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Community and Q&A

Insulation for vaulted ceiling

Kim Howard | Posted in GBA Pro Help on

We re-roofed our house this summer and found a lot of damage over our family room with a vaulted ceiling. We replaced the plywood and are now redoing the underside. The roof has a roof ridge vent and ventilation holes in the soffits. We are creating on-site baffles with 1″ rigid foam boards. We are planning to use R30C insulation which is denser for cathedral ceilings. The question is do we use faced or unfaced, and do we need to use plastic sheeting as a vapor barrier? We want to get it right this time!
Thanks,
Kim

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Replies

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

    Kim,
    It's always easier to answer a question like yours if we know your geographical location or climate zone.

    R-30 ceiling insulation is less than the minimum code requirement for ceilings in all U.S. locations except the southern tip of Florida and Hawaii. In most of the country, you need to install a minimum of R-38 or R-49 insulation.

    There are no locations in the U.S. where interior polyethylene is required. In colder regions of the country, an interior vapor retarder (a less stringent barrier than a vapor barrier) is required by code. This requirement can be met by including kraft facing on the batts or by installing vapor-retarder paint on your drywall.

    In all cases and all climate zones, it's essential to make sure that your ceiling is as airtight as possible.

    To learn more about all of the various ways that you can increase your ceiling's R-value beyond R-30, see this article: How to Build an Insulated Cathedral Ceiling.

    To learn more about vapor retarders, see these two articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    To learn more about site-built ventilation baffles, see this article: Site-Built Ventilation Baffles for Roofs.

  2. Kim Howard | | #2

    I am sorry I forgot to include my climate zone. I am close to Seattle - 4C. I called our local office to find out the restrictions and they told me that since it is an existing structure R-30 would be up to code. The Washington State Energy Code says R-38 in new construction. I looked at many different options and finally settled on R-30 batts and the rigid foam board we used for the baffle which will give us R-35.
    I have read and re-read all the articles you mentioned, which got me to this point.
    Now my concern is that if moisture does happen to reach the fiberglass insulation I do not want it to get trapped. My sense from all my reading is that you would not want to have a vapor barrier on both sides of the fiberglass insulation. Does a vapor retarder follow that same rule?
    The rigid foam insulation is a vapor retarder and that will be on the roof side of the fiberglass insulation. A faced batt would result in a vapor retarder on both sides of the fiberglass insulation. Is that what we want to make it "as airtight as possible"?
    Thanks for your help!
    Kim

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

    Kim,
    One of the articles I linked to, Site-Built Ventilation Baffles for Roofs, explained why you don't have to worry about water vapor condensing on the interior side of your ventilation baffle (even if you choose to use rigid foam as your ventilation baffle). Here is the relevant information from that article:

    "The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusion. Because of this fact, installing ventilation baffles that are airtight makes this type of roof assembly more, not less, robust.

    "What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?

    "The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.

    "That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
    • Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
    • The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
    • Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting."

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Kim,
    If you choose to use kraft-faced batts, the kraft facing will not "trap moisture." The kraft facing is a "smart" vapor retarder with variable vapor permeance. If the kraft facing ever gets damp -- and there is no reason to think it will -- it will become more vapor-permeable, and will allow inward drying.

    Q. "A faced batt would result in a vapor retarder on both sides of the fiberglass insulation. Is that what we want to make it 'as airtight as possible'?"

    A. No. Airtightness has nothing to do with vapor permeance. You can have an airtight layer (like taped drywall) that is vapor-permeable. It's also possible to have a vapor retarder (like kraft facing) that isn't airtight. So stopping air flow and stopping vapor flow are two different things. If these concepts are still confusing to you, you should carefully read these two articles:

