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Community and Q&A

Spray foam directly on underside of roof?

Matt P | Posted in General Questions on

I have a conditioned attic, currently insulated with R19 fiberbatts between the rafters. No airsealing, hence I want to remove the fiberbatts and spray 3 inches of closed cell spray foam in before putting the fiberbatts back in place. The attic is unvented, for instance no ridge vent exists. Should I put any membrane or the like on the underside of the roof sheathing before spraying the foam in ? I was thinking that would help with keeping the insulation intact in case the roof needed repair later on?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mattias,
    First of all, if you have fiberglass batts between your rafters -- with no ventilation channels between the top side of the insulation and the roof sheathing -- your current situation violates building codes and could cause moisture problems. So you need to do something to fix the situation.

    Where are you located? What is your climate zone? Needless to say, R-19 isn't much insulation. In most areas of the country, building codes require either R-38 or R-49 insulation levels for roofs.

    Here is a link to an article that explains everything you need to know about a flash-and-batt job (the type of insulation job that you are contemplating): Flash-and-Batt Insulation. Pay close attention to requirements for the minimum thickness of the spray foam layer.

    If you are worried that installing rigid foam on the underside of your roof sheathing will complicate future roof repairs -- and you aren't alone in that worry -- you can install ventilation baffles, cardboard strips, or strips of asphalt felt in each rafter bay before the spray foam is installed.

    1. Jack Zylkin | | #8

      Martin, I'm getting spray foam installed on my roof deck this weekend, and I have this same concern about adhesion to the roof deck also. However, I'm struggling to figure out how to implement this advice. I'd like to use cardboard so the sheathing can dry inwards if needed. I assume you mean actual 1-ply cardboard (e.g. RamBoard) and not corrugated cardboard like a cardboard box. Should the cardboard be stapled to the sheathing, adhered with an adhesive, or what? What will stop the weight of the spray foam from causing the cardboard to sag or fall out of the bay? My bays are quite wide (30" or so) and I'm only applying 3 inches of foam or so. I also will have to battle the hundreds of nail points poking 1/4 in" through the sheathing -- seems like maybe I could use those nails to tack up a length of fan-fold XPS with the impermeable facer ripped of? Any advice? Thanks so much.

      Thanks

      1. Expert Member
        Dana Dorsett | | #9

        >"I'm getting spray foam installed on my roof deck this weekend, and I have this same concern about adhesion to the roof deck also. However, I'm struggling to figure out how to implement this advice. I'd like to use cardboard so the sheathing can dry inwards if needed. "

        If it's closed cell foam there won't be much drying toward the interior no matter what material is used to prevent adhesion to the roof. Even at 2" most closed cell foam is well under 1 perm.

        If it's open cell foam it may be too permeable to protect the roof deck without an interior side vapor retarder (or a vented deck approach.)

        >"What will stop the weight of the spray foam from causing the cardboard to sag or fall out of the bay? My bays are quite wide (30" or so) and I'm only applying 3 inches of foam or so."

        Spray foam is fairly rigid, and adheres quite well to wood rafters. At 3" closed cell foam is even structural. Open cell foam would still have some modest amount of flex to it at 3", but not much heft- nothing short of a major earthquake would cause it to break free. The chemistry of spray polyurethane foam is remarkably similar to Gorilla Glue. Applied to a clean surface it'll stick. As with Gorilla Glue, misting the wood with water prior to application increases the bond.

      2. Expert Member
        Zephyr7 | | #10

        Don’t use corrugated cardboard. Use something like ramboard or other material with similar properties to the stuff they use on the back of a pad of paper. Attach it to the sheathing using plenty of short staples, making sure to put the stables not only at the edges, but randomly around the surface too. You can press it against any roofing nails so that the roofing nails punch through the cardboard. The cardboard only has to support the weight of the spray foam for the brief time during the initial cure, after which the spray foam will be essentially self supporting and glued to the rafters.

        If you replace the roof sheathing in the future, the staples and nail points will just pull out no differently than if they were being pulled out of an old rafter. The cardboard will probably come apart in lots of places, but that doesn’t really matter — the spray foam is self supporting after curing.

