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Community and Q&A

Condensation with SmartBaffle Insulation

BuildingFun | Posted in General Questions on

I used to build cathedral ceilings with a “hot roof”.  After reading many articles on here, I decided to switch to a ventilated cathedral roof system, as outlined in many articles on here.   I decided to use “smart baffles” SmartBaffle Insulation baffle with 2 inch air channel ( for my air channel/insulation stand off barrier.   I built the entire ceiling, changed the attic venting to a ridge vent and correct soffit ventilation…lots of work that I wouldn’t do with a “hot roof” and felt good about the end product….until I did a spot check and found LOTS of condensation on the underside of the “smart baffles” at night time.  The condensation did disappear by mid-day next day as temperatures increased, but I don’t like the idea of having that much moisture in my roof assembly.   Am I overthinking this?  Should I not be worried that all the smart baffles are completely covered in condensation droplets on cold nights even though it seems to have evaporated by mid-day the next day?

I’m seriously considering tearing it all out and replacing the “smart baffles” with 1 1/4″ SilveRboard XS perforated insulation board that has an R rating of roughly 5.5 at 25 degrees F.  It has a perm rating of about 1.75 at this thickness.   The way I see it, this SilveRboard XS graphite foam board should eliminate or minimize condensation and hopefully allow any moisture that does accumulate to escape slowly through the board and up into the air channel?

Building is in San Diego.  We probably get 10-15 nights a year where the temp drops below 30.  Many winter nights are in the high 30s- mid 40s.  Day temps are usually 50s-60s winter and 70s-90s summers.

Building materials from inside to out are as follows

5/8″ T&G shiplap
5/8″ drywall (fire barrier)
23/32 cdx plywood (I know, not necessary but being used as a nailing surface for the T&G and for psuedo beams)
7.25″ Rockwool (R30)
2″ air gap
20 year old roof assembly (plywood, tar paper, shingles).

No plans for an interior side vapor barrier but the ceiling is AIR SEALED very well.  Zero air leaks through the ceiling assembly.

The ceiling joists are 2x10s (9.25″ tall) so I am working with limited space.

1. Should I rip out the “smart baffles” and replace with 1.25″ SilveRboard that is R5.5 with a perm rating of 1.75? Or simply allow the smart baffles to accumulate condensation on the interior side on cold nights and hope it doesn’t mold before the next hot day makes it evaporate?

2. I’m limited on space, especially now that I may need such a thick baffle material.  If I use the 1.25″ SilveRboard, can I reduce my air gap to 1″?  Or should I reduce it to 1.5″ and then just compress the mineralwool an additional 1/2″ to fit in my now smaller insulation area?  I’m not fond of attempting to cut 1″ or 1.25″ off the top of all the mineral wool insulation that is already installed.   Hoping to simply pull it out, replace baffle, and reinstall same insulation that is already cut to fit.

3. Should I air seal these 1.25″ baffles and rely on the 1.75 perm rating to allow drying?  Or leave them unsealed on the sides (ie: “intentionally leaky” where they meet the 2×10 ceiling joists, to ensure that any moisture can quickly escape?  Or will the small gaps between the foam board and 2×10 ceiling joists actually cause condensation on its own?  I’m not very fond of the 1.5 perm rating of the foam board which may end up more as a moisture barrier over time after getting covered with a layer of dirt/dust over time?

Sorry for the long post and thank you in advance for any knowledge or ideas you may have to share.

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  1. Jeremiah_Sommer | | #1

    I’m in south west Ontario climate zone 6 so not a comparable region but we have used a baffle system like this on cathedral roof assemblies for many years. I have always felt that this assembly was excellent as we have never seen any issues.

    I presume that since you can see condensation on the interior side of your baffles that you haven’t installed insulation yet? I would suspect that once your cavity has been insulated and air sealed from inside your condensation issue would no longer present itself. That said, it looks like the DCI baffles are made from plastic similar to coroplast which makes me think that moisture that does find it’s way into the cavity would have a difficult time drying to the outside. The baffles we use are made from a ridged cardboard which would allow vapour to migrate through and into the air space where it could be carried away.

