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Community and Q&A

How to seal ventilation baffles in rafter bays?

Joe Watson | Posted in Energy Efficiency and Durability on

If I use the “Smartbaffle” in my rafter bays, I would tape across the joints where two baffles meet, but do I need to seal the edges against the rafters also? What would be the recommended/cost effective way? Tape, caulk (messy), spray foam cans like great stuff (would add up with 54 or so 25ft long rafter bays from soffitt to ridge.

Thanks
Joe

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Replies

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

    Joe,
    The answer depends on your goals, and on how obsessive you are with your air sealing details.

    Even if you don't seal the edges of the SmartBaffle pieces, you'll be getting a better air barrier above your insulation than most builders provide, because the SmartBaffle provides complete coverage across the width of the rafter bay.

    Obsessive builders would probably seal the edges of the SmartBaffle pieces with caulk. But sealing with caulk is optional.

    .

  2. Joe Watson | | #2

    Thanks Martin,

    This baffle appears to be great, but at $3.35 each (that is the sale price right now). That is double the accuvent baffle. (rough estimate for my main attic area - $1600 for smart baffle and $820 for accuvent) How are your thoughts on this baffle?

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

    Joe,
    Costs matter. There is nothing wrong with making your decision on cost.

  4. Joe Watson | | #4

    Reality of life...
    I did look at my electric bills for the last 12 months. I am looking at a 330 a month average (this is everything - heat, cooling, well, stove, hot water, lights, etc.) or about $4000 a year. So by insulating my entire attic space my savings (based on 10%-35%) would be $400-$1400 per year. So if I can pull off the two layers or Roxul (including 2x4s, tape, staples, etc) for $6000, I would be looking at a best to worst case payback of 4-15 years! Wondering if going back to insulating the box would be worth it. Let's face it, I live in the south so we are not that extreme on temps. The average annual high is 69.5 degrees, the annual average low is 48 degrees and the annual average is 58.75 degrees......

  5. Eric Habegger | | #5

    Joe,
    I don't have much to add except that one always needs to use the system approach for your house construction. If the roof where it meets the top of the walls is not elevated, similar to my house, it would mean you don't have much room for insulation above the walls. In that case it makes sense to use foam board cut to fit as baffles and then use Great Stuff to seal it between the rafters. You can justify it economically because it would be part of the insulation of the attic and improving the weakest part.

    Of course, if your roof is elevated at the walls then disregard that option as it would be more expensive than is justified.

  6. Joe Watson | | #6

    Eric - Attached is where my roof and walls meet currently.

  7. Eric Habegger | | #7

    It doesn't look like foam board would gain you much used as baffles in your house. My house only has 2x4 rafters so foam boards used as baffles in my house was a good option. Your rafters are so wide that most of the benefits would be canceled by thermal bypassing by the rafters.

  8. Joe Watson | | #8

    The plan is to use the accuvent baffles then Roxul R23 in the rafter bays, then attache perpendicular 2x4s to the rafters and add Roxul R15 in that. Helps with thermal bridging and gets me to the R38 I need in Richmond, VA

  9. KEVIN ZORSKI | | #9

    Joe - Why are you insulating the roof when the floor of the attic already has at least some insulation? Could you just add more to the floor? That would certainly be the most cost effective option. Did I miss something? Is this going to be an attic addition?

  10. Joe Watson | | #10

    Kevin. You did not miss anything. It is a walk up attic with a 20x20 room that can be finished. The other half of the attic is the hvac and storage. Doing all the rafters instead of the "box".

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Joe Watson has posted another question for this thread. Because it was accidentally posted on the wrong page, I'm copying it here:

    "Is there a recommended caulk to use that is best suited for the plastic baffles and wood?"
    -- Joe Watson

  12. User avatar GBA Editor
    Martin Holladay | | #12

    Joe,
    A polyurethane caulk would work.

  13. Eric Habegger | | #13

    Joe, are you going all the way to the ridge vent with the baffles? If you are then I can think of better ways of creating a baffle than using a standard purpose built baffle. It is very difficult to create a good seal with a baffle that is somewhat floppy. Ask me how I know. I think in your special situation the cheapest and easiest way to create a baffle with a good long lasting seal is to create your own.

    Just buy the thinnest thickness of cdx plywood you can find, usually 3/8" thickness, and set up your table saw for the standard spacing between rafters, minus an inch. Run them through and staple 1 inch thick narrow wooden standoffs every so often to one side of the long strips of cdx. That will create the air space between the roof sheathing and the baffles.

    The next thing is a little counter intuitive but actually works. Press the cdx plywood tight against the roof sheathing and then staple a wooden support every so often against each rafter to keep the wooden baffle up there. The plywood baffle shouldn't be so tight that you can't move it side to side a little between the rafters. Position the baffles so there's a half inch space between each rafter along the length of the baffle. Then spray Great Stuff along all the edges along the whole perimeter of the baffles. Once the Great Stuff is in there shouldn't be any movement of the baffle and the Great Stuff will hold the baffle up on its own. It works well because there is not just one or two points of contact of the Great Stuff with the rafters, but along the whole perimeter. Then just remove the wooden supports that were used to keep them up temporarily - or leave them up if you don't quite trust the Great Stuff alone to keep them up over time.

