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Rigid foam board as a vent baffle

tgmuller | Posted in Energy Efficiency and Durability on

In a vaulted ceiling, if using one inch rigid foam board furred out a min of 1″ to provide ventilation from soffit to ridge vent followed by fiberous insulation such as fiberglass batts ……is this proper? would the one inch of foam used as a vent baffle be prone to condensation in a cold climate or does this particular situation not call for the min R-20 of foam? thank you in advance for the help.

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  1. GBA Editor
    Martin Holladay | | #1

    If you are using rigid foam as a vent baffle, the best type of foam to choose is EPS (because it is somewhat vapor-permeable). The next best type of foam to choose would be XPS. I wouldn't use foil-faced polyiso if I were you. As long as your foam is only 1 inch thick, I think you'll be fine.

    Make sure to pay attention to air tightness when installing your vent baffles. For more information, see How to Build an Insulated Cathedral Ceiling.

    1. MichaelAndrew | | #2

      Martin I've been tediously reading many of your posts trying to get answers to my situation. i specifically like one comment you made "The owner of an old house often has to come up with an affordable solution that is the best option, even if that solution is imperfect". As I've been down a rabbit hole figuring out what that best option is. Halting my work..

      this current comment on not using foil faced foam board as vent baffles did you mean inside a wall cavity or above the top plate in the attic? I have started to use foil facing the roof sheathing with a 1.5" air gap at the top plate, they span about 3' to get them past any insulation on the attic floor and are open to the rest of the attic space so I imagine vapor-permeability here should not matter much? should I alter this plan and use other foam board that is not foil faced? Why does vapor permeability matter here in such a small stretch which is open to the rest of the attic and not closed off?

      I also need a air vent channel/baffle for the sloped roof area. I plan to leave a 1.5" gap between the sloped roof sheathing. I was going to use SIKA Foil faced foam board (the same I started to use in the attic top plate baffles) due to it's highest R6 1" board being highest I could find per inch. I was going to leave the 1.5" gap then pack as much insulation board into each cavity I can and then possibly even fur out the 2x4 studs some and then go over top of the studs with one more 1/2" rigid foam board before sheetrock. UNTILL I just saw your comment about NOT using foil faced foam board in this fashion?

      what is your take on radiant barriers? I've seen mixed reviews? both on the foil faced foam board and then sheets of radiant barrier.

      1. MartinHolladay | | #3

        Go ahead and use the foil-faced product for your vent baffles if that is the type of rigid foam that you prefer to use. When it comes to walls, the situation is different that roofs. (For information on wall foam, see "Calculating the Minimum Thickness of Rigid Foam Sheathing.")

        For a thorough answer to your question about ventilation baffles for roofs, see this article: "Low-Slope Roof Assembly Failures."

        The most relevant information from that article has been pasted below:

        One of the reasons that builders install ventilation channels is to help damp roof sheathing dry out. Researchers now realize that ventilation channels can help a little bit at this task, but not as much as some people think. (In fact, during the summer, ventilation channels can actually add moisture to roof sheathing rather than remove it; for an example of this phenomenon, see Image #4, below.)

        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.

        1. MichaelAndrew | | #5

          Thank you Martin. I'm still in the midst of my project I transitioned to replacing some rotten floor joists and ended up doing them all and had some damaged bottom wall plate to replace as well... had some follow up questions. there is so much mixed information out there. I just want to know the best choice for my situation so I can move forward with a plan in place.

          I am happy with the vent baffle situation. Using foam board high enough to get past my ceiling insulation.

          I inset the bed head board area by installing a header pushing the wall close to the slanted roofs sheathing. I am considering now building a knee wall close to the roof rafters/sheathing as I'm realizing I don't have enough stud depth if I use the rafters, I could gain 1.5" if I remove those 2x4 blocking cross members? . I'll still gain about 2 feet of floor space in the areas I am insetting but this is making more need for knee walls which I've read you should avoid because they make insulating and sealing difficult, go figure, hopefully it's still worth my troubles.. was wanting to do it near the bed headboard, the toilet area, and then in the living room to push in the TV a bit..

          my questions:

          1) what is the recommendation for my two exterior walls on each end of the gambrel roof (not the knee wall areas) i see so many mixed insulation recommendations for 2x4 stud bays. the exterior siding is lapped hardie with tar paper & osb sheathing which was done 2 years ago. I am in central florida so I believe climate zone 2. was considering cut & cobble between studs with spray foam and then possibly one more sheet over top of the studs?? I've seen you recommend rockwool but I'm unsure how to install and if I need a permeable barrier ?

          2) in the areas where I have added a header pushing the conditioned space wall much closer to the side roof rafters & sheathing.. is it ok to remove those blocking 2x4 in each rafter bay to get more depth for the ventilation channel? were they just to install the roof rafters or are they needed for support? I was going to run a 1.5" gap behind rigid foam board and continue to add foam board to fill cavity or possibly use some type of batt to go against the rigid foam. Or I can build a knee wall right in front of the roof rafters to create more depth to insulate & provide the ventilation channel.

          3) in photos you can see the sub floor was originally ran just passed the conditioned space wall. should I run the subfloor all the way back?

  2. andy_ | | #4

    One practical bit of advice from having done this is to put a tiny little block of foam in the middle of the baffle so the whole baffle doesn't get inadvertently squished against the roof sheathing when putting in the insulation (or by some other contractor moving things). 1" foam over a 23" span isn't that rigid so having a spacer block in there is good insurance.

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