        I’ve actually done this (cover the sheathing to prevent spray foam adhesion to allow for future sheathing replacement), but in a wall and not a roof. I use house wrap, stapled against the sheathing between the studs. I have two lessons I learned that will help you:
        1- be careful not to overlap the studs too much. If your adhesion-preventer material curls or folds, especially in corners, it will create voids where the spray foam won’t get to. Make sure to install the material tightly against the sheathing and avoid an excess overhanging areas that the spray foam should be contacting.
        2- use PLENTY of staples to hold the material to the sheathing. You don’t want any separation while the spray foam is being applied since separation will create voids. You’ll have less problem with this using something a little stiff like ramboard instead of using housewrap like I did. If I were to do it again, I’d use ramboard myself.

        I’d recommend using 1/4” T50 type staples and a power stapler (I use a pneumatic one) to save your hands the work of installing a bazillion staples.

        Bill

        1. Luke Hagenbach | | #11

          I am also about to spray foam the underside of my cathedral ceiling roof deck and am leery about how this path seems to complicate future roof replacement, especially if a new deck or parts of the deck is needed at that time.

          My differences and concerns are as follows:
          1) I have the original 1" thick skip sheeting under the OSB with radiant barrier, so it's not exactly a flat surface to run a plastic or cardboard sheeting material across, which would leave 1" voids above it in an unvented attic.

          2) A thin stick-on material like carpet protection film seems like it would help mold around the contours of the skip sheeting, but I'm not sure how that would react with the chemicals that form the closed cell foam, if it is a fire hazard, or if it would be cost prohibitive to another option.

          I'm in coastal Orange County, California, which I believe is climate zone 3 (fairly temperate), if that matters.

          My plan is to fill the 2x4 and 2x6 joist bays with the closed cell, and flash the deeper (2x8 and 2x10) rafters with closed cell, followed by open cell or batts for the rest (seems like a wash on pricing and might be easier to deal with just one insulation company).

          1. Expert Member
            Dana Dorsett | | #12

            >"My plan is to fill the 2x4 and 2x6 joist bays with the closed cell, and flash the deeper (2x8 and 2x10) rafters with closed cell, followed by open cell or batts for the rest (seems like a wash on pricing and might be easier to deal with just one insulation company)."

            I actively hate that plan!

            Why?

            * In zone 3 it only takes a flash-inch of closed cell (R5+) to protect the roof deck:

            https://up.codes/s/unvented-attic-and-unvented-enclosed-rafter-assemblies

            ** Anything more than the minimum needed for dew point control is essentially wasted by the thermal bridging of the rafters:

            https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

            *** Closed cell foam is an abuse to the planet:

            https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

            The greenest approach would be the flash-inch of HFO BLOWN closed cell for dew point control followed by dense packed cellulose (which is carbon net-negative) for the rest. For the often high diurnal temperature swings in zone 3B & 3C the comparatively low diffusivity (a thermal mass effect) of cellulose would also make it outperform open cell foam, fiberglass or rock wool, R for R, inserting a real time delay between the peak roof temperature and the peak ceiling temperature, and delivers a lower peak ceiling temperature.

            https://gutex.de/fileadmin/uploads/Produkte/Produkteigenschaften/Content/temperaturleitzahlen-EN.svg

            The time delay and peak suppression isn't as long or low as (even lower diffusivity fiberboard insulation, but it's still pretty good. The fiberboard curves looks something like this:

            https://gutex.de/fileadmin/uploads/Produkte/Produkteigenschaften/Content/phasenverschiebung-EN.svg

          2. Expert Member
            Zephyr7 | | #13

            Dana gave lots of info about the negatives with spray foam. Note also that it’s pricey stuff, closed cell is probably the most expensive widely used insulation out there. I only really use it in a few places where it’s benefits are advantageous. I have only ever used it in a wall one time, and that was a relatively small wall (maybe 25-30 linear feat of a single story), and it was cheap since the crew was already onsite doing a nonvented cathedral ceiling where the spray foam was pretty much a requirement.

            I would normally insulate walls with mineral wool (I like it better than fiberglass), and I would use center roof assemblies wherever possible with a blown insulation on the floor.

