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Community and Q&A

Baffles and Spray Foam for Low-Slope Vented Roof

user-7021433 | Posted in Energy Efficiency and Durability on

I’ve been reading the attic sealing/insulation articles on GBA, and the many posts/questions that deal with this topic, for years in the hopes of finding the optimal solution for my attic, but I haven’t been able to reach a conclusion yet. Here’s my situation.

Split level home with upper and lower attics, both hip roofs, 4/12, no HVAC in attic. Zone 5, north Chicago suburbs, house at present is poorly insulated and sealed and is prone to ice damming. Because of the roof pitch, the vertical gap between the outer wall and the sheathing is 4 inches.

I’m disinclined to go with an unvented assembly, and rigid foam above the sheathing is prohibitive, especially given that my roof is only a few years old. On the other hand, it is impossible to adequately seal and insulate the outer perimeter because of space constraints and accessibility issues. So, is it possible to do a hybrid assembly as follows?

In all rafter bays install full span baffles with 1.5-2″ air channel and blocking as you normally would to prevent wind washing of insulation (though in this case they would serve as a barrier for foam). Rather than use cellulose at the perimeter, use spray foam: 1-2 feet around the perimeter would be foamed from the attic floor to the baffles/blocking. The remainder of the attic would be treated as a standard vented assembly: all penetrations in the attic floor would be sealed, and cellulose installed to code. There would be roof venting and continuous soffit venting to provide for a slightly pressurized attic.

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you could do this, but what are you expecting to gain over just blowing in loose fill cellulose (or similar)? The only advantage I could see to spray foaming out at the eaves in the way that you describe is air sealing the top plates if you don't have any other way to do it (maybe they're too inaccessible to crawl into, for example). Spray foams only advantage here is that it will air seal and insulate, so you accomplish two goals with one step. Other insulating materials won't give that air sealing, but you could potentially air seal the old fashioned way with canned foam and caulk if you can get in there to do the work.

    What I would normally try to do in a situation like this would be to put in air baffles and then fill the eave under the baffles with blown cellulose, trying to get at least 3-4 inches over the top plates and ideally more. The construction of the roof will determine how much you can fit over the top plates though.


    1. user-7021433 | | #2

      Thank you so much for your reply, Bill. Because there's only 4 inches above the top plate, with a baffle the max insulation would be 2.5 inches of cellulose. Additionally, there's no way to get in there to effectively air seal either the top plate or the lower portion of the baffle. I recognize that there's not a substantial difference in R value between 2.5 inches of cellulose vs. spray foam, but whatever increase there is plus the greatly improved air sealing would make for a substantial increase in performance of the assembly at the perimeter. Maybe I'm overestimating how much better it would be.

    2. exeric | | #3

      I think Bill gave a pretty good overview of what's needed. I would add that it's always best to weigh risks vs rewards. Do you know personally of a spray foam contractor that is reliable? I wouldn't use word of mouth from neighbors or friends for this. Statistically speaking, even a poor SF contractor might have a 1:10 ratio of bad installations to good ones. So, your acquaintances might have just been the lucky ones. People in the construction business typically will know the people who have a good reputation because they find out over many years of using them.

      If you don't have access to the knowledge base of reliable installers, then I would stick with canned spray foam. I have a home with a very similar construction to yours and successfully used that method. I ended up using 2" thick polyiso baffles at the perimeter above the top plates using a canned foam cut and cobble approach. It's difficult and not fun if you do it yourself, as I did myself below my own 4/12 roof, but it can be done. You need to take your time and use a good canned spray foam gun that that provides good placement and amount of foam.

      Then just use the same approach to seal any leaks in the ceiling. There should be no need to go to 2-part spray foam for the rest of the air sealing if you have already gone to this much trouble. Then just blow cellulose over everything to the level that is prescribed for your climate.