    It sounds like a lot of work. And it is. But you'll get a much better and longer lasting seal that way and it probably is equal in cost to the Accuvents. Of course, if you don't have the tools like a table saw or pneumatic stapler then all bets are off. I use these kind of projects as my excuse to get good tools.

  14. Eric Habegger | | #14

    I think I should revise my suggestion somewhat. I used this method with 2 inch thick foam board. There is more thickness of Great Stuff holding it up than would be the case with 3/8 plywood. So I would NOT remove the supports after applying the Great Stuff. Otherwise everything should work the same.

    EDIT: After thinking about it some more it may actually be better to staple the 1 inch thick standoffs directly to the sides of the rafters as a permanent connection. Then just attach the baffle to the standoffs. Then apply the Great Stuff or whatever your favorite sealing method is.

    Isn't that the way it works. You change one thing and then you have to slightly revise everything else to match. It still would work and seems to me to be a better method than using Accuvents from soffit to ridge vent.

  15. Joe Watson | | #15

    Eric,

    I am going soffit to ridge vent. 25ft. run and 54 bays of it. Thanks for the suggestion of using 4x4 sheets and ripping them down. I do have a table saw and a brad nailer.
    I wonder if I could get away with using sheets of paneling?
    This then leads to another question - the accuvent baffles are plastic - so if by any chance there is a roof leak, the water would drip down on the baffle and then run down to the soffit vent. (if you overlap them properly). That is not the case with wood - how about foam board? What type would be needed - I am not looking for R value with the foam board so thin is better - would it need to be foil faced toward the sheathing? Would plain foam board absorb the water?

    Thanks
    Joe

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Joe,
    If you haven't read it yet, you should really read Site-Built Ventilation Baffles for Roofs.

  17. Eric Habegger | | #17

    Joe, the Accuvents are made to rest on the top plate and there is no vent spacing built on that end. So it would not work to overlap them. They are built for 4' lengths and that's it. As far as leaks go, there is no easy cure for that but ripping 8' panels cuts down on the joints. There is no reason to use 4' panels. Martin's suggestion of other panel materials than plywood in his article also works. There is no easy solutions to leaks with thicker stiffer materials but personally i would not worry about it. You could use a good double backed tape. Apply it on the top of the lower panel then put up the next panel and stick the other end of the double side tape to the bottom of the next upper panel. I don't have any recommendations for a good double sided tape for that joint.

  18. Joe Watson | | #18

    Eric,
    The accuvent has 2 types - one is type that mounts to the top plate and "bends" up and gets stapled to the sheathing. The other is what the call their "cathedral extension" which is just a 48" long piece that would go end to end up to the ridge vent.
    Doing some quick math, I can get 50 of the accuvent extensions for $80 - that is 48" each or 2400" which equals 200 linear feet.
    The cheapest 4x8 sheet I can get at Home Depot is 9.96 for a .0.106 thick sheet of MDF paneling. One sheet ripped into 3rds gets me 24 linear feet. I would need almost 8.5 sheets to get 200 linear feet to match the accuvent. So I am looking at about $80 in sheets to match the accuvent. Then I have to take into consideration the ripping of the sheet and buying 1x4 strips (ripped in half to act as 1.75" spacers.
    It appears that the accuvent is slightly cheaper and requires less customizing (ripping) than the wood solution. I have to caulk/great stuff foam either solution so I consider that a wash.
    Thoughts? am I missing something?

    Thanks
    Joe

  19. Joe Watson | | #19

    Martin - Thanks for the link to the article Very informative. It does say "The most common materials used to create ventilation baffles are thin plywood, rigid foam, fiberboard, and stiff cardboard."
    So I guess cheap paneling would work fine - as long it is sealed at the edges with can foam/caulk?
    Once I have my baffles in and my 2 layers of roxul, I guess I need to staple some thick poly over the rafters until I get around to finishing the room with drywall, correct? This would act as a vapor barrier to keep moisture from going through the roxul to the baffle?

  20. Eric Habegger | | #20

    Joe,
    Go with the Accuvents then. It seems you have all the information you need to make your decision. Pull the trigger.

  21. User avatar GBA Editor
    Martin Holladay | | #21

    Joe,
    Have you told us your climate zone yet?

    I don't recommend the use of interior polyethylene unless you live in one of the colder sections of Canada or Alaska. For more information on this issue, see Do I Need a Vapor Retarder?

    I suppose you could install polyethylene as a temporary interior air barrier -- as long as you promise to remove it when it is time to install your finish ceiling. (You still need an interior air barrier, of course. The usual interior air barrier is taped gypsum drywall.)

