# Calculating attic ventilation

All too often an existing home has far too little attic ventilation and some difficult choices for adding more.  We all know the existing, although somewhat old, method of one ft² of net free vent area (NFA) for every 300 ft² of attic floor space and double that if the ceiling plane has not been well air sealed.  Whatever that number comes out to be should be roughly divided half high and half low.  60/40 and 40/60 cold country vs ac country are options.  But when you are short on vent area and cash, here is a detail not often mentioned.

The NFA is only part of the ventilation equation.  The other half is the pressure across those vents.  A 10 ft attic will have twice the attic stack pressure of a 5 ft attic.  Now, I would have to do a bunch of reading to find out what height attic those old calculations were based upon, but a simple fudge factor would be, short attics need more and tall attics may get by with less.

Feel free to post any references that show how to adjust for attic height, fudge is good but real numbers taste better.

Bud

Tags: attic, ventilation

Views: 1010

### Replies to This Discussion

Hi Joe, just curious, do you have any other vents, like soffit area to allow air to flow in?  If not, the two "whirlybirds" are competing for the same air and also pulling air from the house.  Even gable vents (in addition to the whirlybirds) would be better than none.

Bud

Whirlybirds are no different from any other exhaust ventilation, they need somewhere to draw the air from in order to work properly.

Hey Bud,

A handful of small rectangular eave vents, and 2 larger gable vents (fins directed for air intake). From bottom to top: eave vents, gable vents, 'birds.

As for soffits, I've yet to see any home with them around here, and never got to work in a home that had them either.

The 'birds are a good solution around here, but I always tell people 1) get the ceiling sealed up or say by-by to conditioned air 2) they won't decrease attic temps much or save on AC costs if the ducts are in the attic. They do move a lot of air if set up right.

One of these days I'll try the NFA calculation on my attic setup and see what the results are.

I don't know what number you could apply to the birds.  What you have sounds fine.  Air sealing and extra insulation would be more productive than adding or changing the venting.

I had a bird many years ago in NJ and it did a good job of helping the house cool down in the evenings.  The fact that I could look right up through the vanes concerned me so I installed a piece of plastic below it.  Never saw any water.  Powder snow seems to be more of a problem in my current cold country, but it doesn't represent a lot of moisture.  I don't see a lot of birds up here in Maine.

Bud

Bud,  Don't sweat any research on attic heights etc.  It has already been done.  William B. Rose. Water in Buildings: An Architect's Guide to Moisture and Mold (Kindle Location 2469). Kindle Edition; traces the requirement of the 1:300 back to

In January 1942, the Property Standards and Minimum Construction Requirements for Dwellings of FHA was revised. It contains the following section.

He goes on from there to reference Frank Rowley's work at U of Minn. Frank didn't work with attic heights, he worked with a cold plate in a lab. His general conclusions, in 1939 were that a cross ventilation rate of 1:576 over 3 years of 40% humidity at -10° F was not enough ventilation to prevent attic condensation on the sheathing; AND that an unvented attic would require 43 days at the same conditions.

Rose goes on with the story.  My take is the 1:300 got picked up and never dropped. It has no real scientific validation.

Get the book, most interesting reading for you.  BTW:  Professor Rowley was the President of the organization we know as ASHRAE in 1932. His research established everything we know about U-factors and R-Values and was the primary basis for this information in the 1st Edition of ASHRAE Fundamentals in 1964.

John, I've never doubted that the research has been done, but we are still being taught and promoting 1:300.  Actually 1:150 where the ceiling has not been properly sealed.  Never have I seen accommodations made for attic height.  Nor have I seen any other method for calculating vent area.  It is another of those super simple solutions that our industry would rather continue to use rather than promote what is correct. Correct being more complex.

Any reference to another method of calculating residential NFA?

Bud

Everything that I have read on the subject agrees with what John Nicholas presented. What appears to be a big misconception by many people is that we ventilate a roof to make it "cold" to minimize ice dams. The real reason is that the fiberglass batts allow air to move through them. The air that is trapped in the fiberglass on a relatively warm day (warmer air can hold more moisture) will cool down quickly at night, primarily from radiational cooling to the sky at night. The cooler air increases in Relative Humidity (RH) until it hits the dew point, where the water vapor becomes liquid. Once a liquid, the water has a hard time getting out of that enclosed rafter bay without some ventilation. I believe that Building Science Corp. has a paper on vented roofs, and concludes that if there is at least a 2" air space, there is a chance of having a "cold" roof.