      1. exeric | | #4

        I should add that instead of placing cellulose under the baffle I think it's preferable to just the canned foam under the polyiso baffle just like you would use for the rest of the cut and cobble approach. I know people will complain about the cut and cobble approach because it tends to fracture over time. But that's only if you try to use it for the entire underside of the roof in a ventless attic. I think it's appropriate here and gives minimal risk for its limited application on the baffles.

        1. user-7021433 | | #5

          Thanks for taking the time to reply to this, exeric, it’s super helpful to talk to someone who has done this. When you say cut and cobble are you referring to the installation of the polyiso baffles and blocking? Not to pieces of polyiso placed between the top plate and the baffle?

          1. exeric | | #6

            In my case I was able to access to the bottom of the baffle even though it was about a 4/12 roof. So that's why I was able to glue, seal, and insulate the polyiso right down to bottom plate with the canned foam. If you can't get access to the very bottom then I imagine you might have to come up with alternative method. A canned foam gun can help immensely to get to that space above the top plate even if you yourself can't get that close. If you can't do that even with the gun, then you'll probably have to improvise a bit.

            A bit more about why I used such thick 2" baffles - It provides quite high insulation value and replaces a smaller value you have if you simply used a plastic baffle along with cellulose. You need as high an insulation value as you can get in such a limited space. A thick slab of insulation that is cut about 1/2" narrower than the rafter bay width allows a more forgiving application of canned foam than a thinner piece of polyiso would allow. You just have to make sure you start foaming on the outermost part of the gaps and work your way inwards towards you. Also, start by tacking it in place on opposite sides of the polyiso so the expansion of the foam doesn't eliminate the gap on the opposite side. All these methods keep you from covering up holidays in the gluing process. For a thin slab you only have one shot at it, and it's immediately covered up if you have a sparse application of spray foam at a particular spot.

        2. Expert Member
          BILL WICHERS | | #10

          With a vented roof assembly, cut'n'cobble isn't really a risk, since the vent channel will carry away any moisture that sneaks though any gaps in your canned foam. Vent baffles don't really need a perfect air seal, they just need enough to hold them in place for physical support. Out at the eaves, you want a good air seal over the top plate, but if you get a crack some day, that vent channel will still keep you from having moisture problems. Vented roofs are far more forgiving and a lot less critical in terms of moisture and air sealing, so the air sealing is more for energy efficiency of the home than for safety of the roof assembly here. Basically that's a long winded way to say I agree with exeric here, that canned foam and some cut'n'cobble at the eaves under the vent channel isn't a problem.

          I would try to get as much insulation over the top plate as possible though. If it's so tight that the baffle is all you can fit, then use some thicker polyiso at the eaves to insulate the top plate, then transition to thinner polyiso (or waferboard) up higher where you don't need the insulating value. If you had lots of space (such as with raised heel trusses), then it's usually easier to just use thin baffles and let the loose fill insulation handle the insulating duties for the top plate.


  2. Deleted | | #7


  3. Expert Member

    User ...433,

    A couple of comments which are unfortunately probably more useful to other readers than directly to your situation:

    - Low sloped roofs are typically defined as those under 3/12. That's the point below ventilation becomes difficult, and codes begin to call for different detailing of the roofing material.

    - The slope of the roof doesn't determine how much space you will have between the top plate and the r0of sheathing. On trussed roofs that's a result of the heel height. On stick framed roofs the higher the slope the deeper the bird's mouth. So you end up with less space than on a lower sloped one.

  4. Expert Member
    Michael Maines | | #9

    Can you add a sloped ceiling at the eave walls so you have space for decent levels of insulation? 2" of polyiso or closed cell spray foam is only around R-12 at best, far from enough in climate zone 5.

    Assuming that's not an option, I think you're right--allowing cold air to wash the underside of the sheathing should reduce the risk of ice damming. Make sure you air-seal the interior to minimize heat loss, and keep the soffit vent as far away from the wall as you can get; Dr. Joe Lstiburek and BSC have shown that soffit vents close to the wall can push sun-warmed air into the rafter vent, increasing the risk of ice dams.

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