Soffit venting
Hi, I’ve read that there should be more pressure in the attic than in the house to prevent the cool air in the house being sucked into the attic, I live in SW Florida. I can’t keep the house below 80 in the summer, I’ve installed a new reflective metal roof and am about to get the soffits replaced. I don’t think the board above the soffit (pic attached) has enough venting so we’re going to add more venting. Currently there’s a 12″x6″ hole every 16′ in the board above the soffit. Is there a formula to determine how much venting should be allowed above the soffits? Pics attached of the existing boards above the soffits. In addition I plan to seal leaks between the house and attic and re-insulate the attic and duct work.
Thanks
Mark
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Replies
You need one square foot of “net free area” of soffit vent per 150 square feet of attic space. That’s the minimum. “Net free area” is the open area of venting you need, so you need to go by the rating on whatever type of vent you use, not just the size of the hole you cut to install it.
Best is continuos soffit venting and a ridge vent. To make a bit of positive pressure inside the attic, you want a little more area of soffit vent than of ridge vent (or whatever type of “outlet” vent you’re using if not a ridge vent).
Bill
> more pressure in the attic than in the house to prevent the cool air in the house being sucked into the attic
On the other hand, it's even worse if pressure in the attic pushes hot summer air into the house. Cool house air already creates a stack effect pressure pulling air from the attic to house (although there may be other pressures that counteract this). Don't make things worse by applying cold weather (when stack effect goes the other way) recommendations. In your climate, I'd stick with balanced.
More attic ventilation can make a significant (perhaps 25%) difference in peak heat loss through the ceiling (but less % for the whole house) . See below:
http://www.fsec.ucf.edu/en/publications/pdf/fsec-cr-1496-05.pdf
Do blower door directed air sealing on the entire house (to reduce latent and sensible load).
You should take the time to read this article.
https://www.greenbuildingadvisor.com/article/all-about-attic-venting'
Please understand As long as you have some insulation in your attic venting your attic will have almost no affect on the temperature in your house.
What would give you a 25% improvement is finding a way to get the ductwork out of the attic and inside the house, think mini split system. If that can’t happen, conditioning the attic is the next best option about 10% better.
Walta
As Bill pointed out, it's not just about having enough venting at the soffit, but also important is having the right balance between soffit vents and ridge vents. You'll have to pay a few bucks for access, but this article from FHB has a sidebar on how to calculate what you need for soffit and ridge vents:
https://www.finehomebuilding.com/2009/07/16/venting-a-tricky-old-roof
Mark,
In all climate zones, it makes more sense to concentrate on sealing air leaks through your ceiling than in improving attic ventilation.
Get an airtight ceiling, with adequate insulation above the ceiling, and you'll be fine -- no matter what venting details you have.