  22. Joe Watson | | #22

    Martin - Yes - I am in climate zone 4a - Richmond, VA. 2 hours south of Washington DC. So we get 100 degree heat and high humidity and we can get low 20's at night in winter. Fun.
    Hmmm - I would take down in the half of the attic that is going to be finished. How about the other half that is storage and not going to be drywalled? I would have two layers of roxul - r23 and r15 (perpendicular to the r23) - I reeaaaaally dont want to drywall the entire attic - especially up high and behind the kneewall.
    The attached pic shows the area that will remain storage - basically behind the stairscase. The thought is the entire roof deck will be insulated and I can get away with interior doors on both sides of the stairs to access the storage area.

  23. User avatar GBA Editor
    Martin Holladay | | #23

    Joe,
    You need some type of air barrier. If it's not 1/2-inch drywall, it could be MemBrain or Tyvek, I suppose.

    It can't be just polyethylene, since polyethylene is a fire hazard and can't be left exposed. Frankly, it's quite possible that MemBrain and Tyvek are also flammable.

    Drywall is cheap and it doesn't burn. I think it's the easiest air barrier to install.

  24. Joe Watson | | #24

    I looked up Membrain and here is what I found:

    Q. What are the surface burning characteristics of MemBrain?
    A. MemBrain has been optimized to meet or exceed the surface burning requirements of the
    International Building Code (IBC) fire resistant classifications for building constructions.
    MemBrain has a Class A or I surface burning designation as specified by building codes and has
    the following average flame spread characteristics when tested in accordance with ASTM E 84,
    “Standard Test Method for Surface Burning Characteristics of Building Materials”, while
    following the mounting methods described in Appendix X1, “Guide to Mounting Methods” and
    Section X1.8, “Thin Membranes”:

    Q. Is MemBrain suitable for exposed applications?
    A. While MemBrain meets the fire rating requirements for exposed applications, the film’s
    material properties have not been evaluated for long-term ultraviolet light exposure. Therefore,
    MemBrain should only be used in exposed building applications where the product will not see
    any direct or indirect (reflected) ultraviolet light exposure due to solar or electrical sources.
    CertainTeed is currently developing an enhanced MemBrain product for ultraviolet light
    exposure applications, such as basement wall insulation."

    Looks like leaving it exposed is feasble. This is only in the storage area.

  25. Joe Watson | | #25

    and I just talked to the company and the rep said he would not advise leaving it exposed.....

  26. User avatar
    Dana Dorsett | | #26

    Even if there is no other reason to cover it, 2-mil nylon isn't really all that rugged, perhaps too easy to puncture/slash if left exposed.

  27. Joe Watson | | #27

    True Dana. The more I analyze the project and view the costs, it makes me almost want to go back to insulating the room only and forging the rafters. The sheer volume of materials - with me doing the work is getting $$$$.

  28. frasca | | #28

    Facing a similar situation to Joe but with only 2x4 rafters and living in an area where it rains a lot, I decided to optimize for roof leak detection and repairability. I decided on SmartBaffles for the whole run and started overlapping them from ridge working my way down to soffit. I'm 75% done, and now I'm noticing the weight of each baffle is making the bottom part sag almost flush with the bottom edge of the 2x4 rafters.

    I was planning on having 2" of sprayfoam applied to these, and now I'm worried that the weight of the foam will weigh the baffles down even more and I won't have a very sound installation.

    I could put cross bracing in the bays at about the midpoint of each baffle to hold them up but that would add wood where I would rather have foam, not to mention being a ton of work and after spending a weekend shimmying out into each bay on my back I'm not sure my abs could take it... does anybody have other thoughts?

    Am I too worried about the weight of the sprayfoam; i.e. will it be totally fine once it cures stiff?

  29. User avatar GBA Editor
    Martin Holladay | | #29

    Frasca,
    Are you getting closed-cell spray foam or open-cell spray foam? Closed-cell spray foam is denser and more structural than open-cell spray foam. If your baffles are firmly secured, and if your spray-foam contractor is OK with proceeding, and if you have specified closed-cell spray foam, I wouldn't worry, especially if the foam will cover the exposed edges of the top chords of your roof trusses. The top chords will provide structural support as the foam cures.

    If you are paying for 2 inches, let your spray foam know that you'll be expecting a full 2 inches of foam, and that you will be measuring in the center of each bay. That way, if you get 2 inches there, you'll end up with 2.5 inches nearer the top chords of the trusses.

  30. frasca | | #30

    I’m going with closed-cell foam. Great to hear that you think it will be structurally sound enough, and I appreciate the advice on the installation.

    My other though was to rip 1.5” rigid foam board so it would fit tightly in each bay, and press it in so that it was flush with the bottom face of each 2x4. That would get me a few mor R-value, but I wasn’t looking forward to that work either and I would be worried that it would further reduce the surface area of the 2x4 that the spray foam would have to bond to.

    Thanks Martin!

  31. Shakeyray2000 | | #31

    Hey Frasca, have you done this yet? Did it work out. I was thinking about doing something similar as stated by joe. I am in florida and have solid brick home. I too would like the water to drain down as a layer of protection.

    Ben

  32. Shakeyray2000 | | #32

    Oh...I found another thread where Martin talked about shingling the Accuvent.....that's what I'm gonna do.

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