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.
I have not done the math but my guess is if the goal is to keep the attic within 10° of the outdoor temps one would need to replace 100% of the attic air every 5 minutes or so.
That is simply not happening with passive vents. 1 square inch per 150 square feet is the recommendation that is almost nothing really and the passive flow is driven by the temperature differential between inside and outside. If somehow the temp equalize the air would stop moving.
Yes, 10° is a very ambitious goal but what is the worst case 60° maybe if one did no venting. Does it really matter that much if the attic is 120 or 180 if there is an R 38 blanket on your ceiling?
Walta
Walta,
"Does it really matter that much if the attic is 120 or 180 if there is an R 38 blanket on your ceiling?
Isn't heat flow through insulation proportionate to the difference in temperature on both sides?
I agree that is the formula.
How MUCH it matters with R38 is the question.
Walta
Yeah I think those are the big questions --
Will adding eave vents + ridge vents significantly cool my attic more than my two tiny gable vents?
Will that cooling actually impact my cooling load if my ceiling insulation is at R38 but my ducting is in the attic?
Will that impact be worth the effort of pulling out my bird blocks with my roof still on? Alternatively, maybe I save this project for when I need to re-roof in the next 10 or so years...
> air seal the attic ...
Did you mean just "air seal the attic floor" (to isolate it from the conditioned space below)?
In that case, your heat pump is going to have to fight the extreme SoCal summer attic temps, so make sure the installer insulates the air handler really well (they can wrap it in insulation and reflective foil).
Another option if you have the space is to make an "insulated cabin" in the attic to house the air handler, and then bury the duct runs in the ventilated parts of the attic in cellulose. I did this at my place it and it seems to work fine.
Otherwise "air sealing the attic" and "adding attic ventilation" seems like a contradiction.
Actually, air sealing the duct work that will remain in the unconditioned attic the top priority.
As any leakage is the coldest air in the system and under the highest pressure. Every cubic foot that escape into the attic is lost and replace with unconditioned air from outdoors.
Walta
Got it. Thank you all for the replies.
So yeah by "air sealing the attic" what I meant was, air sealing the attic floor from the conditioned space. Unfortunately I just don't think a fully sealed/conditioned attic is going to happen in this case.
I have a team that's promising <5% leakage from my ducts, and they say most installs they can get pretty close to 0% leakage. I can ask for extra insulation on the air handler as well, and all the ducting will be buried in cellulose.
However the question still remains -- will adding proper ventilation to the attic help significantly? I could also wait til it's time to re-roof the house, which might be a ways away...