            If you need to find out if spray foam will adhere to a protective membrane you want to use, get a piece of that membrane and ask your spray foam contractor to spray it with the foam they’d use on their job. Chances are the spray foam contractor can spray a sample for you easily within a few days if they can take it to some other project sites. Wait a few days and test the adhesion. That’s the safest way. That said, spray foam is basically a spray apply urethane foam, and tends to adhere to most things VERY well.

            You could also do a cheap pseudo-test using regular ol’ canned spray foam (great stuff), which is a similar, but NOT THE SAME, product to what your contractor would be installing. If the great stuff doesn’t stick, I wouldn’t expect the “real” spray foam to, either. If the great stuff DOES stick, then I’d think the real stuff will probably be ok, but I’d still have the contractor test it for me.

            Bill

          3. Luke Hagenbach | | #14

            @Dana
            A lot of really helpful information there. Thank you. I'm days away from pulling the trigger on a decision here, so it's pertinent and might affect the path that I take. I wish I had someone near me that could consult on this (almost no spray foam contractors in my area, and just as few of dense pack cellulose).

            In terms of the environmental impact of HFO closed cell, that equation is always a tricky one for me to calculate, because there are a lot of "net impact" factors to consider such as increased energy efficiency achieved by a higher R-value than could otherwise be achieved in shallow, 3.5" bays, lower air exchanges from making the roof assembly super tight, etc. I did not see open cell spray foam on the chart. Is it safe to assume it's somewhere between HFO closed cell and fiberglass?

            "Anything more than the minimum needed...is essentially wasted by the thermal bridging of the rafters"
            I read the linked article, and it appear that adding more spray foam into the bay increases the total R-value of the assembly. Not by as much as I would have intuited, but the comparison was only the difference of 0.5" of closed cell vs open cell, and therefore it's not exactly clear on how that would compare to your suggestion of 1" HFO closed cell followed by 2.5" of dense pack cellulose? Is it even possible to do dense pack cellulose in such a tight area that would likely vary in terms of the space available in the cavity?

            Regarding low diffusivity of my climate zone, this is the first I've heard that cellulose could outperform mineral wool or open cell spray foam. I'm assuming this would also be the case for exterior walls? My plan was to use mineral wool there for thermal and acoustic benefits.

            Any additional information would be helpful. I'll call around tomorrow and see if I can find a dense pack cellulose installer in the area that could come in after the spray foam. Thanks again.

          4. Luke Hagenbach | | #15

            @Bill
            It's less about finding a material to get spray foam to adhere to (as you rightfully mentioned, it adheres to everything) as much as it is finding a material that will wrap the contours of the skip sheeting and roof sheathing an inch above it, and stay in place long enough to get the spray foam applied. My biggest issue with spray foam is that it will adhere to the roof deck. I do not have a roof that will accommodate soffit/ridge vents, so there will be no air barrier to create.

          5. Jack Zylkin | | #16

            In my case I don't think the ramboard will work. I have so many nails poking through -- and not all of them are very securely embedded in my sheathing boards (some of them have been nailed to a toungue or a groove, and not to the full thickness of the wood). Slapping ramboard up there will probably cause nail pops unless I pre-drill holes for those nails. woof. Plus I don't like the potential for voids, given that my T&G sheathing consists of loosely interlocking boards and not simply a flat plane of OSB.

            I am wondering if anyone ever considered applying "mold-release agent" spray to the roof deck. Mold release is a silicone spray specifically designed to prevent polyurethane casting material from sticking to its mold. One mold-making company here in Allentown (Pour-On) suggests sealing wood molds with shellac or wax, then applying their Universal Mold Release spray before applying uncured polyurethane: https://www.smooth-on.com/page/sealers-releases/. I suspect just the wax by itself might even be enough. Or a spritz of dry silicone lubricant from Home Depot. As far as chemical compatibility, In principle I don't see much difference between two-part pourable polyurethane and two-part spray polyurethane foam, and silicone/wax are pretty inert anyways. I know Gutex contains some percentage of wax, and I imagine it can be sprayfoamed easily...

            There is the issue of flammability and heat resistance to take into account, since the foam heats up as it cures.

