Hi all,

 I am the customer service guy for our energy improvement company.  Recently, a customer asked what difference the orientation of his ridge vent made in relation to the prevailing wind.  In other words, if the vent axis is north-south and the prevailing winds are east or west, how does this affect the performance of the ridge vent?

thanks,

Bob

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When the wind blows it usually exceeds any natural stack effect ventilation, but in most cases it simply adds to what ever is already happening. Direction does make a difference, but it's not enough to justify rotating a home, even at the planning stage.

There are ridge vents that are designed to work better with the wind than others. But if the prevailing winds are reliable one could consider adding a gable vent on the leeward and/or windward sides, rain being one of the deciding concerns.

Bud

Ridge vents generally don't work well. Do some performance testing with a 'smoke bucket'.

Ridge Vent Smoke Test

Hi Bob,

I watched that video and visited their web page and I have to admit they are coming around, all-be-it extremely slowly.  But they missed on this demo.

Sunny and 80 degrees outside with 95 to 109 degrees in the attic isn't a very good setup to demonstrate attic ventilation.  That abundant sun is keeping the air above the roof and around the house much warmer than 80 degrees so the actual delta T is very low.  If we assume a delta T of 10 degrees and call that attic height 10 feet we would have 0.7 Pascals of total stack pressure.  Divide that half high and half low and you have about 1/3 of a Pascal to move the air.  If they had waited an hour the smoke would probably have been gone.

I love their statement "according to accepted common knowledge, this smoke should be rising straight up and out the vents at the peak of the roof".  If they know where the air should be going they should say so and not put the blame on "accepted common knowledge", but their thinking still wants to believe that the warm air is going to rise by itself and pull in its replacement air, not.  They have it close is a couple of spots on that web page where a couple of years ago,, well I won't yell at them for following the "then accepted common knowledge".

The bottom line is that attic venting shouldn't have to do a lot if we correct the other problems, air sealing and insulation, and it is a good thing, since passive ventilation is at best minimal. 

BTW, 95 and 109 are rather reasonable temperatures to start with.

Bud

Show me a single study/video where ridge vents work properly that ISN'T and ad from a manufacturer. I've look high and low, never have been able to find one. most designs are simply too restrictive to be effective. If there is any type of "filter", forget it. IF some air does find it's way through the filter will quickly become clogged with dust.

The old school whirlybirds DO work since they have minimal resistance to airflow. They basically act like an open hole in the roof when no wind is blowing. When the wind blows the whirlybirds really start pulling the air.

I'd like to think that there are some things that we can accept without having to run a study to prove the obvious.  Passive attic ventilation pressures are very low, but it works. 

Bud

Passive ventilation can work. It's the ridge vents that don’t 90% of the time. Very restrictive considering the very low pressures involved. A more "open" solution works better.

So Bob are you more worried about the temp in your attic or the energy  efficiency of your house.  If you have a poorly insulated and air sealed attic what you will do is create a negative pressure in the attic.  Some of the make up air will come from the house.  So you created a situation in which you are pulling conditioned air out of the house. Thus more hot humid air will be pulled in that needs to be cooled. You will also be causing more wind washing of the fiberglass insulation

If more air flow is good why not a powered attic fan. Georgia has outlawed powered attic venting because it does more harm than good.

In the winter when you are not concerned about the attic temp you will still be sucking air out of the house. 

In the summer the roof gets hot from radiant heat from the sun  but it also give off more of its heat  in the same way.  The radiant heat is penetrating the porous fiberglass insulation which in turns heats the ceiling raising the meant radiant temperature of the room making it uncomfortable.

I think we can agree that attics should be air sealed and have an adequate amount of insulation.  Preferably cellulose too.  The air sealing will stop the warm moist air from entering the attic during the winter so the need for venting is kept to a lower level.

If you look at FSEC studies you will see that radiant barriers make a measurable difference in poorly insulated attics which would also be poorly sealed.  But when the attic insulation level was brought up to an R38 the effect of the radiant barrier was barely measurable and definitely not economically feasible.