Don't forget that with that ventilating air space, you will still have heat transfer to the roof sheathing by radiation.

A roof cavity that is dense-packed with cellulose should NOT be vented for this reason. Air does not move through dense-packed cellulose. If the cellulose is not dense-packed properly or if there is an intentional air space above it, you will get condensation when there is a large temperature swing between day and night.

The concern with using a fan or other assistance to move air out of the attic space is that it may be sucking warm air from the house as well.

Bud,  Read the book.  Rose never found any research regarding attic height. He only found cross ventilation. He didn't find 1:300 or 1:150 in anything research wise.  He found the first mention of 1:300 in the 1942 guidelines.  He found the 1:576 was not enough to prevent under the conditions he stated.

This guy has been researching attic ventilation among other things for years.  If he hasn't found the research to support anything, then I have a \$10 spot that says it doesn't exist.  Notice he specified cross ventilation. Not High or Low or some of each.

Hi John,

I have to apologize but the chance that I'm going to read that book are rather slim.   After a months of emergency rooms, echocardiograms, EKGs, blood tests, and a pile of horse pills every day, I'm afraid my backlog of reading is beyond hope.  They have me all fixed up, but it will take awhile for my spirit to recover.

I specifically haven't addressed cross ventilation (wind driven) as whenever the wind blows ventilation will be better than whatever the static conditions can generate.  Therefore, static is basically a worst case condition.  If it can provide a minimal acceptable level, all else is better.  As for that minimal level, it has many variables, far too many to go into here even if I knew them all.  My point with this discussion is that we are taught 1:300 to provide a minimal amount of NFA.  That NFA provides static air flow based upon the height of the attic.  If we are to ignore height then we (Rose) are saying that static air flow is insignificant and only the wind should be considered.  Unfortunately that doesn't work everywhere.

Bud

Rose was not saying anything about height, NFA or cross ventilation. Rose was summarized the history of attic ventilation research as he found it.  Rose did no actual hands on research. He read what Rowley and other conducted.

Rose concluded, based on his research, there was not much research done to supper the 1:300 or most other things we have written into the codes over the years.

He was not saying this is good or this is bad.  He was saying this is what I found. "No research to support …  "

Sorry for the delay John.

If we accept that there has been no research to support the 1:300 (or other) guideline, then why is this such a definite number we should follow?  Some of the recent discussions on vapor retarders (there have been many) are good examples of accommodating a variety of climates and weather conditions.  Shouldn't we be doing the same with attic ventilation?

I have never lived in an area of heavy fog, but I cannot imagine inviting that rain cloud into my attic.  Attic height is just one of a number of variables which should be discussed and considered when determining how much (if any) natural ventilation is required.

Then there is the "often discarded" option of some form of mechanical ventilation.  A modest level of mechanical ventilation with a reasonable amount of NFA produces very little change in attic pressures, and in the case of an ac dominated hot climate, that change reduces the house to attic pressure.

Have you or anyone seen a comprehensive list of what should be considered when deciding how much NFA to install?

Bud

We use it because we are stuck with it!  No one likes change! No one really understands why attic ventilation needs to happen. How to make it work correctly or the problems with incorrect work on the attic enclosure.

We do not realize the problems we create for ourselves because we do things the way we were taught, they taught us that way because they were taught that way and so on.

It all goes back to 1942 and the FHFA picking out a number of 1:300 because it was half of a number and twice another number. Like "Why do we test houses with a blower door at 50 pascals?"  Answer: "Because we couldn't get results at 75 pascals!" Stolen from someone, probably Dr. Joe.

It has gotten entrenched into code and we are stuck.

--

It is also indicative of a larger problem.

In codes, everyone likes simple prescriptive stuff like: "30 amp circuit requires X gauge wire!"  Easy to read, remember and assess when you inspect it.

With Energy, not so easy.  Is more insulation in the attic OK?  Does it make up for eliminating the insulation (the type I don't like) HERE?  Maybe, maybe not!

It is the difference between assessing an end result "No condensation in the attic, so vastly reduced chances of Mold" AND the prescriptive "1:300"!

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