            Seems like a far-out idea to experiment with on my own home though. Any spray-foamers out there willing to try this for me on a scrap piece of wood and a froth kit? Maybe I could just do this experiment myself with great stuff. Any thoughts @Dana?

            P.S. my project is using HFO foam (Lapolla 4G) and I have weighed the cost/benefit of my insulation stackup carefully.

  2. Expert Member
    Dana Dorsett | | #2

    At 3" closed cell foam would have to be installed in two lifts to be fire-safe during curing and have maximum installation quality. At 3" it could catch fire while curing, and would be more prone to shrinkage cracks or adhesion problems.

    And there really isn't any need for 3".

    It only takes 2" to protect the roof deck in almost any climate, even 1" would be adequate in all both the coolest US climates.

    In US climate zone 3 or lower it only takes an inch for dew-point control at the foam/fiber boundary, for up to R43 of fiber insulation.

    In zone 4C (Pacific Northwest) it takes 20% of the total R as exterior foam to keep the fiber insulation dry, so you'd be good for up to R24 of batt under your flash-inch of foam.

    In US climate zone 4 it takes 30% of the total-R to be exterior foam for dew-point control, so with just 1" / R6 you'd be only good for R14 of batt insulation (say, an R19 compressed to ~4" thickness), but at 2"/R12 you'd be good for up to R28 fiber. and would be running ~R38, which was code-min for zone 4 prior to IRC 2012.

    In zone 5 it takes 40%, so at 2" of foam you'd be good with R18 of batt (an R19 compressed to 5.5", instead of it's labeled-test 6.25" loft), which would leave you at R30.

    If you have more depth than 7.5" to work with (greater than 2x8 rafters) it's still worth going for as much fiber insulation that will fit. At 2" the vapor permeance of the foam is about 0.5-0.7 perms, a class-II vapor retarder, which is low enough to protect the roof deck from wintertime moisture drives. To keep the fiber insulation dry, use a broad sheet smart vapor retarder such as Intello Plus (available online) or Certainteed MemBrain (available in some box stores as well as distributor outlets) detailed as an air barrier on the interior side, between the fiber/rafters layer and a layer of half inch wallboard (painted or unpainted.) MemBrain is pretty cheap- less than 15 cents per square foot, much cheaper than an extra inch or two of closed cell foam. Smart vapor retarders won't create moisture traps the way polyethylene sheeting does- they become vapor open if the moisture inside the fiber becomes high enough to support mold growth, letting that moisture dry toward the interior.

  3. Matt P | | #3

    I am in climate zone 4a, Washington DC. It seems like I should go with 2 inches of spray foam, to avoid needing two layers, followed by as much fiberbatts as will fit. With 7.5 inches rafter depth, I can only fit R19 batts back in. From Martin's linked article, it seems like I need R15 insulation value from the closed cell spray foam, but per inch I only get 6.5 r factor from closed cell hence R13 at two inches. Would you be worried about condensation on the rafters due to falling short of R15? Or is that the reason you add the vapor retarded on the interior? I am planning on asphalt felt as the first layer directly under the sheathing (which is just wooden planks). Thank you, please let me know if you see anything wrong with the plan.

  4. Expert Member
    Dana Dorsett | | #4

    The R15 prescriptive in the IRC presumes that the total R value is R49. (R15/R49= 30.6% ) There is no need for R15 unless you have something like R34 of fiber, and you only have half that.

    In a 2x8 rafter bay you have 7.25" of total depth. With 2" of foam it leaves you 5.25" of space for the batts. An R19 compressed down to 5.25" would perform at about R17. (See: https://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass.jpg ) With R12 foam + R17 fiber the total is R29, and R12 / R29= 41%, which means you would have HUGE dew point margin.

    If you kept it at flash-inch (R6) and installed 6.25" of celluose (R23) or an R25 batt compressed to 6.25" (~R22) you'd also be at about R29, but with a ratio of (R6/R29= ) 20.7 %, you'd be a bit shy for dew-point control at the foam/fiber insulation on R-value alone. But the extra inch of closed cell foam is going to run you a buck a square foot or more, yet a 12-15 cents per square foot smart vapor retarder would be sufficient for keeping the fiber insulation dry.

