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Looking for guidance on ventilation and insulation for attic to living space conversion

kelchm | Posted in General Questions on

I purchased my first home in central PA earlier this year. One of the projects I’m planning for 2019 is to convert the attic space above the first floor into living space. Currently there are soffit vents on the north west and south east overhangs approximately every 4 rafter bays along with two ventilation fans.

For the portions of the roof sheathing that are currently insulated, it seems to be two 1″ thick sheets of foil backed polyiso insulation. These have been spaced out from the sheathing to create a ~2″ channel between the insulation and the sheathing. I have no way to verify, but I assume that the existing cathedral ceiling on the opposite end of the house is done in a similar manner, just without the attic fan. Other than knowing it was done ~10 years ago, I don’t know anything about the corrugated metal roof or what kind of underlayment was used when it was installed.

My dilemma is to what extent do I worry about ventilation when finishing the space given then there is no ridge vent, and even if there was one, it wouldn’t be effective for a significant portion of the roof?

A local contractor I used for an energy audit proposed the following:
– air seal all top plates and penetrations between first floor and attic
– add blocking above/bellow the existing knee walls then seal with closed cell spray foam
– fill joist bays of finished space with 3lb/cuft dense packed cellulose
– Apply high density cellulose to exterior wall and knee wall cavities.
– Add soffit vent chutes and fill unfinished attic areas with loose cellulose on top of existing fiberglass and cellulose.

In general the research I’ve done points to using dense pack cellulose in a cathedral ceiling like this is a bit controversial/risky, but has been used successfully in some areas. I’ve not seen much discussion on having only part of the roof ventilated as this contractor has proposed.

Any thoughts or recommendations the community can share?

EDIT: Was not able to upload any images to GBA. Please see the following link for images: https://photos.app.goo.gl/zq28mMkfPFCHeh9L8

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Replies

  1. GBA Editor
    Martin Holladay | | #1

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

    If you want to install cellulose insulation between your rafter bays, you need a ventilation channel between the top of the insulation and the underside of the roof sheathing. The existing rigid foam serves the same purpose as ventilation baffles -- so the rigid foam can stay where it is. But you need to retrofit a soffit vent in every single rafter bay, and you need to retrofit a ridge vent.

    If for some reason you don't want to install soffit vents and a ridge vent, you'll need to install closed-cell spray foam from the interior, directly against the underside of the roof sheathing, to create an unvented roof assembly.

    More details on these options are explained in this article: "How to Build an Insulated Cathedral Ceiling."

  2. kelchm | | #2

    Hi Martin,

    Thanks for the link -- that is helpful.

    Were you able to view my photos? Adding ridge vent wouldn't work for front half of the roof given the design of the roof.

    I'm hesitant to use spray foam for the entire roof because of the coast involved and the fact that portions of it behind the knee walls will remain a traditional attic (see photos).

    Would it be reasonable to follow the plan the contractor outlined above but simply replace the dense pack cellulose with closed cell foam in the joist bays above the finished area?

  3. GBA Editor
    Martin Holladay | | #3

    Matthew,
    It's possible to install vents at the top of a shed roof that dies into a wall. Here is one of several brands: Cor-a-Vent Roof-2-Wall Vent.

    It's also possible to use closed-cell spray foam (or the "flash-and-batt" approach) if you want to create an unvented assembly.

  4. Expert Member
    Peter Yost | | #4

    Martin has given great guidance here. You might also want to check out a relatively recent 2-part article I wrote for JLC Online: https://www.jlconline.com/how-to/insulation/avoiding-wet-roofs_o.

  5. kelchm | | #5

    Thanks for the further thoughts and links.

    I did find one other paper which I've found helpful:
    CCBST-2017-Review-Unvented-Sloped-Wood-Frame-Roofs-Cold-Climate.pdf

    Additionally, I did find one source which discusses that vents like the roof-2-wall vent could promote leaks under certain conditions (like snow drifts against the wall), which I think does make logical sense: https://www.finehomebuilding.com/1996/01/01/venting-a-shed-roof

    I also found it interesting that in this post William specifically calls out that "soffit vents and air chutes but no high vent, is a suitable design, except for homes in the coldest climates with high indoor humidities." given that this is the configuration of the existing cathedral ceiling on the other end of the house.

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