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

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I never fail to learn something from the information that Mr B Poll provides.  Thanks again from St.Louis!  Go Cardinals!

I'm convinced the attic height is a big factor.  Have a problem I need to address.  New cabin in mid Michigan having problems with ice dams.  So large they knocked off commercial grade gutters last year. Cabin is 30x60 with 4-12 pitch and 3-12 vaulted ceilings. 7 skylights, 3 solar tubes, 16 IC can lights into the attic space and a high efficiency wood burning fire place so lots of heat sources going into the attic. 18"-24" insulation through out and built with energy trusses.  Skylights wrapped with insulation.  For ventilation, soffits are vented their full length and width and baffles run in every gap between the trusses.  Ridge vent used but we get quite a bit of snowfall here.  Bottom line, I'm convinced that I am not getting enough air exchange due to the short attic height. Considering adding gable vents but I've seen mixed reviews on their effectiveness. Thought I may need to go with a power vent. Planning pulling off the gutters rather than installing a very expensive heating system. Winter is quickly approaching so I need to make a move...any suggestions? Thanks.

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Hi Mike,

Air leakage into that vaulted attic is probably your number one issue.  The IC recessed lights need to be AT as well.  They may be, but test them to see if air is leaking past.

I see T&G pine on the ceiling.  That is a potential problem area, even with plastic under (over) it.  The plastic is often stapled up with lots of resulting holes.

Your pictures are showing you where heat is getting into that attic space.  With a 2 1/2 story, the pressure at the ceiling is high, so a really good effort at sealing is needed.  But to be honest, access issues and that T&G will make it difficult.

Power venting will depressurize the entire attic which may remove more heat, but will also pull heat and moisture through any remaining leaks.  Adding gable vents would increase the air coming in the soffits, but not enough to make a real difference.

Make sure bath and kitchen vents are blowing to the outside and not into the attic.

As a last resort, if this years fixes do not solve the problem, remove the snow after every storm.

That will get you started.  I'm sure others will add their advice.

Bud

Following on Bud's comments - How you define problems tends to color the approach used in design solutions.

Einstein said something about how if he had 60 minutes to save the world, he's spend 55 minutes thinking about the problem.  Rushing to solutions helps insure you'll get continuous work, and nothing fixed.

"Keep Roof Colder" leads to either ventilate more, or to "why is the roof warm?"   Maybe efforts should directed toward not heating the roof in the first place.  Stack and depressurization driven leakage from the house to the roof deck once diagnosed and addressed might avoid a lot of "poke n hope".

I do not think NFA is your "cabins" biggest problem. With there being so much insulation between a 3-12 ceiling & a 4-12 roof the only area to ventilate is the upper 1/2 of what little "attic" there is. Having lived in the Western NC area I can see some issues a builder or architect might overlook. Only having a continuous soffit & ridge vents won't do when the house is on a hill side surrounded by trees! If the home was between two ridges with the winds moving up or down that slope most of the year you might get away with it. The photos. tell me that is not the case. You cannot rely on static pressure or the thermal rules to move that much air. It needs some help. From VA down to the tip of FL, gable vents aren't an issue; they are going to be there unless you have a hip roof (like mine). The reason - who knows from where the wind comes & where it is going? In your case unless it is flowing up/down that slope your attic's NFA #'s aren't going to be the whole answer. If the wind is moving across the slope gable vents will assist in the pushing/pulling air through the attic. Turbines may help. Solar powered attic fans may be the best overall solution though (locate them on the South facing roof). Your surrounded by trees! They block wind flow & add  moisture to the surrounding air. Houses surrounded by trees (close to the dwelling) typically have more mold/mildew issues, etc...

The issue that jumps out at me most is you appear to have balloon framing in the walls. The roof shows less snow on the upper half and directly above the outer walls. That lower area indicates a possible combination of air sealing & thermal (stack effect) issues. I'm wondering if the builders air sealed and insulated the juncture of wall to ceiling joists and rafters (using Energy Star, BPI, RESNET, etc. standards). A thermal scan of the home coupled with blue prints, building materials list & how those junctions were constructed may be very revealing! The fix - if that is the cause can be addressed by going into the eave & upper portion of the wall & finishing it right.

Pics 19 & 21 also show the grounds slope along what I take to be your garage. My experience of living in the mountains tells me your will have some water issues. The garage, back door threshold and possible under the siding into the living area may be future problem areas of water penetration. The ground slope HAS to be down and away from anyplace you do not want the water to penetrate! It isn't a good situation once it happens. Water issues in homes don't usually show up until someone smells the tell tale odors. The water running down from the hill sides behind & on the sides need to be channeled away from your garage & house foundation!!!

God Bless you in your new home.

