How much can soffit vents help?

Hello,
I have a house from the 60s. It is 30 feet deep and 60 feet long. I have 3'x3' gable vents in each end of the house. Plus there are two roof vents about 8 to 10 inches round.
There are no soffit vents what so ever. Reading on the Internet only talks about the benefits to roof its self. I would like to know how much it would help with the temperature in the rooms just below the attic.
The walkout basement family room and bedroom are always a good 15 degrees below that upstairs and the A/C runs all the time.
Will the vents help?
Thank you,
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Frederick Wilson

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The vents would help. Hot air rises, but if there is no way for cooler air to enter so the hot air can exit the higher vents it gets trapped. That said, without knowing the A/C configuration for supply and return vents what you'll actually observe in the effect on cooling may not be that great. The basement rooms that are at least partially surrounded by the cooling effect of the earth will always be cooler than the upstairs rooms that are not. Improving the air flow within the house (drawing the cool air up to the upper levels and the warm air down to the lower levels) will probably have a more dramatic effect on the cooling.
That said, there is also the risk of moisture build-up in the attic with insufficient ventilation. Installing the soffit vents will help reduce this, keeping mold, rot, and other moisture related problems to a minimum.

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Frederick Wilson wrote:

Depends on your local climate and wind direction. Look at the Building Science Corporation web site. TB
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I live in Indiana. So it does get quiet cold in the winter and it has been 90 degrees F, for the past couple of days.
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Depends how much insulation is in your attic. This is more important in winter. In summer if your ceiling is warm there is no convection (warm air rises), in winter if the ceiling is cool than cold air sinks.

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Frederick Wilson wrote:

The standard for venting is to have 1/2 of the vent space near the top and 1/2 near the bottom of the area being vented. My house had end gables and a powered roof vent. When I reroofed the inspector said I needed more venting, so I added vents in the soffits. The change was remarkable in controlling heat gain (single level ranch style).
To answer your question, you need the soffit vents. Vents in just the upper area is like having a car with brakes only on the rear axle.
The comparison of the walk-in basement with the upper story is not a good one because the ground is a heat sink and provides a lot of cooling. If you A/C is sized for the house, you should have no problem keeping the upper story any temperature that you want providing you don't live in some area where the temperature goes to 110 day after day. And, the A/C should not run constantly. A key component in keep the house cool is how much and how well the attic is insulated from the ceiling below. You should have at least 12" of insulation and depending on the severity of your winters and summers the insulation should be increased 3" to 12" more.
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George E. Cawthon wrote:

Huh?
As long as you establish air currents that remove the hot air, it doesn't matter how.

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CJT wrote:

Didn't like that comparison? I thought it was pretty good.
You get good circulation only between the lower and the higher vents. If there are no lower vents, the circulation in the lower part will be poor to non-existent.
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George E. Cawthon wrote:

If you bring in cool air at the top, convection will do the rest. It doesn't have to enter at the bottom.
I'm not knocking soffits, but they're not a panacea, and they're not the only solution.
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CJT wrote:

The point to having soffit vents is that hot air naturally rises, creating lots of natural convection for free. If you bring cold air in at the top, sure it will help, but first, you're gonna need a powered fan to do that, and it's not nearly as effective as bringing it in a the bottom.

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CJT wrote:

Why would you want to? That requires power. Natural convection, heat rising, will not require any power.

Of course you are. Don't know what you mean by panacea but soffitts plus ridge or end gable vents is the system that is most used. Nope, not the only solution, just the most practical and economical.
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better venting will prolong the life of your roof
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Frederick Wilson wrote:

Basic rule of thumb: You can't have too many or too much soffit vents.
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You really need lots of soffit vents and ridge vents.
On Mon, 03 Jul 2006 19:29:47 -0400, Frederick Wilson

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wrote:

My House is like the OPs. The vents and soffit vents helped a little. I think I need more vents.
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