    Questions and Answers About Air Barriers

    All About Vapor Diffusion

  5. Kim Howard | | #5

    We made baffles from rigid foam boards (XPS) and sealed them with caulk and Dupont Tyvek tape. Next, we put up the insulation and sheetrock, then had to move furniture back into the room, use the room and leave the project until after the holidays (about a month). We were just about to start taping the sheetrock a couple of days ago when we noticed a drip coming from the ceiling. We pulled off the sheetrock in that area and found that the room side of the baffle was covered with drops of water. We have now pulled off most of the sheetrock in that room and there is moisture on the baffles across most of the ceiling. Each side of the ceiling is 10 feet from wall to peak. The condensation is on the bottom 8 feet in all but three bays, which are mostly dry. We went down to 15 degrees on several days, and our wood stove was working full time so the difference in temperature inside and out was very high. We also had our Christmas tree in there and the water from it was evaporating into the room. Not sure if those things had any effect, but thought I would mention them to give the full picture.
    Our first thought is that we didn't work quickly enough to get a vapor retarder on, but based on my reading of your site it would seem that it is the air barrier and not a vapor barrier that makes the difference.
    Once the baffles and insulation dry, our plan is to put the insulation back in and then put some kind of a smart vapor retarder on the indoor side of the insulation before we put the sheetrock back on. We are also considering using vapor retardant primer to be double certain, but I wonder if that would negate some of the value of a smart vapor retarder.
    Here are my questions:
    1. In your articles you state “The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusion.” Also, “Although most walls and ceilings don’t need an interior vapor barrier, it’s always a good idea to include an interior air barrier. Air leakage is far more likely to lead to problems than vapor diffusion.”
    We had the sheetrock up, just not taped. If the problem is air, does the lack of taping really allow enough air through to create the moisture we are seeing?
    Also, I am confused. If air is allowed to flow through the vapor barrier, wouldn’t the vapor barrier prevent the air from carrying the moisture through? Is so, where is the moisture coming from?
    2. Is there anything else that we should be thinking about that may have caused this? We want to make sure we address all possibilities before we get it all sealed up.
    3. I have found two vapor retarders that I am considering. Any recommendations or caveats on these:
    a. CertainTeed MemBrain Air Barrier with Smart Vapor Retarder
    b. FSK paper which says it will also add to the R value of our insulation
    4. Would it be a good or bad idea to also use vapor retardant paint? We just want to be certain to do all that we can to protect our house, which is why we would go above and beyond. However, above and beyond is not helpful if it will cause other issues.
    5. Is there any reason we should be concerned about the roof side of the baffles based on what we are seeing? Should we pull the baffle off one bay and see what it looks like underneath?
    6. We put Ice and Water Shield over all the roof sheathing before we laid our shingles. Would that have an impact on any of this?
    7. How quickly do we have to work to get to the point of taping the sheetrock before we get condensation that would be trapped in the ceiling?
    8. Would it make a difference if we do it on warmer days where the temperature differential is less?
    Thanks!!!
    Kim

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Kim,
    I'm really sorry to hear about your experience. Almost certainly, the source of the moisture was exfiltrating indoor air. As you suspected, untaped drywall isn't an air barrier. Large volumes of indoor air can escape through the seams between the sheets of drywall if they haven't been taped, and evidently that's what happened in your case.

    Unfortunately, it's very common for leaky ceilings to drip during the winter. Airtightness is the key to stopping this type of air leakage and condensation.

    It's essential to tape the drywall immediately after it's been installed -- especially in winter.

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

    Kim,
    I've been thinking of your story since you posted your latest questions. Again, I want to say that it is distressing to hear of your problems.

    You came to GBA for advice; you tried to follow the advice; but you still have a major headache on your hands. What's the lesson?

    Well, the lesson depends. The first question is, "Do you have a contractor?" If you have a contractor, then the contractor screwed up, and the contractor has to fix the mistakes at no cost to you.

    If you are doing the work yourself, the story you have shared demonstrates why a little bit of knowledge may get you into trouble. Remodeling is a complicated trade. Homeowners who think they can tackle a job like this often underestimate the years of study and practice that are required to become a knowledgeable and skilled remodeler. Most of us don't do our own dental work -- wisely -- and maybe most of use should leave the remodeling to professionals.

    In my first answer, I advised you that "it's essential to make sure that your ceiling is as airtight as possible." Based on the information you have shared, it seems pretty clear that you should have paid more attention to this advice. If you had read more articles on GBA -- for example, Questions and Answers About Air Barriers -- you would have read about examples of dripping ceilings. These stories explain how the cause of these dripping cathedral ceilings is air leakage.

    In case you're still wondering about how much air can leak through unsealed seams, it's important to remind you about the driving force behind this air movement. The driving force is the stack effect. Remember, air near your ceiling is at a high pressure with respect to the pressure outdoors. It is eager to escape. It is pushing against your ceiling all the time, looking for cracks. That's the driving force.

    I'm sincerely distressed by the outcome of your remodeling efforts, and wish you the best of luck. At least your story may be instructive for GBA readers, and prevent a similar problem for someone else in the future.

  8. Charlie Sullivan | | #8

    I'm going to attempt to answer all your questions:

    Q1. In your articles you state “The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusion.” Also, “Although most walls and ceilings don’t need an interior vapor barrier, it’s always a good idea to include an interior air barrier. Air leakage is far more likely to lead to problems than vapor diffusion.”
    We had the sheetrock up, just not taped. If the problem is air, does the lack of taping really allow enough air through to create the moisture we are seeing?

    A. Martin already answered this. Yes.

    Q1, continued: Also, I am confused. If air is allowed to flow through the vapor barrier, wouldn’t the vapor barrier prevent the air from carrying the moisture through? Is so, where is the moisture coming from?

