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Community and Q&A

To add soffit ventilation or not?

cambridgethreedecker | Posted in General Questions on


I have a MA ( Climate zone 5) : 3 Family Flat Roof ( Low pitch inverting in to center) with an attic space ranging from 2 1/2 feet to 16 inches. Last year we had a blown-in insulation (to a max of 12 inches) with air sealing through a state incentive program. I do not believe the sheathing has any insulation  attached to it (above or below). The attic space does have some HVAC duct work running through it so I believe it counts as conditioned. Currently we have a 6  Pre-existing Roof vents ( avg 15 inch diameter). 2 are defunct Solar ventilator. This year we are modernizing the building with new siding and decided to put in a new cedar soffit. In the past we had a prefab sheet metal soffit ( built in corbels etc ) that seems to be non-vented except some interspersed sections with four holes. After some research I am still a bit confused. My question is should we put in continuous soffit ventilation based on those attic ventilation calculators? If so which ratio (1:300 etc)  to be used or do I have calculate based on existing roof vents etc?  It seems since we already have roof vents we are compelled to balance the system with soffit vents? Thanks in advance for any insights.

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  1. Jon_R | | #1

    If I understand you correctly, you have a small vented unconditioned attic under a nearly flat roof. Code says a *minimum* of 150:1. In your case, I'd put in as much soffit ventilation as reasonably possible, no less that 100:1 (including existing vents). And install something to monitor attic Winter humidity (so you get advance warning in the unlikely event it is inadequate).

    If you don't have an interior side vapor retarder (< 1 perm), apply vapor barrier paint (it will reduce attic moisture). And spray foam the attic ducts.

  2. cambridgethreedecker | | #2

    I believe my roof would count as conditioned because HVAC ducts run through it? I am surprised for the 100:1 to because it is a flat roof? or because of the number of existing roof top vents I have? Probably down the road I will need to remove some roof top vents especially the defunct ventilators. I was confused because some articles talk about not even venting a flat roof so I was wondering since I already have roof vents I might have "pressurize" my attic with soffit vents.

    Thanks, Al

  3. GBA Editor
    Martin Holladay | | #3

    The fact that you have ductwork in your attic doesn't mean that you have a conditioned attic. It means that your HVAC contractor made a mistake, and put the ducts outside of your home's thermal envelope.

    The best way to proceed would be to add a continuous layer of thick rigid foam insulation above the existing roof sheathing -- but that work is expensive, because you would also need new roofing. At the moment, your ducts are outdoors -- and that's not good.

    Another factor to consider: attempts to vent your attic won't be very successful, because you have a low-slope roof.

    More information can be found in these two articles:

    "Solutions to the Attic Duct Problem"

    "Insulating Low-Slope Residential Roofs"

  4. cambridgethreedecker | | #4

    Thanks Martin,
    Because of my existing roof top vents which I can over the long term remove during a future re-roof job. It seems like there is not additional benefit at this time adding soffit vents or it might likely be a be a barrier to creating a un-vented attic space down the road?

    1. GBA Editor
      Martin Holladay | | #5

      It's hard to speculate on whether adding soffit vents will help under the current circumstances. Soffit vents might help, might hurt, or might leave the situation unchanged.

      Here's what's clear: The best way to proceed would be to add a continuous layer of rigid foam insulation on the exterior side of the existing roof sheathing, followed by new roofing. If you go this route, you'll also need to seal all the vents, and insulate the vertical walls of your cramped attic.

    2. Expert Member
      Akos | | #6

      If your existing soffits had vent, I would not block them with the new install. If your roof deck was in good shape during the last re-roof, it means whatever is there already mostly works, don't mess with it. I would make the new ones the same size as the existing ones and not worry about it.

      You can always block off the soffit vents down the road.

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