Thanks for the insights. What appears as balloon framing I guess essentially is that on the front of the cabin. The entire front of the structure 30'x30' is open from floor to ceiling. Few more shots attached (I was glad when I got all the pine up with out incident!). My builder suggested the solar vents as a remedy but what about the snowfall? Big storms bring it in by the inches per hour...that's why we are there. My unheated garage usually has 12 to 24 inches on it from late Nov until early May. The turbine sounds interesting. I checked and all can lights are AT and IC.  My next step is to get the scaffolding back out and spray foam all openings around can lights and at the joint between the wall and ceiling. I have a vapor barrier under the pine but yes, it is full of holes where I nailed in the pine. Thought another coat of Poly might do some good but a tough task so it would be a last resort.

Relative to ground slope, the appendage on the cabin is actually the original structure...kitchen, dining and two bedrooms. Grading on the back side is being addressed.  I have all the water captured in the gutters and routed underground down the hill, that is why I would really want to keep the gutters up so I do not create a different problem.

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If I had to name your problem, from what I see it is air leakage into the attic.  The height of the building will result in a high pressure across the ceiling to attic boundary. With only plastic above the pine, it expands every time the pressure in the house changes, doors open and close, the wind blows or the temperature varies.  Essentially, the flexible plastic pumps warm air into the attic.  Between the staples and nails, combined with all of the lights there is probably a 6" equivalent hole from house to attic.  24/7 that will move a lot of heat up there.

Baffles help to keep the insulation out of the soffits, but at the expense of air flow.  It works when the ceiling plane is well air sealed, but static ventilation cannot handle a leaky ceiling.  Adding gable vents will increase the air flow somewhat and will increase the natural pressure at the soffits.  But I don't think it will be enough to end the ice dams.

That large air cavity formed by the web trusses between the first and second floor will feed air into any electrical or plumbing holes that were not sealed and behind any wall coverings not glued air tight.

Bud

Before spraying those can lights do some research1st. Some have to vent their heat - some don't. If they do have to be vented there are manufacturer directions on air sealing/insulating, yet allowing them to vent at the time. The holes in the vapor barrier. Assuming the ceiling is T&G, wouldn't the planks & barrier be drawn up tightly to the rafters?! The new pics show it is balloon framing. Stack effect is more pronounce in these walls. Did you air seal & insulate the outer wall under that 2nd floor? Air will infiltrate between those joists under that floor at a faster rate than at a sill plate. We see it here in SC all the time and we never have the lower temps. you have. Good luck.

You can't ignore the effect of the prevailing wind which has a MUCH bigger impact that the natural draft that occurs from temperature differences.

Net free area also has it's issues, especially with ridge vents. Net free area on ridge vents does NOT take into account the very restrictive "filter" present in most designs. And what happens when the filter loads up from dust caused by the airflow?

A simple hole cut 12x12 is NOT the same as 144sqin "net free area" on a vent that requires the air to make multiple turns before exiting. Back pressure is created anytime you force air to make 90 degree turns.

Bob, I agree.

I just want to add, when the wind blows, ventilation usually gets better and that is good, except when it brings in rain or snow.  Getting the static ventilation to work is a worst case design.

As for NFA, it is hard to trust the mfg numbers. About 12 months ago I took your question directly to the top dog of the roofing ventilation group.  * Using the opening area and not accounting for the filter material the air has to pass through seemed a bit of an exaggeration.  He was blunt and insisted they used an approved standards testing organization to determine that number, end of conversation.  The bottom line is, NFA is directly related to the calculations we do and we really should have a high degree of confidence in their number, I don't.  There are other NFA numbers that seem to vary widely, but I think I'm too low on the totem pole to be able to suggest improvements.

Bud

*18 square inches per linear foot with a 3/4" opening on each side, hmmm!

A 12" roof turbine is rated @ 113sqin, and IMHO it moves every bit of it. Enough airflow moves through the turbines to turn the vent even when the wind isn't blowing!! When the wind blows the turbines really move the air. Unfortunately they do poorly in the aesthetics department and you don't see them much anymore, form over function.. Just upsize the AC unit and call it a day...

"Whirlybirds"... I've got a pair on top of my house, they are on the back slope of the roof so they really can't be seen from the street. They are such a common item around here aesthetics usually isn't an issue. Don't see too many new homes with them though.

Mine are on the same plane, and equidistant from the roof ridge. Interestingly, there are times when one is spinning away and the other is barely moving. Most of the time, thanks to onshore flows and the usual amount of late afternoon wind we receive in this area, they are pretty much moving most of the late afternoon and a bit after sunrise. Never had a problem with moisture getting in either. I've been in attics where the 'birds were frozen (as in not moving) and haven't run into any moisture issues in those either.

Warm dry climate, no snow, mild winters

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