Replies

1. 
   mike_guertin | Sep 19, 2003 03:49am | #1

   The standard roof ventilation formula used by roofing manufacturers, ARMA, and model building codes is 1 square foot of ventilation per 150 square feet of ceiling area IF you unequal venting (only gable vents or only soffit vents).   Alternatively, if you have roughly half your venting at the soffit / eaves edge and the other half at the top (ridge vent, mushroom roof vents, gable vents....) then you only need 1 square foot of venting for every 300 square feet of ceiling area.

    Ceiling area is the ceiling of the dwelling space - not the roof surface area.  A flat ceiling of a building that's 24 x 40 = 960.  If the ceiling is vaulted then you have to measure the vault surface area.

   Now this is a 'standard'.  It may be overkill in some situations like when there's a well detailed air barrier between living space and the attic and all holes are air sealed.  But the ventilation may not be enough in an overly leaky ceiling. 

   MG

1. 
   seeyou | Sep 19, 2003 02:23pm | #4

   This is a story and a half house. I've always wondered how that rule applies to this situation. Do I use the upstairs ceiling only or the downstairs ceiling area behind the kneewall as well, and what about the sloped part where the ceiling is applied directly to the rafters?

1. 
   mike_guertin | Sep 20, 2003 03:24am | #9

   Count only ceilings that communicate directly with attic or rafter spaces.  Where drywall is direct to rafters - measure surface area of ceiling - treat it like a vauted ceiling.  Behind kneewalls I'd count the rafter area rather than trying to determine the ceiling area of the first floor.  It will be a slightly greater number so that's OK - you aren't reducing ventilation but rather increasing it.

2. 
   rjw | Sep 19, 2003 04:14am | #2

   You also need to figure out where that mositure is coming from.

   In my area (NW Ohio) most tiomes that I see significant condensation problems on the roof sheathing, I'm also seeing significant moisture problems in the crawl or basement.

   _______________________

   Why Don't Blind People Like To Sky Dive?

   Because it scares the bejabbers out of the dog

   Your mileage may vary ....

1. 
   seeyou | Sep 19, 2003 02:30pm | #5

   This didn't appear overly moist. There was some staining on the bottom of the copper. The copper was installed w/o any sort of underlayment. In the two wings which have gable vents (but no soffit vents), the underside of the copper was still shiny. No sign of any damage to the sheathing.

3. 
   goldhiller | Sep 19, 2003 04:41am | #3

   Here's a couple of sites with good info.

   http://www.certainteed.com/pro/ventilation/pdf/PrinciplesAtticVent.pdf

   http://www.buildingscience.com/

   Knowledge is power, but only if applied in a timely fashion.
1. 
   seeyou | Sep 19, 2003 02:30pm | #6

   Yeah, that's what I'm after - thanks.

1. 
   brownbagg | Sep 19, 2003 03:36pm | #7

   they have been talk in the deep humid south of  not venting the roof at all 100% air tight. They thinking keep the moisture out and let the intrerior cook

2. 
   goldhiller | Sep 19, 2003 03:42pm | #8

   Well, here's another that comes to mind.

   Scroll a ways down the page and you'll see links to some very good articles.

   http://www.umass.edu/bmatwt/publications/by_author.html

   Knowledge is power, but only if applied in a timely fashion.