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Community and Q&A

Is it OK to still install a soffit vent with an unvented spray foam attic?

Danny Kelly | Posted in Green Building Techniques on

I realize that a soffit vent is not really required as part of the roof system but was thinking that we should think twice before removing it – it may allow moisture to escape from the overhang if any gets in.

Last time I did a foam roof I asked the installer this and he told me no – that they filled the entire soffit full with foam and that venting defeated the purpose of an unvented attic which I of course understand the theory behind an unvented roof assembly. But when it was time to install the foam, the boxing was not installed yet, so the cavity was not completely filled with foam (not that that really matters). I recently asked the technical support at Icynene the same question and he explained to me why you do not vent an unvented roof assembly – he obviously did not get my point nor understand my question.

I am thinking it is better to be safe than sorry in this instance. I am planning on blocking between the rafters to give something for the foam to back up to. I am worried that if moisture ever gets into my soffit/overhang through solar vapor drive or a clogged gutter that overflows that there will not be any way for it to escape if there is not a vent there. I do not think I am hurting anything by venting this small space. Any thoughts?

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Replies

  1. Riversong | | #1

    Danny,

    Take your thinking to the next logical step. You're concerned that a closed soffit might get wet someday and, not having an escape route for moisture, become damaged by decay organisms. Good so far.

    But the roof deck is far more important a structure than the external soffit box, since it protects the entire house from rain, sun and wind. What happens if the roofing leaks (happens eventually on every house) and the roof sheathing doesn't have an escape route for moisture, particularly given that there will be almost no outward vapor drive but a high inward solar drive?

    You say "I of course understand the theory behind an unvented roof assembly" but do you understand the reality of a non-vented roof with an impermeable and non-hygroscopic layer attached to it?

    Except perhaps in the most humid environments, roof venting always contributes to a more durable roof structure.

    However, second only to an unvented closed-cell foamed roof with bituthene membrane, a vented roof with fibrous insulation and no vent channel from soffit to ridge is a recipe for disaster. Better to have a vent channel and no vent openings than vice versa, as the air gap allows some evaporative drying and hygric redistribution.

  2. GBA Editor
    Martin Holladay | | #2

    Danny,
    Q. "Is it OK to still install a soffit vent with an unvented spray foam attic?"

    A. It's OK, but it's unnecessary.

  3. HDendy | | #3

    I'd like to resurrect an old thread here, to see if anyone has lessons learned since this came up 6 years ago. Scenario- unvented spray foam roof assembly, closed / boxed roof soffits. Is it better to A. leave them as trapped air space with no venting, B. fill them solid with foam, assuming there is blocking between the rafters so the attic roof spraying isn't already filling it, or C. add soffit vents.

    A client recently had soffit boards rotting (spray foam roof, unvented soffit). It was localized so I suspect the source of moisture was from some other construction defect.

  4. Chaubenee | | #4

    My spray foam man thinks vented soffits are a good idea under that circumstance. We happened to have a general discussion of it and he claimed to have seem sealed soffits that experienced some sort of moisture issues that he believed a soffit vent would have cured, even though there was no pathway to the ridge and the attic was entirely sealed and within the building envelope. I personally am skeptical of such enclosures and prefer a cold attic with soffit and ridge venting. Zone five, upstate NY.

  5. GBA Editor
    Martin Holladay | | #5

    Hunter,
    Soffit boards aren't going to rot unless there is a source of moisture. If there is a roof leak or an ice dam, of course soffit boards can get wet enough to rot. Otherwise, there is no reason to worry about your soffit boards rotting.

  6. HDendy | | #6

    Thanks Martin. I just heard from someone that visited the house recently that the problem was widespread across the house and on N, E, and W exposures so it wasn't localized like I thought at first. If localized I could rationalize that there must be a defect in the air barrier in one area. Since it was happening at most of the soffits it must be a problem with the detail (trapped space) or, like you mentioned ice dams which are likely to happen along an entire roof. I'm a little surprised by the leake since the roof has ice and water shield underlayment, so the water intrusion must be happening at the fascia board.

  7. HDendy | | #7

    If ice damming is the problem, then obviously cavities filled with foam would not prevent the problem. Now the questions is, would it be worse? Would the sheathing and rafters have been rotting for a longer time before the problem was noticed, assuming not as much moisture would find it's way down to the soffit.

  8. GBA Editor
    Martin Holladay | | #8

    Hunter,
    It's hard to diagnose what's going on at the house you describe without a site visit. But if the soffit is protected from rain, why is the wood rotting? Soffit vents won't change the relative humidity of the outdoor air.

  9. HDendy | | #9

    "relative humidity of the outside air" I think that's getting to the root of the question, and hypothetically speaking now (not just this specific example). If relative humidity is a potential problem (assuming no sources of leaks), then what is the best method for handling it? Wouldn't ventilation be your only chance to promote drying on both sides of the wood?

  10. GBA Editor
    Martin Holladay | | #10

    Hunter,
    Assuming that a building (for example, a barn) has a roof, the wood won't rot. For wood to rot, the wood has to get wet due to rain, ground contact, or splashback.

    Ventilation sometimes helps wood to dry -- for example, when you ventilate a damp attic with cold outdoor air during the winter. Ventilation can also make wood wet -- for example, when you ventilate a crawl space with hot humid outdoor air during the summer. So it isn't true that ventilation always helps wood to dry out.

  11. PaKettle | | #11

    Condensation is a known cause in vented roofing so why ignore it in an unvented one?...

  12. GBA Editor
    Martin Holladay | | #12

    Brian,
    Your comment is confusing.

    You wrote, "Condensation is a known cause in vented roofing." A cause of what?

    You wrote, "So why ignore it [condensation] in an unvented one?"

    Who is ignoring it?

    And what type of condensation to you think might happen in an unvented roof? If an unvented roof assembly is designed properly, it won't have any condensation.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Hunter,
    I don't know much about hot humid climates, but here in the PNW, where the outside air is wet enough that well protected siding on the north face of houses can grow mold, I have never seen any difference between the materials on unvented or vented soffits outside the building envelope. The renovations to houses with both conditions haven't yielded any difference in the amount of rot that I've found. Some other factor always accounted for problems.

  14. PaKettle | | #14

    I didn't mean to be confusing - The discussion was about moisture getting into the roof area and causing problems. Condensation is one of the largest causes of moisture problems right after the more obvious leakage issues.

    No matter how many precautions you take or barriers you put up warm moist air from the interior will eventually migrate into the roof area. Unless it is removed it will eventually cause a problem. Without some form of venting the success of the roof becomes very dependent on both the quality of materials used and installation. A simply ceiling crack covered up by molding can mean premature roof failure.

    I don't mean to be a drama queen about it. I just find it odd that condensation appears to be frequently left out of consideration.

  15. GBA Editor
    Martin Holladay | | #15

    Brian,
    Actually, the discussion on this page is about soffit boards, not moisture in roof assemblies.

    You are right that moisture can cause problems in roof assemblies, and that a properly detailed ventilation channel can sometimes reduce the likelihood that sheathing will rot. However, the existence of a ventilation channel is far less important than a feature that is often neglected: a bulletproof air barrier on the interior side of the roof or ceiling assembly.

    For more information on these issues, see All About Attic Venting.

    While it's true that ceiling air leaks that bring warm, moist interior air in contact with cold roof sheathing can lead to problems, what happens isn't condensation. If the sheathing is cold enough, the moisture is deposited on the underside of the roof sheathing as frost. In warmer conditions, there is no condensation; what happens is sorption. Under these conditions, the moisture content of the sheathing tends to increase when it is in contact with warm interior air.

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