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Insulating kneewall with no soffits

user-7114034 | Posted in General Questions on

The articles I’ve read talk about air flow coming from the soffits. My house doesn’t have soffits. The only air flow is from the roof vents. Do i insulate the roof and make the space conditioned space or insulate the knee wall and floor leaving the space as unconditioned?

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  1. User avater GBA Editor
    Martin Holladay | | #1

    User 7114034,
    First of all, can you tell us your name? (I'm Martin.)

    Does your house lack soffits or does your house lack soffit vents? Plenty of homes have soffits without soffit vents.

    The soffit is the horizontal board or "ceiling" under a roof overhang. If your house has soffits, you can hire a carpenter to install soffit vents in your soffits.

    If you house lacks soffits, you have no roof overhang, and the water from your roof dribbles down your siding every time it rains. There are a few houses like that.

    If you want either (a) a vented attic, or (b) a vented insulated roof assembly (a vented cathedral ceiling), you need soffit vents.

    If soffit vents are impossible, you will end up with either (a) an unvented attic, or (b) an unvented insulated roof assembly. Both of these are possible, but they have to be detailed correctly.

    For more information on these issues, see these three articles:

    “Two Ways to Insulate Attic Kneewalls”

    “How to Build an Insulated Cathedral Ceiling”

    “All About Attic Venting”

  2. user-7114034 | | #2

    Hi Martin, I'm Ellie.
    I don't have soffits as I have no roof overhang but i have roof vents.
    "Two ways to insulate attic knee walls" article doesn't mention what to do with the sides of the triangular area e.g. one method mentions what to do with the sloped roof and the other method mentions what to do with the floor and knee wall.
    "If you plan to insulate a kneewall and the attic floor behind the kneewall, protect the insulation with an adjacent air barrier. The air barrier should have no leaks, especially in the areas where the floor meets the wall and the wall meets the roof." I don't understand what this means. Is the poly on the triangular inside area behind the knee wall? Or is the poly on the bedroom side of the knee wall?
    If I have roof vents, what do I do? Have them closed off? I'm about to get new shingles, do I tell the roofers not to install new roof vents?

  3. User avater GBA Editor
    Martin Holladay | | #3

    Ellie,
    It sounds like you have a Cape Cod house (1 1/2 stories), but I'm not sure.

    In a typical Cape, there are attics on the second floor (these are the triangular attics behind the kneewalls) as well as a third-floor attic above the horizontal ceiling on the second floor. Which type of attic are you talking about?

    For the vented approach to work, you need a way for air to enter near the bottom of your roof. These could be wall vents in the second-floor attic, in theory -- although this wouldn't be as good as soffit vents. But this is just one component of a venting system. You would also need baffles in the sloped ceiling section to allow air to pass between the top of the insulation and the underside of the roof sheathing. This air could then escape through the ridge vents.

    If you have these baffles in the sloped ceiling, you can go ahead and insert wall vents in the triangular attic behind the kneewall, to admit outdoor air to your second-floor attic.

    Q. "Is the poly on the triangular inside area behind the knee wall? Or is the poly on the bedroom side of the knee wall?"

    A. The air barrier I am talking about is on the attic side of the kneewall. Polyethylene would be a bad choice to use as an air barrier (because it would become a wrong-side vapor barrier). The article you quoted from includes better options for an air barrier at this location: rigid foam or housewrap, for example.

    Q. "If I have roof vents, what do I do? Have them closed off?"

    A. If you have (or intend to install) all of the components of a vented roof assembly, keep the ridge vent. If you lack (or are unable to achieve) all of the components of a vented roof assembly, you will have to convert your roof assembly into an unvented roof assembly, as described in the articles I linked to.