  5. GBA Editor
    Martin Holladay | | #5

    Matthias,
    To comply with the building code, the spray foam layer should have an R-value of R-15. That's about 2.5 inches of closed-cell spray foam (assuming R-6 per inch). The formula in the building code is somewhat more conservative than Dana's recommendation. Take your pick -- as long as your building inspector approves.

    In your climate zone, the 2012 IRC requires R-49 of total insulation -- so in addition to R-15 of spray foam, you need R-34 of fluffy insulation under (and in direct contact with) the spray foam.

  6. Matt P | | #6

    Thank you, Dana and Martin. How could I possibly get R49 with 7.5 inch rafters and without reroofing and putting exterior insulation? I could extend the depth of the rafters and put more batts? What would you do? The attic height us not too large, so I would not want to lose too much interior space.

  7. GBA Editor
    Martin Holladay | | #7

    Matthias,
    Your options are laid out in this article: How to Build an Insulated Cathedral Ceiling.

    Most builders would scab on deeper framing members (on the interior side of the rafters) to provide enough depth for insulation that (at least) meets minimum code requirements for R-value. Of course, this necessarily lowers your ceiling.

    Your three options are:
    Extend the rafter depth and lower your ceiling.
    Add rigid foam insulation on the exterior side of your roof sheathing (an option that requires you to remove the roofing).
    Accept the fact that your ceiling assembly will have a low R-value (less than minimum code requirements).

  8. Expert Member
    Dana Dorsett | | #17

    @ Luke, #14:

    >"my project is using HFO foam (Lapolla 4G) and I have weighed the cost/benefit of my insulation stackup carefully."

    You did the math on the additional R-value of closed cell only vs. flash-foam + fiber? What did you come up with? How much energy or carbon savings is that achieving per year/decade/century?

    An inch of HFO blown foam is sufficient for adequate air sealing and dew point control. The whole-assembly performance difference between filling the remainder with more foam vs. fiber insulation is likely to be a single-digit percentage, whereas the environmental impact difference will be several hundred percent. The difference between R10 whole-assembly and R11 whole-assembly for the 2x4 framing part isn't much in energy or carbon in a location as temperate as Orange County CA.

    I'm with Martin on #7- adding depth to the 2x4 rafters would be more valuable and less damaging than encapsulating them in closed cell foam.

    A flash-inch of HFO blown foam uses as much polymer as 4" of half-pound open cell foam, but open cell foam isn't sufficiently vapor retardent for dew point control. If you're planning to re-roof at some point in the next decade, as little as 3/4" (R5) 0f foil faced polyiso on the exterior could provide that dew point protection for the roof, and in the mean time plank sheathing is a bit more tolerant to moisture cycling than OSB. Long-nailing shingles through the 3/4" foam board is usually sufficient for not-so-windy locations.

    Fatter exterior foam is of course more useful, and can be relatively green if reclaimed foam is being used rather than virgin-stock goods.

    If dense-packed cellulose simply isn't an option, either cellulose batts or cotton batts have similar characteristics in terms of diffusivity, and as a DIY would likely be cheaper than closed cell foam (and a heluva lot greener!)

    https://cmsgreen.com/sites/default/files/ecocell_batt_sheet.pdf

    http://bondedlogic.com/pdf/denim-insulation/ut-denim-insulation-brochure.pdf

    It's likely that the batts would have to be special ordered, but I'm pretty sure the Ultra-Touch cotton can be ordered through the big box store chains (though it may be cheaper from other sources.)

    1. Jack Zylkin | | #18

      @dana Actually that was my comment you are responding to. I’m in zone 4/5 so 3 in of foam seems appropriate as it is more or less the code minimum. By saying I’ve already weighed my options I only meant I’m specifically looking for feedback about my mold release idea and not about my choice of insulation.

      By the way my spray contractor suspects the spray would simply drip off the roof deck if we tried it that way. He suggested rosin paper, which is way thinner than ramboard. That is what I will be doing.