  2. BuildingFun | | #2

    Thanks for the reply. The r30 insulation and 23/32 (fully air sealed) CDX plywood were already installed when the heavy condensation was noticed on the inner side of the “smart baffles”. We were making a last minute electrical change when the condensation was noticed.

  3. sazerac | | #3

    Apologies I have no answer for you, but instead a question. I assume you mean "hot roof" / "warm deck" as in exterior insulation on the roof deck. In most every thread I've read on here, the recommendation is to insulate on the outside. I also live in San Diego and I'm about to re-roof with a plan to convert to an unvented roof assembly with exterior insulation. Is there an article on here you can point me to that explains why that might not be preferred? I want to avoid a costly mistake

    1. BuildingFun | | #4

      No, by "hot roof" i mean no air vents and simply insulate from drywall to bottom of roof with Rockwool. This creates a warmer roof which in theory can wear out the roofing materials faster, but as I'm learning, seems better than dealing with this condensation issue.

      Hoping a cathedral ceiling pro will weigh in on my condensations issues... maybe I'm overthinking this new found moisture.

  4. Expert Member
    DCcontrarian | | #5

    The SmartBaffle website says it's appropriate for cathedral ceilings, but I don't see how. It looks like they're made of coroplast, which is pretty impermeable. The function of venting is to allow moisture that has leaked through the ceiling to dissipate, as you're finding that's not happening.

    The SmartBaffle seems more appropriate along the eaves of a vented attic where the attic floor is insulated, which seems to be their main thrust.

    There are a lot of building materials that are sufficiently permeable to serve as baffles in this situation -- OSB, drywall, unfaced foam insulation.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Martin makes the point that with good interior air sealing foam baffles should work fine, but like you I've never felt comfortable using impermeable baffles either.

      1. Expert Member
        DCcontrarian | | #7

        Well, he makes the claim.

        A surface can be air-tight and still vapor permeable. What is 1/2" drywall, 50 perms?

        I took OP to mean the drywall was in and he was seeing condensation: "We were making a last minute electrical change when the condensation was noticed."

        1. Expert Member
          MALCOLM TAYLOR | | #8

          Impermeable baffles sure don't help. We know roofs vented above the sheathing can work well just through sorbtion. Why not take advantage of the large surface area of the baffles to do the same thing?

          1. BuildingFun | | #10

            DCContrarian- sorry for the confusion. I removed part of the ceiling to do some last minute electrical work, when I noticed LOTS of condensation where the "smart baffles" and Rockwool insulation met (inner side of the smart baffle)

            MalcomTaylor- Agreed. No need to tempt fate here.
            Do you think a material with a perm rating of 2 is enough to dry outward(I'm thinking EPS or XPS here)? As much as I don't like the idea of completely impermeable baffles, I also don't like the idea of drywall or any other easily moldable material that could remain damp for days on end...

        2. Deleted | | #9


        3. Deleted | | #11


  5. DennisWood | | #12

    It would seem a lot safer to have the air sealing at the membrane in this case as you're in a damp, warm environment and dry to the interior, no?

    "Many winter nights are in the high 30s- mid 40s. Day temps are usually 50s-60s winter and 70s-90s summers."

    During today for example, temp in San Diego is 50 F at 73 % RH. That air is in your vented air space, and without perfect air sealing on the "smart vent" (coroplast) that air can migrate into your insulated assembly. Moisture will drive from hot to cold, moist to dry right?..and this is happening between your vent space and insulated space particularly during the day. Dew point for 50 F at 73 % RH is 41 F. I see a lot of days right now in your forecast where the daytime high and night's low would cover these conditions. It follows that you would see condensation on the underside of that coroplast baffle at night...and you may see even lower temps in that vented area due to nighttime radiative cooling.

