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Attic ventilation; ridge vent with[out] gable vents?

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Jul 08, 2013, 09:58 PM
SturdyNail
Attic ventilation; ridge vent with[out] gable vents?
When we had our house re-shingled around seven years ago, we had a ridge vent installed.

We still have gable vents too and I'm wondering if they are still useful or if they are inhibiting the function of the ridge vent. Our air conditioner seems to run nearly constantly in Summer.

An addition on the house covered some of the soffit vents in the the back of the house (please see the sketch I've linked to).
Sketch of house

With that in mind, I thought it would be best to keep the gable vents.

If I had the money and the time, I'd put a temperature and humidity sensor in the attic crawl space and take readings over a couple of weeks and compare them with outdoor conditions. I'd like to take readings with the gable vents open and then with the gable vents covered. I don't have the time or the money though, so I'm wondering what you think the best combination of vents would be.

I will tell you that I was on a ladder a couple of days ago and the heat was rushing out of the attic crawl space through the gable vent.

So, what is the best combination? Cover both of the gable vents? Cover only the North gable vent and keep the gable vent on the South end open (the end where some soffit vents are blocked)? Keep both gable vents open?

Thanks in advance.
Jul 08, 2013, 11:23 PM
Jaybee
The ideal is to have equal total SF openings for letting hot air out (the ridge vents or the gable end vents) as you have allowing cooler air it (the soffit vents). You probably already know this but it's a simple convection system: Hot air rises and exits out through the ridge vents. Since air pressure must remain the same, the air is replaced by pulling outside air in via the soffit vents. Since that air is outside the house, it is relatively cooler in relation to the attic air temperature. With soffit vents scattered all around the house and the ridge vents running the entire length down the middle, an even and equal amount of cooler air is distributed through the attic, cooling it evenly.

While you could say that more openings are better, the problem with having ridge vents in addition to the gable vents is that since the gable vents are located up high, they allow hot air to escape from the attic. However, some of that hot air can be sucked back in through the lower part of the gable vent. So instead of getting cooler air spread evenly, you get hot air reentering the attic and making a shorter path to the ridge vents.

I would block off the gable vents - especially if they are the rectangular style shown in your sketch. While you are in there, make sure that you have a clear air path from the soffits to the main attic space.

I've found that if the natural convection of a ridge and soffet vent system is not enough, then the best next step is to install a thermostatically controlled power vent fan. This would be mounted up high near the ridge and basically works as a powered ridge vent.

I should add though that if you want the best 'bang for the buck' to take a load off your A/C unit then the best thing you can do is to add more insulation to the attic space. Just make sure that those soffit vents can still work.


Jaybee
Jul 09, 2013, 09:43 AM
Sparky617
There is a fair amount of discussion on powered roof vents and whether they actually save money on cooling costs.

The fans take electricity to run. Unless you have meticulously sealed EVERY opening from between your conditioned living space and the unconditioned attic you are likely pulling conditioned air out of the living space and into the attic. This sealing would include the attic hatch or door and every penetration into the attic for plumbing vents and electrical wires and fixtures. Without the fan pulling air you will lose some conditioned air through these penetrations but a powered fan will create a negative pressure in the attic and pull a lot more air through the holes. If you go into your attic and look for dark streaks on the insulation you can find many of the penetrations.

My own house built in 1999 has 3 triangular gable vents (main house and the lower garage/bonus room roof) and continuous soffit vents. I don't have a continuous ridge vent.

Here is a good article on the subject, link opens a PDF.

https://www.dom.com/about/cons...ttic_ventilation.pdf


General Disclaimer

Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.

My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
Jul 09, 2013, 10:48 AM
swschrad
fans are also a maintenance item. they gunk up and screens need to be cleaned. sleeve bearing motors eventually gunk up and the motor stops. ball bearing motors start out a little noisier and get more so with age. on a really hot day, the thermal breaker might open and you have no attic ventilation -- if it didn't, the motor could overheat and potentially burn out.

we fought it all at the folks' house. I have two wind turbines on the top of the roof and soffit ventilation at the bottom. while I would not recommend sleeping in the attic, it's not killer, and the hot air does escape. I've pulled wire in June, and it was not bad. all from natural convection and a little help from a passing breeze.


sig: if this is a new economy, how come they still want my old-fashioned money?
Jul 09, 2013, 12:57 PM
Conrad
Just to add?
Do you have enough insulation in your attic for your heat/cooling temp zone? Since you mentioned the AC runs a lot, assume it is sized right for your home?
And it is not unusual for even properly sized central AC units to run more than one thinks they should.
Jul 09, 2013, 01:18 PM
SturdyNail
Jaybee, Sparky617, swschrad, and Conrad, thanks for your ideas.

Jaybee, you've reinforced what I suspected would be the better way to go (i.e., close both of the gable vents).

Sparky617, I'll find some down time to read the article you linked to.

swschrad, I actually like the look of those "chef's hats" on a roof. I don't know how they'd blend with the ridge vent though.

Conrad, we definitely could use more insulation in the attic. Also, the AC is kind of limping at the moment. The service guy says it's not holding the pressure it is supposed to. So we're evaluating whether to spend money to see if they can find and fix a potential leak or spend more money to get a new unit. Our current unit is over 10 years old. It's hard to say if the unit is sized correctly, because there is an area of the house that does not cool/heat effectively. That is probably more a function of lacking returns and long supplies (I had another thread on the duct-work issue duck-work thread ).
Jul 09, 2013, 04:51 PM
Sparky617
Around here, where AC is the bigger load than heating it is pretty standard for 2 story homes over 2000 square feet, built in the last 15 years to have two units one for the first floor and one for the second. It costs more than one bigger unit but there are a number of benefits, no floor space chewed up with chases between floors for heat ducts, better ability to zone the house and as I found out once again over the weekend when a capacitor on my downstairs unit went bad, we still had half the house with AC until my HVAC guy could come and replace the capacitor yesterday.

I'd probably bump up the insulation in the attic, make sure any ducts in the attic are well sealed and insulated and read the article I linked to on my earlier post.


General Disclaimer

Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.

My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
Jul 11, 2013, 01:08 PM
SturdyNail
That's an interesting (short) article Sparky617.

Don't you love it when science disproves what science thinks it proved? It makes me leery of anything "state of the art (or science)", like foam, house-wrap, butyl flashing, PVC lumber, etc...
Jul 11, 2013, 02:19 PM
Sparky617
quote:
Originally posted by SturdyNail:
That's an interesting (short) article Sparky617.

Don't you love it when science disproves what science thinks it proved? It makes me leery of anything "state of the art (or science)", like foam, house-wrap, butyl flashing, PVC lumber, etc...


Part of the challenge is what works in hot and humid Florida is probably not what works in hot and dry Arizona or cold Minnesota. Air conditioning becoming nearly as universal today as central heat was in the 1950s also changes the equations in the house. Homes being built tighter has lead to lower energy use but increased problems on indoor air quality.

This website has some good regional information on how you should build a home based on the local climate. http://www.buildingscience.com...es/designs-that-work


General Disclaimer

Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.

My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.
Jul 11, 2013, 03:08 PM
SturdyNail
A big challenge in my part of the country is that, with respect to weather, we get it all. We have spells of hot and humid weather (like we're in now) and sub-zero blizzards and everything in-between.