      1. Luke Hagenbach | | #20

        @Jack
        What about using a spray adhesive like Spray-Lock for the rosin paper or whatever you use to avoid adding thousands of staple holes into the roof deck and hammering on it with the tack hammer (unless you have a pneumatic stapler).

      2. Expert Member
        Dana Dorsett | | #21

        OOPS! Sorry, it gets a bit confusing scrolling up and down to re-read stuff. I was primarily responding to Luke's...

        "In terms of the environmental impact of HFO closed cell, that equation is always a tricky one for me to calculate, because there are a lot of "net impact" factors to consider such as increased energy efficiency achieved by a higher R-value than could otherwise be achieved in shallow, 3.5" bays, lower air exchanges from making the roof assembly super tight, etc."

        ... and then mistook your comments as a continuation of that thinking.

        The center cavity R doesn't matter as much as the whole-assembly R, and no matter how much high-R foam you stick between 2x4 rafters 16" o.c. it's never going to "pay off" environmentally or financially compared to a flash + fiber solution or extended depth.

    2. Luke Hagenbach | | #19

      @Dana
      I believe you replied to Jack's post rather than mine, but I still gleaned some insight from it. It sounds like Jack and I are trying to achieve the same thing, but there's not clear way to avoid this pitfall of spray foam in this application: it adheres to the roof deck.

      My roof was re-roofed 3 years ago, and I hope it doesn't have to be replaced in the next 10 years, but who knows. I'm not 100% confident in the roofing job that was performed. Pretty sloppy with the radiant OSB applied, with a lot of missed nails blowing holes in the OSB to the inside. I'd like to be able to re-roof, but it seems like even with a 1" flash of closed cell I'm sort of screwed here for pulling off the OSB from the skip sheeting (I think it's about 1"x5" with about the same gap in-between). I wish I knew then what I knew now. I would have added the polyiso to the exterior side.

      If I could drop my 2x4 joists, I would have. The issue is clearance because these are on low ceilings where clearance is a big issue. I'm stock with 2x4s and 2x6s in half the house. The 2x4s might have to stick with all closed cell unless there's a more environmentally friendly option that's marginally close to the same performance.

      1. Expert Member
        Dana Dorsett | | #22

        >"If I could drop my 2x4 joists, I would have. The issue is clearance because these are on low ceilings where clearance is a big issue. I'm stock with 2x4s and 2x6s in half the house. The 2x4s might have to stick with all closed cell unless there's a more environmentally friendly option that's marginally close to the same performance."

        With 3.25" of HFO blown foam you get an center-cavity R of R22-R23, but it's a "so what" when you have 10% framing fraction.

        With an inch of HFO blown foam (R7) and 2.5" of compressed R13 fiberglass batts (R10) or 3lbs cellulose (R9) you have ~R17 at center cavity which SEEMS like a ~25% performance boost, but the whole assembly-R barely moves due to the R4-ish thermal bridging.

        How much environmental BENEFIT is there from a 10% reduction in heat loss/gain? The comfort benefit difference will be negligible.

        To be sure there won't be a whole lot of thermal diffusivity benefit from only 2.5" of cellulose compared to foam, but on the deeper cavities it will be real enough. And yes, in a 2x6 wall it's enough to make a measurable difference in locations where the night time temps are often below the heating/cooling balance point temps, and daily outdoor temps are higher than the balance point temps, which is most of the year in say, Irvine, assuming balance points between 60-65F:

        https://weatherspark.com/y/1855/Average-Weather-in-Irvine-California-United-States-Year-Round

        It makes a greater difference in roof assemblies than in wall assemblies, since the roof temperature is usually well above the outdoor air temperature during sunny days due to direct solar gain, and well below the outdoor air temperature during clear nights due to radiational cooling.

        1. Luke Hagenbach | | #24

          Your logic makes sense, and I have even more wood than a typical 24" OC roof assembly (even more than 16" OC) due to leaving the old 2x4 joists in place when the new 2x6, 2x8 and 2x10 joists were added, so the R-value of the entire assembly will be even less impacted due to substantial thermal bridging.

          I'm going to strongly consider your recommendation IF I can find a blown-in cellulose contractor in my area in the next couple of days. It seems every insulation company I call in my area either installs fiberglass batts, or does blown in loose-fill cellulose in attics.

