Attic ventilation
The information available on your site is awesome, so thank you for making it available to others.
I’m located in 2A region, a coastal county in Texas. I recently purchased an approximately 1120 sq ft farmhouse built in the 1940’s. The home currently has three turbines in peak areas, each within about 10 feet or less from a gable vent. There are no soffits; the roof hangs over about 8 inches (in most areas) and has about 2 -4 inches, ( most areas) above the top plate under the roof interior. The HVAC is in the attic space.
The house is basically a “T”:
-The original house frame makes up the “T”‘s top bar with a gable vent and turbine at each end.
-A later addition, the lower bar of the “T” has a gable vent and turbine at the end.
-The 18 foot long portion of the house – between- the ‘top T bar’ and the room at the ‘bottom end of the T’ currently has no turbines or gable vents. This is also the location of the HVAC.
The previous owner bought it from the original owner, I’m assuming, to flip it. He put on a new roof, but did not obtain a WPI certificate (required in a coastal county) which means it wasn’t inspected to meet coastal roofing requirements for insurance coverage. Since this is my residence, I need the insurance coverage, so I’m putting on a new roof. $$
The roofer is coming this week, he will remove the turbines, add 80 ft. of ridge vent, and leave the gable vents. He suggested I add soffit holes and insert soffit caps (a screen base that fits in the holes) under the edges of the roof just above the top plate. The ridge vent will include the space which currently is without direct ventilation & the HVAC location).
After reading your information, it appears adding soffit holes would be a good idea. Would I leave the gable vents? If this is recommended, it will be a DYI project, and done after the roof is completed, unless completely not recommended.
Also, 80 feet of ridge vent doesn’t sound feasible; would you recommend less? I’m estimating that is every inch of roof ridge that I have.
I will also add, it appears the A/C unit may actually be the wrong size for the house because there was condensation on the interior duct vents when the A/C stops blowing; of course it may be related to the work being done – doors constantly open, etc. It was left to be determined later since I can’t replace it now anyway.
Any suggestions on the attic ventilation is greatly appreciated.
Thanks very much.
NM
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Replies
N. Madrid,
There is no justification for venting attics in a hot, humid climate like yours. Building scientists understand that fact -- but some building inspectors and code officials don't, unfortunately.
For more information on this issue, see All About Attic Venting. (For you, the most relevant section of the article is the section under the heading "Hot, humid climates." That section includes an explanation of one of your problems -- the problem that you described this way: "There was condensation on the interior duct vents when the AC stops blowing.")
In your climate, you don't want any attic vents. Of course, you want an airtight ceiling. You also don't want any ducts or HVAC equipment in your attic.
Since you have HVAC equipment and ducts in your attic, the best measure it to transform your vented unconditioned attic into an unvented conditioned attic. Here is an article that explains what you need to know: Creating a Conditioned Attic.
Once you decide what you want to do, make sure that you get your plan approved by your local building department. Good luck.
What Martin said- on the gulf coast a ventilated attic brings more moisture into the house than it removes, and creates higher decompression/uplift condition during hurricanes than would occur with a sealed, unvented attic.
If there are air conditioning ducts and air handlers up there vented roof also increases the risk (& quantity) of liquid condensation during the cooling season due to the humid outdoor air in the attic. With a sealed attic the dew point (but not the temperature) of the attic air tracks that of the conditioned space- the air conditioning dries the attic out, even though it doesn't cool the attic.
An unvented roof may make the attic somewhat hotter- less than what you might think, but from an energy use point of view it lowers the whole house infiltration rate, and the latent load along with it.
More information on it than you ever cared to know about the relative merits of vented vs. unvented attics in a hot humid climate can be gleaned from this document compiled by the Florida Solar Energy Center:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
Thank you both very much for your input. It makes perfect sense, I cannot make a change to an unvented attic; there's not enough time to get the planning completed. If the roof requirement wasn't immediate, things might be different. I may however look at changing that a bit down the road, esp since I don't have a tall attic, and I may have condensation issues.
I got about a third of they way through the Florida extremely informative article before I needed to stop. It has so much information, and the information listed for ridge vents was helpful since I will be using a vented attic for now. I will take the information I have so far, and try to put it to use.
You've both been extremely helpful. I will follow up on this thread later, when I have more information to add.
Thanks again for your time and all of your valuable input, I appreciate it.
NM
Good