's not as big a deal as I anticipated. At least the ridge vent. The soffits would be a bigger deal, but once we start on some of the outside work, it shouldn't be too big a deal. I'll see if I can get the wife to buy in, as long as we can find some reasonably matching shingles. We have to do some repairs to some flashing anyways, so not terribly out of the way.

From what I've read, it's better to get the externally baffled ridge vents, i.e. shingle vent II vs. others... how big of a deal is that? This video is pretty convincing.



Lowes seems to have it, at 10$ a linear foot... we'd probably need ~50 feet, so that's almost $500?

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Old 03-21-2013, 08:34 AM   #11
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How to plan for attic ventilation for central air when modifying gable vents?


That's the one I would buy.
There not sold by the ft. there sold by the piece.
Might want to call around for pricing.
Any real roofing supply, lumber yard, HD as well as Lowes will have them.
Hard to tell from the picture but it look like you have plain old 3 tab white shingles.
It would be great if you could get a picture of that soffit area.
Not fun trying to run a ciruler saw up side down trying to cut out for a vent, there may be a better way.

Last edited by joecaption; 03-21-2013 at 08:57 AM.
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Old 03-21-2013, 08:38 AM   #12
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How to plan for attic ventilation for central air when modifying gable vents?


My bad, that's a 12" WIDE piece, 4' long, so I'd only need 12-14 of them, @ 11$/pc that's about $150. That makes it a done deal. I'll have to stop by a real roofing supply to find the shingles anyways. I prefer to use the real building supply stores over the big box stores when I can. But that's good to get a rough estimate to tell the wife it's not terribly expensive to do. Now just to figure out the soffit portion. Thanks again!

http://www.lowes.com/pd_14697-228-SH...ent&facetInfo=
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Old 03-21-2013, 08:41 AM   #13
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How to plan for attic ventilation for central air when modifying gable vents?


Quote:
Originally Posted by Ocelaris View Post
I'll have to stop by a real roofing supply to find the shingles anyways. I prefer to use the real building supply stores over the big box stores when I can.
Don't rule out negotiating with them if the box store price is better. They've got wiggle room but most folks don't bother to ask. Better to give them the sale AND get yourself a bit of a deal too.
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Old 03-21-2013, 08:50 AM   #14
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How to plan for attic ventilation for central air when modifying gable vents?


I personally think that ridge vents are extremely over rated in how much air they move. A powered gable end vent moves a lot more air through the attic, and can be triggered by a thermostat humidistat combination. Without soffit ventilation, ridge venting is pretty much useless. If you do decide to install ridge venting make sure you get a vent that is designed for homes with a ridge pole, as they make them for truss configuration, and ridge pole, the latter being wider. A tunnel fan located in the gable end, suspended from the rafters can be used to cool the attic, and with a powered louver in the ceiling can serve as a whole house fan, when its not too hot out. Thats the way I would go. I had that in a former colonial, and it worked really well. I'm not a real fan of air conditioning until it gets really humid, as I don't like the closed in canned air feeling of AC for the most part, so a whole house fan is great for me. I have one in my ranch house now. You really need to get some more insulation in that attic, that is priority one.

That is a really nice attic too. I would put a floor in up there.

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