          I was researching on the 475 website and came across the blog post, "The 8 Golden Rules for Foam-Free Unvented Asphalt Shingled Roofs," which was pushing the INTELLO Plus smart vapor retarder they sell.
          https://foursevenfive.com/blog/the-eight-golden-rules-for-foam-free-unvented-asphalt-shingled-roofs/

          If I can find a dense pack cellulose insulation company in my area that will also work with this product, do you see that as a viable alternative to the 1" flash of spray foam? They seem to tout its benefits, but they are also the exclusive supplier as well. I'm guessing there are similar performing alternatives as well.

        2. Luke Hagenbach | | #25

          @Dana, in the context of using dense pack cellulose, is there any particular reason I should flash with 1" of HFO closed cell rather than use a smart vapor barrier like Intello Plus on the interior side of the ceiling? It would seem that this method would:

          A. Eliminate spray foam from the equation, and by extension a big CO2 hit.
          B. Prevent winter warm air vapors from ever even making it to the insulation, let alone the cold roof deck.
          C. Reduce future complications with re-roofing.

          Some voices out there are quite anti spray foam for this application and have stated that spray foam forms microcracks with normal building movement, and consequently loses the vapor barrier advantage over time. I have only heard this from suppliers that provide alternative solutions to spray foam, and I have not come across data that supports the issue of monolithic stability over time. I would certainly be open to avoiding the issues associated with spray foam that I outlined above.

          1. vap0rtranz | | #26

            It sounds like you have more options for avoiding spray foam than others. I can say we are happy with the decision to spray foam despite the carbon factor. We took all things into consideration and I don't think most folks do.

            A lot of foam-free folks are building new, not retrofitting, or doing a deep retrofit; and it's easier start "new" with foam free in the plans. I know because we had gotten to the bid phase of a new build with a green architect who had experience attaining LEED for clients ... before we decided to buy used. It was much easier to build new and avoid foam ... at least above grade. And just take a step back: a lot of folks (even on GBA, ) push heavily to build new, and even my Partner and I were on that path, but do folks really mention the carbon impact of new builds? Several studies are out there that have called new builds into question, and I suspect deep energy retrofits that go down to the studs also suck up a lot more carbon than folks realize. Some of the obvious problems are:

            -you cut down more trees for new lumber for stick frame
            -you mine more quarries to lay down new cement and concrete
            -foam still gets used below grade - unless someone knows a natural alternative?*
            -most homeowners are still burning petrol for their commutes home, so that's awash
            et cetera

            * even Dana and Martin admit this so it's an exception to the Foam-Free folks: https://www.greenbuildingadvisor.com/article/building-a-foam-free-house

            Those of us retrofitting/renovating old homes are told not to use foam b/c we would be terrible stewards of the environment?? HOGWASH! I think we are in a MUCH better position to talk about carbon footprints and embodied than folks building new homes. I smell an unfairness in the bias towards new build.

            Putting that observation of bias aside, it sounds like you have more options that our house did.

            > I have even more wood than a typical 24" OC roof assembly (even more than 16" OC) due to leaving the old 2x4 joists in place when the new 2x6, 2x8 and 2x10 joists were added,

            That sounds like a range of 2x4+2x6 to 2x4+2x10? so a lot more rafter depth. We didn't have this option in our old farmhouse.

            >If I could drop my 2x4 joists, I would have. The issue is clearance because these are on low ceilings where clearance is a big issue. I'm stock with 2x4s and 2x6s in half the house.

            Exactly. Original oak 2x4's here like your house has and the previous owner had just re-roofed. And our 2nd floor ceiling height was already short w/ sloped ceilings. Not of a lot of folks talk about the clearance issues in older homes. Do folks think we're going to rip the roof off and build a new one to gain clearance?! That's possible but then you factor in all the carbon to build a new roof.

            >My roof was re-roofed 3 years ago,

            Same here, so we would have been ripping off new(er) asphalt shingles, and though we don't like asphalt for several reasons, there were no leaks found by our contractors. So what to do?

