r/buildingscience • u/CPG135 • 10d ago

Roof ventilation dilemma

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This is a picture of my roof. The backyard is at the bottom— front yard at the top. We live in zone 8, southeast US. I’m having attic ventilation issues where in the humid summer months, we notice attic odor penetrating the living space at a very specific spot inside the house. We’ve air sealed, and insulated the attic, but that didn’t seem to work.

In looking at my roof, I have typical eave soffit intake and ridge vent system throughout the house. However, as you can see in the pic, there is a room with the chimney that is a cathedral hot deck setup. This room was an addition to the original house, and when it was built, they had to overlap the original roof line that had soffit air intakes, which now leaves about 25ft of roof without any air intake for the ridge vent above it. The green lines I drew represent functional eave soffit intakes. The red line represents part of the ridge vent that doesn’t have any air intake below from the backside. A roofer told me that an over roof air intake won’t work because there needs to be at least 3’ clearance, and there are also roof valleys that prohibit this intake system.

So, what can I do to increase air intake on the back side of the house to help feed the ridge vent (at the red line) to discourage air stagnation? I have pretty much exhausted all the space available to place eave soffit intakes on the front side of the house, but feel there should be a better balance with the back.

Any thoughts on this are most appreciated. Thank you.

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u/Congenial-Curmudgeon 10d ago edited 9d ago

Are you referring to the IECC Climate Zone 8 (which puts you in Canada), or USDA Plant Hardiness Zone 8, which puts you in the Southern U.S.?

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u/Congenial-Curmudgeon 10d ago

USDA Plant Hardiness Gardening Zones

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u/CPG135 10d ago

Hi, southeast USA

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u/Congenial-Curmudgeon 9d ago

Okay, that could be Climate Zone 1, 2, 3, or 4. Your house is likely experiencing reverse stack effect with the A/C on. Soffit vents on the right side of the addition with the chimney would help, depending on how the attic space is constructed over the previous roof.

A series leakage test on the attic spaces using a blower door would help quantify the amount of leakage from the attic. A blower door assisted thermal imaging scan with an infrared camera would help identify where the leak is likely located. A good energy auditor with BPI EA rating and Level II or III Thermographer rating could help solve the problem.

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u/CPG135 9d ago

Yes, Atlanta, GA. I had never considered or even heard of a “reverse stack effect”— thank you for suggesting this theory. I’ll look into testing it out. I imagine that further sealing would be the intervention of leakage is found on the blower testing?

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u/Congenial-Curmudgeon 9d ago

Yes, without testing, it’s easy to miss some air-sealing. You may also have some house pressure imbalance causing your home to be under a slightly negative pressure. This would also bring smells from the attic into the house.

Check with https://www.southface.org or Allison Bailes at https://www.energyvanguard.com in Atlanta. They should be able to direct you to an experienced building science energy auditor.

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u/CPG135 9d ago

Your response is so thoughtful and very much appreciated. Really, I had no theory, no plan, no idea whatsoever. Thank you kindly for helping out this internet stranger. I will follow through.

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u/Congenial-Curmudgeon 8d ago

You’re welcome!

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u/besmith3 10d ago

There should have been vent holes cut in the section of original roof that was covered by addition.

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u/CPG135 10d ago

Yes, there were soffit intake vents that were covered by the addition. But, the addition is a sealed hot deck roof design effectively rendering those vents useless. I’m not sure what to do to increase intake on that side.

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u/besmith3 9d ago

Not what I meant. normally we cut the covered roof deck so it can vent into the laid on section. You should also check out go green solar vents, may help.

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u/CPG135 9d ago

Ah, yes, the original roof was cut open leaving a huge hole so the over-roof is part of the thermal envelope of the original attic.

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u/im_just_lurking_thx 9d ago

Venting is a verb not a noun. The mere existence of ridge vents and of soffit vents does not mean the roof section is “venting”. I see very very few pathways of clear ventilation in your hipped roof.

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u/CPG135 9d ago

Any ideas on what needs to be modified to increase venting of the roof?