Updating my attic + HVAC — should I add ventilation?
howdy ya’ll,
I’ve got a 90 year old home in sunny 3C climate zone. I’m updating my HVAC and attic, and wondering if I need to add ventilation.
Right now I’ve got a 25 year old ducted AC/Furnace system in the attic, which is a “ventilated” in that it has 2 small gable vents. No other ventilation. Roof is clay S-tiles AKA barrel tiles, over tar paper and wood, with a 6/12 or 7/12 pitch. The total attic footprint is probably about 1000 sq ft, and on a hot day like today it probably gets up to 130 degrees, maybe more.
I had an energy audit done, and we’re going to pull out the attic fiberglass batt insulation, air seal the attic, put in a ducted heat pump system, and then blow cellulose in.
I’m wondering if I should do anything else smart while I’ve got the attic cleaned out and before I blow in insulation and make it impossible to work in. Most importantly, should I add more ventilation?
The 2 gable vents are tiny, maybe 1.5 sq. ft. each. The exterior has exposed beam eaves without soffits, so it wouldn’t be easy to add pretty ventilation to the low part of the roof. I know Cor-A-Vent has a system for adding ventilation to the “bird blocks” between rafters, but it seems like it’ll be a pretty difficult project to cut out the bird blocking, attach the cor-a-vent, and then re-fit the blocking and redoing the stucco…
How much of a difference will adding ventilation make in this situation? Is it worth the effort?
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Replies
Venting does almost nothing to cool an attic. The amount of air moving thru the attic is too small to have any real affect on the temp of the attic.
The point of attic ventilation is in the cooler months that attic venting does move enough air to disperse the moisture in the attic before it can become a liquid and get things wet.
If you want to save energy and lower your electric bills then get the HVAC out of the attic. OK, you are in the south that is simply impossible and a personal insult to suggest so I am Sorry. LOL
Consider going ductless or ducted in a dropped ceiling.
Burring the ducts is now a possibility and Vapor diffusion port has been approved.
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
https://www.greenbuildingadvisor.com/article/roof-for-a-mild-coastal-climate
Walta
Walta,
Thanks for jumping in here. Curious if that's true that "venting does almost nothing to cool an attic."
According to Lstiburek, "In a hot climate, the primary purpose of ventilation is to expel solar-heated hot air from the attic or roof to reduce the building’s cooling load and to relieve the strain on air-conditioning systems."
https://buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view
I'm in 3A/3C zone, definitely a hot climate.
I think in our case we're planning to mostly bury the ducting in the cellulose regardless, blowing at least 12" of cellulose to reach R-30. I had also considered radiant barrier, but also according to Lstiburek, once attic insulation reaches R-30, radiant barrier won't be effective.