            So to get more depth we would have needed to a) become small people b) not use the 2nd floor, c) re-roof with a thick stack (and I mean >1' sandwhich of rigid foam) on top. None of options were really doable for us :) So we bit the bullet and went with spray foam on the underside. Have to say the upstairs, closest to the roof, is much warmer this "winter" than last winter -- I say "winter" because of that cold snap that just blew through even though it's still autumn.

          2. Luke Hagenbach | | #27

            @vaportranz It sounds like I have the same options you had; I'm discussing an alternative solution to the problem that is being suggested by a company that believes there is always an alternative solution to spray foam because they are 100% against it and claim it actually fails by forming micro cracks. I'm not sure about that claim, but I do like the idea of being able to re-roof some day without an incredible amount of retrofit work from the closed cell spray foam being bonded to the skip sheathing and radiant OSB.

            To clarify, I have some areas that are only 2x4, mixed 2x4 and 2x6, mixed 2x4 and 2x8 and mixed 2x4 and 2x10. Essentially, the entire roof was 2x4, and where we could we beefed that up, but some areas were too low to and we had to leave them as 2x4.

          3. Expert Member
            Zephyr7 | | #28

            Wow, you have OAK 2x4s? That’s unusual. I’m guessing it’s a pretty old place you’re renovating. Almost all stick framing lumber is pine, that’s why you see “SPF” stamps (spruce/pine/fir) on everything. Sometimes “SYP” for southern yellow pine. Hardwood is almost exclusively used for fine finishes, and even there you usually see other things due to cost.

            I should mention that essentially all framing lumber is farmed these days, so it’s a crop like corn or wheat. Building new isn’t cutting down a forest somewhere, it’s tree-planted cropland and gets replanted and reharvested. One of the reasons lumber these days isn’t as dense as the. lumber of old is that the newer lumber is harvested from younger trees and faster growing species to allow shorter cycles (fewer years between harvests) on these “tree farms”.

            Regarding foam, if your primary concern I carbon footprint, a term I’ve never really liked because energy sources tend to vary by region so you can’t accurately say “this product released x carbon”, remember to take into account all of the energy savings you’ll get from that insulation over the life of the structure. Sometimes you may come out ahead because the insulation is going to reduce your total energy use for both cooling AND heating for decades to come. There are a lot of advantages to rigid foam on renovations and new structures, although building new does open up options that aren’t really practical to consider for renovations (like thick double stud walls).

            Bill

          4. vap0rtranz | | #29

            @Zephyr7

            >Sometimes you may come out ahead because the insulation is going to reduce your total energy use for both cooling AND heating for decades to come.

            Exactly. You said it better than I could.

            Our house was built before 1940, thought I'm still researching the exact date in local archives. We found knob and tube electric when renovating so it was around during the big rural electrification push, and I found field stone in the 2' foundation walls when replacing the basement windows. Several contractors have said it looks to be oak. I'm no expert on hardwoods but nobody has said our house was built with pine.

  9. Jack Zylkin | | #23

    I just want to point out that, to avoid condensation and mold, the dew-point calculations should be applied to the rafters themselves. In my house, the rafters are full 3x5 oak beams. These oak rafters have a very low R-value (maybe R4-5) , and so I plan to fir them out perpendicularly and cover the front of them with some amount of fibrous insulation (as per Martin's suggestion in another post which I can't seem to find a link to right now.) However, to keep the exposed face of each rafter above the dewpoint in my climate, I can only add about R6 of fibrous insulation in front of each one. Furthermore, since the sides of the rafters will be even more buried in fiber, and closer to the sheathing, they are even more susceptible to condensation than the faces. Therefore in my project the sides of the rafters will be completely foamed up all the way up their sides.

    In a normal flash-and-batt job, the foam/fiber ratio you set also has the effect of creating a more or less acceptable ratio of rafter buried in foam to rafter buried in fiber, thereby avoiding condensation on the rafter itself. If you get fancy with it by furring out perpendicularly to the rafters, the calculation has to change, in favor of more foam than the code requires.

    Just saying.

    This is all to say that there are complications to the flash and batt approach if you have hardwood rafters that are very shallow. It is not as simple as saying "use the bare minimum amount of foam the code says to"

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