    A. This is kind of a moot point, but just to clarify, most vapor barrier materials, such as polyethylene sheets, block air and water vapor. There are also vapor permeable air barrier materials such as drywall, that block air flow but some water vapor through. And there are smart vapor barriers that let varying amounts of water vapor through under different conditions, but completely block air flow. But there are no materials that block water vapor while allowing air flow.

    Q2. Is there anything else that we should be thinking about that may have caused this? We want to make sure we address all possibilities before we get it all sealed up.

    A, It's possible that there is some other air leakage path into that cavity, for example from the living space into the wall cavities and from the wall cavities into the ceiling cavity. It's worth checking and perhaps sealing better at that junction.

    Also, there must be some path for air to exit, the insulated cavity to the vent space above, probably near the ridge. I think it's likely to be more effective to attempt 100% air tight drywall than to attempt to seal whatever leaks you have up there better, but it can't hurt to improve that. It could be at wood-to-wood joints, wood-to-foam joints, or foam-to-foam joints. Without looking at it I don't know which to suspect, but you probably know which of those are least well sealed.

    Q3. I have found two vapor retarders that I am considering. Any recommendations or caveats on these:
    a. CertainTeed MemBrain Air Barrier with Smart Vapor Retarder
    b. FSK paper which says it will also add to the R value of our insulation

    A. FSK paper only add R value if it is facing an open cavity. I assume you won't have that, so that advantage doesn't apply. Also, it's a complete vapor barrier, and won't have the capability that the MemBrain does to help the space dry out in the spring and summer. Given that you might have some residual dampness from this this problem left there when you are done, that ability to dry is a good thing. So I think your choice is easy.

    Q4. Would it be a good or bad idea to also use vapor retardant paint? We just want to be certain to do all that we can to protect our house, which is why we would go above and beyond. However, above and beyond is not helpful if it will cause other issues.

    A. If you've got the MemBrain in there, I think you'd be better without vapor retarder paint, because drying in the summer will be faster without it.

    Q5. Is there any reason we should be concerned about the roof side of the baffles based on what we are seeing? Should we pull the baffle off one bay and see what it looks like underneath?

    A. No. Any moisture that diffused through the XPS would have easily evaporated, assuming the space above it is vented.

    Q6. We put Ice and Water Shield over all the roof sheathing before we laid our shingles. Would that have an impact on any of this?

    No.

    7. How quickly do we have to work to get to the point of taping the sheetrock before we get condensation that would be trapped in the ceiling?

    The most critical is to get the drywall up quickly after the fiber insulation goes up. Next most critical is to get the drywall seams sealed quickly after the drywall goes up. I don't have any good basis for recommending any particular time frame for those but I'd guess that 24 hours would be safe.

    8. Would it make a difference if we do it on warmer days where the temperature differential is less?

    Yes. Not only that, but also having the room cold would help, and having the room dry would help. If you can close off the room from the rest of the house and open the windows, that would give you the ability to work more slowly. Or you could run a dehumidifier in there. Given that you want to go all out, you might be tempted to do both, but conventional dehumidifiers don't work well at low temperatures, so it's really one or the other.

    Doing this could buy you more time to complete the sealing of the ceiling.

  9. Charlie Sullivan | | #9

    A general comment about this situation and the way Martin describes it:

    "Remodeling is a complicated trade. Homeowners who think they can tackle a job like this often underestimate the years of study and practice that are required to become a knowledgeable and skilled remodeler. Most of us don't do our own dental work -- wisely -- and maybe most of use should leave the remodeling to professionals."

    My own experience in being a homeowner who initially underestimated "the years of study and practice that are required to become a knowledgeable and skilled remodeler" includes many instances of hiring "professionals" who were not adequately knowledgeable and skilled. Simply hiring professionals is not a sure solution, especially for homeowners who underestimate the necessary capabilities.

    In principle, one should be able to ask around about reputable builders and find someone who does it right without learning all of the construction techniques and building science yourself. But if you ask other uneducated consumers, you will find people who are very happy because the trim looks good and the work was done with relatively minor cost over-runs and delays. That doesn't guarantee that the builder got the details right for energy efficiency and longevity.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Charlie,
    You're right. There are many categories of people who underestimate the years of study and practice that are required to become a knowledgeable and skilled remodeler.

    One category, as I noted earlier, includes homeowners.

    Another category, unfortunately, is remodelers.

    For a homeowner like Kim Howard, there is a major advantage to hiring a contractor for a job like the one under discussion. If Kim had hired a contractor and the ceiling started dripping, it would have been the contractor's responsibility to fix the mistakes -- at no cost to the homeowner.

  11. Charlie Sullivan | | #11

    Martin, that's well put. We agree.

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