    It would make some sense either eliminate that vented air gap altogether, or air seal the underside of your baffle using 2" of close cell spray foam and allow drying to the interior. I have zero experience in your climate zone so hopefully some experienced voices will chime in here.

  6. BuildingFun | | #13

    oops. Duplicate...

    Anyone have insight to air sealing vs not air sealing these baffles if wind wash is not a concern? I know a few of the articles on here mention air sealing the baffles to prevent wind wash in fiberglass batt insulations.... I'm not worried about that at all with my R30 rockwool.... I'm only asking from a condensation point of view (the ceiling lid is very well air sealed btw).

    With that being said, strictly from a moisture point of view, what are the benefits or cons of air sealing the R5 foam board baffles (with a Perm rating of roughly 2) to the ceiling joists? I know NOT sealing them will be an even faster way for any accumulated condensation to escape... but will it also allow condensation to enter the ceiling assembly faster with air leakage from outside to in or be a non issue? Thanks again for all the replies. I'm getting closer to wrapping this project up.

    1. Expert Member
      MALCOLM TAYLOR | | #14


      From a moisture accumulation perspective, I think sealed or un-sealed will have a negligible difference. With a good warm side air-barrier the mechanisms moving moisture into the vent space are diffusion and sorbtion, not air movement.

      1. BuildingFun | | #15

        That makes sense and would likely explain why the cavity below the baffles was dry by 5pm when it was full of condensation droplets at 6am earlier that day, even with the "smart baffles" that were vapor impermeable. I suppose changing the impermeable baffles to the SilveRboard R5 foam board with Perm rating of 2 ish will merely be an extra measure of insurance. Although, I'm still hesitant to believe the micro-perforated foil surface of the SilveRboard will retain its perm rating once dusty from outside air and the microperforations fill up with dirt/dust?
        I wonder if its ok to add more perforations myself or simply remove the foil face from the GPS board to increase perm rating? I know it will lower R value slightly, but I'm already above required/desired R value so maybe it will be worth it?

        1. Expert Member
          MALCOLM TAYLOR | | #16


          Have you considered just using house-wrap for the baffles?

  7. BuildingFun | | #17

    I did! Not a bad idea. The way I saw it (maybe flawed thinking) the house wrap would be more of a headache to install, would allow bowing up in the centers over my 24" wide spans, and would almost always allow condensation on the inside of the house wrap do it being so thin. The foam board would hopefully put the condensation point somewhere within the 1" board on most nights (mid 40s-70sF) , and on the 20-30 nights/year the temps were in the 30s, would still allow the condensation to dry both outward and inward. Just not sure the Perm rating of 2 is enough to do that, although its certainly better than the impermeable "smart baffles" . I'm all ears as my thinking may be bass akwards on this one. Probably just overthinking things, but hey, this seems like a forum of over-thinkers, which I appreciate.

    1. Expert Member
      MALCOLM TAYLOR | | #19


      Usually the house-wrap is stapled to blocking on both sides. It might bow with cellulose, but I can't see why it would with batt insulation.

      From the perspective of moisture accumulation, I wouldn't worry about using house-wrap. Condensation occurs on what Building Science Corp. term the "condensing surface of interest". That needs to be both cold enough for the vapour to condense on, and vapour-closed enough that the wetting exceeds the drying. Permeable house-wrap baffles don't meet those criteria.

  8. Expert Member
    DCcontrarian | | #18

    Have you looked into getting roofing polyiso which is faced with paper rather than foil?

  9. DennisWood | | #20

    In your position (and given my previous post of dew points/temps in your area), I would air seal the back side of the installed baffles with 2" of close cell spray foam, and then back fill with your rockwool and leave the ceiling assembly free to dry to the inside. I don't see any downside as you'd address both the air sealing and dew point issues in one step.

  10. handyhomehacker | | #21

    Highly interesting. I hope you eventually update us with your success in dealing with this issue OR in how you learned to not "overthink" it. I suspect that nobody ever got fired for overthinking moisture in a roof.

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