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Open ceiling in small cabin

dlbrannon | Posted in Energy Efficiency and Durability on

Hi there,
we are owner-building a 16×34 cabin in Upstate SC (zone 3). We have a vented metal roof system with metal sheeting, double bubble, and 2×4 purlins over 2×6 rafters. The intention is to put 1×8 Knotty Pine tongue and groove boards over the rafters to create a ‘vaulted’ ceiling inside.

Since we would like to keep the ceiling open, and we need to only insulate between the joists, but didn’t plan accordingly during that phase. Oops.

So, that being said, our inspector directed us to ResCheck site where we can pass with a R19 in the ceiling verses R30, b/c of the whole house thermal envelope Trade-off. Great.

I am getting confused, however, about what needs to be done, in addition to the insulation in between the joists.

I found a Roxul brand R23 batt that is 5.5″ deep, that will work perfectly in the 2×6 space. However I don’t know exactly what else we will need to do:
1) do we need to seal all the seams/edges with expanding foam to eliminate leakage?
2) do we need to install baffles between the unfaced insulation and the underside of the roof to maintain the venting system?
3) do we need a vapor barrier and if so, what and where?

Thank you in advance for your insight!!

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I'm confused by your reference to "joists." I'm guessing that you are really talking about rafters, not joists. (Rafters are sloped framing members used to support a roof. Joists are horizontal framing members used to support a floor.)

    You are trying to insulate your sloped roof assembly, right? We're not talking about a flat ceiling or an attic floor, right?

    Of course, your building inspector is correct that every house needs roof insulation or ceiling insulation. (The "double bubble" product you installed is basically worthless as insulation. Unfortunately, it represents a wrong-side vapor barrier, so you should probably cut it up into small pieces with a utility knife from below, removing as much of it as you can, and putting it in a dumpster before you insulate your rafter bays.)

    Here is a link to an article that explains everything you need to know about insulating this type of roof assembly: How to Build an Insulated Cathedral Ceiling.

    Unless you decide to insulate with spray foam, it's essential that you include an air barrier on the interior side of your roof assembly. Tongue-and-groove boards leak air like a sieve, so you can't depend on your finish ceiling to limit air flow. The usual solution to this problem is to install gypsum drywall with taped seams as your interior air barrier. (This work is done after the insulation work is complete.) Once the drywall is installed and taped, you can install any type of decorative ceiling you want.

  2. dlbrannon | | #2

    thank you for your reply - yes, you are correct, I meant rafters, not joists - the hour was late! :P

    I know the double bubble has no insulation advantages, but there is nothing else under the metal roof, other than that - I am confused as to why we need to remove it, I thought it was basically a sound dampener? It sounds like you're saying its actually detrimental to leave it?

    If I read the article correctly that you posted (thanks for that, btw) it looks as though I should have the metal roofing, then the baffles, then the insulation. If we are using the R23 5.5" batt insulation I found, can the insulation and the baffles fit into the 5.5" rafter width?

    Also, we have already had the rough electrical in the ceiling - if we added drywall, we would have to have all those moved. Is there anything else we can put up behind the T&G, like a film that would block the air from flowing through?

    The R23 batt I found was the most affordable option, so that is the most appealing for me, verses switching to a Foam insulation, which was twice the cost. The article seems to say that the Unventilated option is actually preferrable - do you think that would be the best option for the long run, paying the money now, but less headache later?

    Thanks in advance!

  3. GBA Editor
    Martin Holladay | | #3

    Either the ventilated or the unventilated option will work, as long as the details are correct.

    The double-bubble product, I assume, is made of polyethylene plastic -- perhaps with a metalized finish, perhaps not. It is a vapor barrier. You don't want to install a vapor barrier on the exterior side of your insulation, because a vapor barrier traps moisture and prevents your roof assembly from drying to the exterior.

    If you decide to install spray foam, your roof assembly won't need to dry to the exterior, so the double-bubble product can stay. However, if you install the Roxul mineral wool batts, then:

    (a) The double bubble product has to be removed.

    (b) You need to install ventilation baffles. Ideally these ventilation baffles will also be an air barrier; I suggest that you install site-built ventilation baffles. (For more on this topic, see Site-Built Ventilation Baffles for Roofs.)

    (c) You need to install the mineral wool insulation. Ideally, you would have extended your rafter depth to allow enough room to meet minimum code requirements (R-38 in your climate zone), but evidently you don't want to do that. (Note to GBA readers: if you are planning a house in Climate Zone 3, aim for R-38 roof insulation, not R-23.)

    (d) You need to install taped gypsum drywall as an air barrier on the underside of your rafters.

    (e) If your electrical boxes are now recessed, you need to install electrical box extenders (sometimes called goof rings) on all of your electrical boxes. By the way, electrical boxes in your ceiling are usually a bad idea, because they leak air. Do your best to seal the air leaks at the back of each electrical box with caulk, and to seal the air leaks between your drywall and the electrical boxes.

    (f) Install your tongue-and-groove ceiling.

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