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: 2994

Reply to This

Replies to This Discussion

<We use it because we are stuck with it!>  I agree that's what we are doing, but that doesn't make it right.

<No one likes change!>  As you know, I've already encountered that!  But it would be hypocritical of us to complain about other contractors following old habits while we have in front of us a standard from the same "hand me down" origin.

<No one really understands why attic ventilation needs to happen.> Although home owners may not understand, we (all auditors and related contractors) should.

<How to make it work correctly … with incorrect work... > That is often the challenge with a retrofit and add to that, the solution will often have a very limited budget.  Thus it is good to know what is needed in each situation and whether there exists any wiggle room.

<It has gotten entrenched into code and we are stuck.> Stuck is not stopped, it just means we are going to have to dig a little to get out, and get a little muddy in the process.

There are few problems that a bunch of money can't fix, but very few owners will pop for those expensive solutions.  If a house needs more ventilation, the owners deserve to know if the estimated solution is a wag or one based upon their home and their location.  The 1:300 is a wag and we can do better.  I was hoping someone had.

Bud

Bud,  I know you believe every house is a unique structure and worth individual attention.

If you have a 900 sf ranch with a full basement, or 1800 sf conditioned space. ca 1965.  No vaults etc. The 1:300 is 3 sf of attic ventilation. That would be NFA.  Ridge Vent is 11 NFA inches per foot.  Turtles are rated at 50 NFA for a 10 inch diameter (I don't buy that one, but I'm not rating them.) A 6 x 16 soffit rates out at 64 NFA.

3 sf = 432 sq in NFA.  Does not take much.

The 1:300 was about 2x of the 1:576, which Rowley said was OK. It is half the 1:186 that he thought was too much.  He was also talking about total ventilation -  Gable to Gable.  No high / low.

Food for thought.

John, to be honest, I don't think we can consider any of the Rowely work as relevant, 1939, cross ventilation only, and all done in a lab.  One could say we have had 70 years of field work to add credibility to his conclusions, however, there isn't a one size fits all solution.

If we look at just the cross ventilation limitations, as compared to what we experience with a good ridge and soffit combination, there is no way equal vent areas will produce the same effective air flow.  That alone should allow one to install substantially less NFA for a ridge/soffit install.

Then, 1939 is even older than I am.  A lot has changed since then and our understanding of attic issues has improved, I hope. 

Your example is for 1:300, which assumes an air sealed ceiling, but that is not always the case and certainly wasn't way back then.  So air sealed or not is a factor of two, height can be another factor of two and other variables can probably push that 3 ft² to anywhere between 10 ft² to zero.  I'm exaggerating, but there is a cost that goes with ventilation and we need a better way to pick a number.

Bud

RSS

Featured Forum Discussions

What's the Most Profitable HVAC Job for Your Company?

Started by Wayne Melancon in HVAC. Last reply by Wayne Melancon 31 minutes ago. 3 Replies

Lighting replacement - 300 watt LED flood light

Started by Mary Sinkler in General Forum. Last reply by Jan E. Graveamker 16 hours ago. 4 Replies

Glass Block Windows

Started by Ken Kalke in Best Practices. Last reply by Sean Lintow Sr on Saturday. 1 Reply

EZ Green Home is Hiring!

Started by Chloe Chapman in General Forum Jan 5. 0 Replies

Videos

  • Add Videos
  • View All

Latest Activity

Wayne Melancon replied to Wayne Melancon's discussion What's the Most Profitable HVAC Job for Your Company?
"I'll agree with you there Ken. We'll get shopped by residential when the customer already…"
31 minutes ago
Horace Douglas Hunt, Jr. commented on Horace Douglas Hunt, Jr.'s photo
Thumbnail

IMG_3138

"and....this was the best looking spot in the air handler. Seal the unit if you want to have any…"
2 hours ago
Horace Douglas Hunt, Jr. commented on Horace Douglas Hunt, Jr.'s photo
Thumbnail

IMG_3139

"Beginnings of mold....due in some part to leaks in air handler pulling hot humid air from…"
2 hours ago
Horace Douglas Hunt, Jr. posted photos
2 hours ago
Eric Kjelshus replied to Wayne Melancon's discussion What's the Most Profitable HVAC Job for Your Company?
"What is your "Key Performance Indicator" dollar per hour? gross dollar per man day? % of…"
11 hours ago
Jan E. Graveamker replied to Mary Sinkler's discussion Lighting replacement - 300 watt LED flood light
"Hi Mary, What is the purpose of lighting the backyard ?  Sense of safety, night time…"
16 hours ago
Chris Laumer-Giddens liked Chris Laumer-Giddens's blog post Rock Wool Insulation for Floor of High Performance Tiny House
17 hours ago
Joe Davis liked Wayne Melancon's discussion What's the Most Profitable HVAC Job for Your Company?
20 hours ago

Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported by the U.S. Department of Energy) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network. Please honor our Guidelines

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service