Why did I bring up radiant barriers when talking about attic vents.  It has to do with controlling the attic environment.   If you have air sealed and insulated like you should then the attic venting will not have an effect on the energy efficiency or comfort in the house.

Go to Buildingscience.com , Greenbuildingadvisors.com  and energyvanguard.com and do some research on the history and benefits (or lack of) of attic venting. 

Attic venting is about keeping the equipment and ductwork that lives up there cool in the summer. Of course it's best to get it all into conditioned space so that attic venting won't matter, but retrofitting is costly. If builders would be willing to give up 4sqft for equipment closet and get ductwork into conditioned space they could cut equipment size by 25%.

Wind turbines allow unrestricted airflow, there is no motor that creates negative pressure like an electric fan. 9/10 ridge vents simply don't work. If you know of any INDEPENDENT research that shows they do work, please post a link.

If it were me building a house I'd spray foam the attic and make it conditioned space, NO VENTING in the attic/roof. Roof would be metal, so no concern about shingle life. With the entire attic conditioned space it wouldn't matter if the equipment/ductwork is up there.

My friend and colleague Dick Tracy (who was an early adaptor in building science with a blower door and scanner in 1979) told me something interesting a few years back.

I commented on several times seeing snow sitting on top of the attic insulation down the center of the attic just below the ridge vent and how I couldn't understand how the snow could get in through the ridge vent.

He said it didn't.  He said that he had made a video of the snow coming in through the soffit vents and heading towards the ridge vent with some of it dropping out to fall in the center of the attic.  I did not think to ask what the conditions were at the time, but obviously it was cold with no sun.  So the ridge vent works under some conditions.

I wonder how much a tight attic floor effects the attic temperature for a given set of conditions, and therefore the effectiveness of the ventilation?  Of course the tight attic floor would put less moisture in the attic and therefore reduce the need for ventilation.

And I agree the conditions under which the smoke test were done were probably about the least likely to produce results - why not do the test under an extreme, and under more than one set of conditions?

Ed

Hi Ed,

I do agree with Dick, the wind can drive snow, especially the very light powdery variety, just about anywhere the air can go.  Fortunately, that light powdery snow doesn't have a lot of water content, but it is unnerving to see it snowing inside ones attic.  But snow, rain, and the prevailing winds are the real considerations that need to be on the table when deciding which vents should be go and which should stay.

As for a tight well insulated attic floor, the winter driving force for natural ventilation almost disappears, but as stated, so does the need.  Yet we are given one shoe to fit all, well two, 1/300 or 1/150.  Just look at attic height.  A low pitched roof will have very little stack effect while a 12' attic can have more than a single story home. 

One of the conclusions I drew from my efforts on understanding air flow was, when the wind blows, the stack effect is irrelevant.  One or two Pascals of stack effect quickly gets lost in the breeze.  It is when the wind isn't blowing that we need the passive pent pressures.

Bud

It really doesnt take much to vent an attic.   The biggest benefit is to remove moisture from building up and to vent excessive heat.  I say excessive heat....some people think you need to get the air temp close to the out door temps.  That is not true. 

From an energy standpoint the heat in the attic should not be a big deal. Insulation should keep that out of the house.  The roof has more of an effect from radiant heat transfer versus convection.  The insulation chosen should be of sufficient depth to protect against the radiant heat .  Cellulose does a better job of that than does fiberglass.  Cellulose also does a better job of trapping air than does fiberglass.

If the attic floor is not well sealed then more ventilation will suck more air out of the house. It gets even worse if you added turbines or attic fans. 

If you do a good job of air seal you block the source of moisture during the cold part of the year.  There will be little moisture escaping in to the attic. 

What I see neglected is the use of baffles in all the rafter bays.  Baffles prevent wind washing of the insulation and ensures that all parts of the attic receive venting.

 

Turbines don't pull near the static pressure that electric fans do. Either is only a concern when there isn't proper intake ventilation.

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