  4. user-7114034 | | #4

    You're right. It's a cape cod house (story and a half). And it's in climate zone 7/8. The attic is not vented. Here's what I'm thinking of doing: ripping out all the drywall etc on the second floor and running new electrical, installing new insulation and drywall.
    Here's how I'd put the upstairs back together (starting with the inside of the rooms (vertical walls) and working my way out: drywall, poly, roxul, plywood and house wrap. The cathedral ceiling (horizontal part of ceiling) would only have drywall, poly and roxul. As for the floor of the inside of the knee wall, working my way towards the ceiling of the room on the main floor, it would go in this order: house wrap, plywood, roxul, poly and drywall. I would try to use the blocking method you mention as best I could. The electrical and the two heat ducts would have the vapour barrier pockets around them.
    I'm not sure what to do with the part of the roof between the knee wall and the cathedral ceiling. I know it would have drywall and vapour barrier and I guess roxul next then the ship lap roof.
    I hope you can understand this. I'd rather you try to understand my description and agree disagree with some or parts of it. I'm an older woman in my mid-50s and this is all new to me - some of your language is lost on me. But I'm trying!
    I am confused as to how I can load up batts of roxul in between the floor joists (floor inside knee wall) or in between the wall studs of the knee wall for that matter. Also confused as to the gable end sides inside the knee wall portion of the attic. I know you said it's in the article, but I couldn't find it. I'll try again. And sadly, here's a pic of the underside of my roof. Should I be scared?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Ellie,
    Q. "I'm not sure what to do with the part of the roof between the kneewall and the cathedral ceiling. I know it would have drywall and vapour barrier and I guess Roxul next then the shiplap roof. I hope you can understand this. I'd rather you try to understand my description and agree disagree with some or parts of it."

    A. The approach you suggested will only work if you include a ventilation channel between the top of the Roxul mineral wool and the underside of the shiplap roof sheathing. This ventilation channel needs to be connected to soffit vents at the bottom of the roof and a ridge vent at the top of the roof.

    Ignoring this advice is a building code violation, and risks moisture damage to your roof.

    If you want this roof to be unvented, it will need to be insulated by installing closed-cell spray foam on the interior side of the roof sheathing.

    Once again, I urge you to read "How to Build an Insulated Cathedral Ceiling." You wrote, "some of your language is lost on me." If this article is hard for you to understand, you should follow the advice of an experienced insulation contractor with good references.

  6. user-7114034 | | #6

    Yes I will check with an insulation contractor. I would like to know what my options are prior to talking to someone and that's why i'm coming to this site as we all know different contractors do things differently and not all of them are correct. RE: vented roof assembly. I saw a really interesting video on this: DOE Building America - UMN NorthernSTAR Research: Roof Overcoat. A roof in northern Minneapolis was waterproofed and insulated above the roof sheathing. They used Perm-A-Barrier Wall Membrane on the roof to waterproof it followed by two layers of polyisocyanurate - one layer was three inches thick and the other layer 2 inches thick. (Based on other articles i'm reading it sounds like the polyiso may not have been the best choice for cold climates given polyiso's reaction to cold temp.) They vented the roof by creating an intake at the top of the wall and a ridge vent at the top. I doubt that I'll be doing this...just found the video really interesting.
    At the end of the Insulate cathedral ceiling article is says: Remember, an insulated sloped ceiling isn’t always a good idea. Sometimes a good old-fashioned unconditioned attic is the best way to cap your house.
    To clarify, an unconditioned attic is this, correct?: https://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads/2016/04/09111814/021230089-1_xlg.jpg
    The portion of the pic starting at the top of the kneewall is what you're talking about using spray foam in, right?
    Then for the attic on the second floor (the triangular attics behind the kneewalls) I am thinking of roxul batts
    For the third-floor attic above the horizontal ceiling on the second floor Im also thinking of using roxul batts. When the room is put back together, the only part of the room that would not have poly would be the section the roof that was done with spray foam, correct?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Ellie,
    The illustration you linked to depicts the triangular attic behind a kneewall. This isn't what I want thinking of when I wrote about an old-fashioned unconditioned attic. I was thinking of the type of attic shown in the illustration below.

    You have a Cape Cod house, so it's too late for you to design a house with an old-fashioned unconditioned attic above all of the living space. The type of attic I'm talking about is found in a two-story house, not a 1 1/2-story house like yours.

    You are correct that the part of the attic where I discussed the use of spray polyurethane foam is the sloped roof assembly from the kneewall up to the horizontal ceiling above your second floor. You could make that sloped roof assembly insulated if you used one of the "unvented assembly" methods in my article, "How to Build an Insulated Cathedral Ceiling."

    For more information on installing rigid foam on the exterior side of your roof sheathing, see this article: "How to Install Rigid Foam On Top of Roof Sheathing."

    .

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