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I have the AC blower in the attic and my attic gets extremely hot. One side of the house has regular 8 foot ceilings while the other has cathedral ceilings. Some of duct work that runs in the attic is very close to the roof sheathing, since there isn’t much space. My roof shingles are black and the sun beats on them all day long. The attic has very little ventilation, I believe I have a small vent ridge but no side vents and possibly no soffit vents either. The house was built in the early 90's.

While testing my delta T at the return and registers, on the low ceiling side I get about 13/15 difference, but on the high ceiling side its less. The AC on the high ceiling side struggles to stay cold. This leads to my questions and concerns.

As far as I can tell the duct work is using R3 maybe 6 insulation, see attached pictures, the blower unit itself has a grey cover over where the coils are. It does have some gaps on the insulation but it’s not leaking air and I taped it up as much as I could. I’m concerned that the air coming out of the registers is not cold enough because of the lack of insulation on the ducts since the attic is very hot. I did check for the freon levels and they where fine.

The short duct runs have a nice flow with cold air but the long duct runs have a noticeable difference in temperature. On some of the long duct runs the flow is also weak, which I assume is either a leak I’m not aware or kink.

Should I replace the ducts with R8 ducts? This will not be easy due to the low crawl space.

Should I add R8 insulation between the roof rafters?

Should I add soffit vents?

Do I need to insulate my suction line in the attic ? I insulated it going to the compressor.

I’m basically looking from recommendation from the experts on here on what I can do to improve efficiency and wanted to know if the hot attic, lack of insulation and venting is an issue on the air temp coming out of the registers especially on long runs. What is the biggest change I can do that will make the biggest difference?

TIA, Paul enter image description hereenter image description here

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    You believe your AC is not doing proper job. Based on delta T. The guidance says the delta T should be 15-20 F. You have 15 so on the lower end, but still. I would not spend $20,000 on new roof.
    – DIY75
    Commented Aug 10, 2022 at 17:59
  • "Possibly" have no soffit vents? Should be obvious whether you have them or not. Pictures would help. I assume that the ducting in the attic is all accessible so that there is no ducting above the cathedral ceiling or is there? What is the depth of the space between the cathedral ceiling and the roof decking? What is the pitch of the roof? Where is this located?
    – Jim Stewart
    Commented Aug 10, 2022 at 18:02

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wanted to know if the hot attic, lack of insulation and venting is an issue on the air temp coming out of the registers especially on long runs

Cooled air running through a hot space in minimally insulated duct will heat up. End of story.

A black roof will absorb more heat than a white one. You may consider replacing the black shingles with lighter colored ones. (No, not a cheap option.) Steel roofing is becoming quite popular and many lighter colors are available. (Still not a cheap option.)

You might look into better venting for your roof since it sounds like you many not have enough ventilation. According to OwensCorning*, you need 480 square inches of venting (both intake and exhaust) per 1000 square feet of attic space for proper ventilation. If you have at least that on the ridge, that's a good start, but you need some sort of soffit level air intake, as well, to get actual air flow.

Since proper attic ventilation will help your whole house, that might be the best place to start, then see what impact this has on your AC outlet temperature. If you need to take further remediation steps after that, then do so.

*First response to a search for "calculate roof ventilation". No recommendation or endorsement intended or implied.

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You need an attic fan. It doesn't have to be a "whole house fan" (although those are awesome) it just needs to be able to vent the excess heat out of the attic. The smaller ones are attached to a thermostat and activate automatically when the attic reaches a set point (say, 110°F/43°C) and their only job is to bring the attic temperature down to the outdoor temperature.

Once that's done, yes, you can and should insulate your ducts as much as you can, but bringing down the ultra-high temperature in the attic ought to be the first task.

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  • Thank you for the reply. I also was thinking the first thing is to bring the temperature down.
    – Da P.A
    Commented Aug 10, 2022 at 18:03
  • Effective attic exhaust ventilation depends on proper amount of soffit ventilation. Exhausted hot air must be replaced with fresh air at ambient temperaure.
    – Jim Stewart
    Commented Aug 10, 2022 at 18:07
  • What type of roof do you have? Do you have enough ridge length for ridge vents? Do you have a hip roof or a gable roof?
    – Jim Stewart
    Commented Aug 10, 2022 at 18:11

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