Installing Attic Ventilation

9 years ago

We are completing a remodel and both the AC contractor and the insulation contractor told us that our attic was way too hot and we needed better ventilation.

We live in the Phoenix, AZ metro area. Single story asphalt shingle home, built in 1985. The builder installed an attic fan that the insulation contractor says is electric. It has not kicked on during this plus 100 degree weather, so is probably no longer functional.

Have read quite a bit and the one thing that stood out is many attic ventilation systems are set up wrong and therefore, not effective. I had one roofer out today who want to put in a ridge vent, but with nothing else. I think we need both air inflow as well as outflow. He said we could hire someone else to add some bird stops for soffit venting. Shouldn't the layout be designed for placement and numbers, but the same company

Since this is not a contractor we will want to work with, I would like to look for another roofing company. I am just not sure the correct questions to ask and how to know if we are working with someone who can get the air flowing correctly. Can anyone steer me in the right direction? Thanks so much for your help!

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Comments (4)

jackfre
9 years ago
I just put in two low profile vents in the new roof. We haven't seen the really high temps yet, fortunately! I guess I'd get a min/max thermometer and install it in the attic. Put in the ridge vent and see if the temps come down to an acceptable level. If not add mechanical ventilation. You can do this in steps.
User
9 years ago
The ridge vent would be installed by a roofer. The soffit vents would be installed by a carpenter. The size of the vents are dictated by the building code. The required clear area is reduced considerably when both ridge and eave vents are installed.

The attic ventilation you describe is primarily for removing moisture but in a hot climate it will reduce the temperature a bit. Adding a fan can pull conditioned air out of the house into the attic or pull moist air into the attic at night.

To reduce the attic temperature you can add foil faced insulation between the rafters and install air baffles against the sheathing to allow venting to occur at the underside of the sheathing.
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Will it help in cooling your top floor? It could, although if the attic is properly insulated, it really shouldn't make all that much difference. What determines whether or not your attic needs a fan is the temperature and air flow in the attic itself. If the attic is getting too hot, it will eventually damage the roof. And if you have electric wiring running in the attic (and nearly everyone does), the combination of a very hot attic and a heavily loaded circuit could cause a fire. An attic fan may pull air out of the cooled space if there isn't enough air opening to let the fan draw outside air into the attic. When we moved into a house that we lived in previously, it had a fan for the attic (no other outlets other than a single gable vent). I noticed the first summer that when the fan was on, I could feel air being drawn out of the house through and around some can lights in the living room. I went around the house, counted the soffit vents, and did a rough calculation. I figured it needed about six more, so I added three in front and three in back. I also moved away insulation that was blocking several of the existing ones. That helped a lot, and it made the attic a bit cooler too. (Even though during the summer, with the thermostat set at 100F, the fan ran around the clock except for a few hours in the early AM.) Our current house has a hip roof, but the entire length of the ridge is equipped with ridge vent. And the soffit has continuous venting all the way around the house. I've been up in the attic in hot weather, and although I haven't measured, it didn't seem excessively hot.
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First put your inventive mind to work considering whether adding an attic ventilation fan is a good idea if you have not doe that already. In most homes it is not a good addition. If you are in a hot, dry climate, insulation, passive ventilation and a radiant shield are good things. In a hot, humid climate, sealing the attic and insulating the roof deck can serve you well.
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It sounds like there may be a few different things going on, that are likely inter-related... Here is a good initial approach solving the issue: 1. Stop the source(s) of humidity - Attic ventilation is only a back-up system to exhaust any humidity from the attic. In cold climates, the home's inside air is warm and has a higher relative humidity than the unconditioned attic air. That warm/humid air wants to go to the cool/dry spaces, like the attic. Once it hits cool surfaces in the attic is condenses, leading to mold. Here are the most common causes I see during inspections: a.) Make sure the bath fan exhausts do not terminate in the attic space, they need to exhaust outside. b.) Seal any penetrations between the attic and living space. Chimney, flue and gas vent pipes should be sealed with special materials devices, by a professional. 2. Provide a balanced amount of lower to upper ventilation. If you do not have the proper amount or ratio of soffit to ridge ventilation, this can actually make problems worse by de-pressurizing the attic; and drawing more warm/moist air from the living space. Check out AirVent Inc., they have many resources for calculating the right amount of ventilation or finding a local pro to do that for you. It's hard to tell without seeing the space, but 3x 18x60 vents may not provide even ventilation and create dead air zones in the attic. Maybe you could post some pic's of the attic and roof and I can give you some more insights. (specifically attic overalls, around the HVAC unit, at the perimeter by the heels, exterior overall and by the snow fence) .
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Comments (1)
The air vent is used to cool the roof during the hotter months so cooler air flows up thru the attic and drops the temperature and the reverse during the winter. This is a good solution.
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User
9 years ago
Proper ridge vents---(with sufficient soffit venting) work constantly. They work on the principle of hot air rising---the soffit vents let in cooler air to replace the air rising out of the ridge vent.

Nothing else needed---just proper area for air to escape/enter and clear passageways from the soffits to the attic.

A competent carpenter/handyperson can create the soffit vents.

The formula is basically to have slightly more soffit venting area--equally spaced around the roof---than the total area of the opening in the ridge vent.

Example---30 square feet of ridge vent opening would need 32 square feet of soffit openings---spaced equally on each soffit area---maybe 10 soffits per side that were just over 1.5 square feet in area.

There are actual tables for figuring the amounts.
worthy
9 years ago
Forced attic ventilation sounds like a great idea. That's why you see all those whirlygigs atop homes, even in our cold climate. But in practice it usually uses more energy than it conserves.

As linked above, the Florida Solar Energy Center observes,"Detailed simulations suggest that the heat transfer in an attic to a residential building interior in mid-summer is dominated by radiative gains from the hot roof decking directly to the insulation surface ... This mode of heat transfer is more effectively limited by 1) increased attic insulation, 2) a truss-mounted radiant barrier or 3) a white reflective roof surface that limits solar gain to the attic structure." Photovoltaic attic ventilation is more energy efficient than equipment connected to the electrical grid, but it is more expensive to acquire.

As noted by handymac, providing more intake at the eaves than exhaust at the top
works better than a "balanced" system.

R8 correctly points out the peculiar division of labour between the installation of roof and soffit vents.

This post was edited by worthy on Sat, May 31, 14 at 8:38

Installing Attic Ventilation

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jackfre

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