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Roof is R-42 instead of R-49. Big Issue?

tundracycle | Posted in General Questions on

In trying to figure out what is causing the snow melting on our roof I also discovered that at least one section of our roof insulation is only R-42 (6″ of closed cell rather than 7″). We are in St Paul MN so Zone 6 bordering on 7.

This cathedral ceilinged loft (attic) is sheetrocked and trimmed with painters waiting to get in. Finding out if the entire roof is sub-par would require cutting out strips of sheetrock to take depth measurements. Not too difficult to patch.  If it is under insulated then we’d need to pretty much strip out all of the sheetrock and add 1″ or so of foam which would obviously be a much bigger project along with significant delays.

Of course we may need to go this route to fix the snow melting issues but that’s another problem.

Worth it for the extra 1″?


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  1. tundracycle | | #1

    How I discovered this... The pictured truss (photo is at gusset #11) has a 2x8 top cord so 7.25" thick. There are foam durovents in the spaces between the trusses so 1.125" vent + 7" insulation s/b at least 8" or about 1+" over these rafters. Didn't look like that to me and some probing confirmed that it is only about 0.125" over the rafter or about 7/8" shy of where it should be.

    BTW, I'd welcome suggestions for the best thing to use for measuring the depth of the foam. The depth gauges available only go to 6.25". It needs to be thin enough to push through the foam but stiff enough to not bend or deflect. It can't have too sharp of a point that would push in to the sheathing.


  2. MattJF | | #2

    Spraying more foam over all this likely won’t fix the issue. The r42 vs r49 is pretty much no concern. The r10.5 framing section might be a concern.

    I wouldn’t touch a thing until I had enough blowers to crank the place up to 100pa+, fog and a good ir camera.

    Edit: Any piece of steel wire makes a decent probe, no need for something labeled “foam depth probe.”

  3. MattJF | | #3

    This roof is R21 ccSPF/ R30 rockwool, 1” polyiso furring on 2x10. The box in red is missing the rockwool. Snow is about a week old and R30 doesn’t make an enormous difference.

  4. Expert Member
    BILL WICHERS | | #4

    I don’t see 1” of spray foam thickness variation causing you any issues with snow melting. R49 is a code requirement, not a specific “any less than this and you’ll have big problems” kind of number.

    If you want a better depth gauge, to get some 9 guage steel suspension wire at a hardware store. Straighten some of it out (it usually comes as a coil), then cut it at a bit of an angle to form a point. That’s usually all you need. Put some colored tape or Mark it with a sharpie marker to make the “gauge” part. The straight part of a wire coathanger will usually also work. If you need something really beefy, use a piece of thin steel rod from the hardware store and cut a diagonal “point” on one end.

    If you use anything that pokes a big hole, I’d stick some canned foam in to seal the hole when you’re done testing.


  5. tundracycle | | #5

    Thanks. My primary question here is general energy efficiency, not the snow melt problem (that's in the other thread). E.G., how much difference would that 1" make and is it worth trying to figure out if we are generally short 1" and if so then spraying an addition 1" on. Or do we forget that problem and focus only on the snow melting?

    Seems like we should probably not worry about the missing 1"?

    1. Expert Member
      BILL WICHERS | | #6

      The extra energy savings you'd get putting in another 1" of spray foam over your existing insulation will probably not pay for the cost of the spray foam over the life of the house.


      1. tundracycle | | #7

        Thanks Bill. The $ cost to us would be zero since this is a deficiency that the builder and insulator would be required to correct. The cost to us would be delay in completing the project which we'd really like to avoid but would do it if necessary.

        1. Expert Member
          BILL WICHERS | | #10

          If it's a free adder, and especially if its free due to the need to correct someone else's mistake, then I'd do it. Depending on your goals, you have two possible downsides:
          1- Project timeline disruption and mess from the spray foam overspray. This is your primary concern if your primary motivator is cost. Is the potential project disruption worth it to you for the relatively small energy savings you'll get from the extra spray foam?
          2- Environmental impact. This is your primary concern if you are trying to be the most green and minimize your impact in terms of expended resources on your project. Is that environmental cost (admittedly difficult to quantify) worth it to you for the relatively small energy savings you'll get from the extra spray foam?

          If it's #1, you'll probably decide it's worth it, since it's a one day job and they can tarp things to minimize the problems from overspray -- assuming you don't have final finishes up yet. If it's #2, that's a tough one, since it's difficult to quantify the costs/benefits on either side of that argument.

          You might find a happy medium of doing some spot checks, just having the spray foam guys hit the worst areas. Make sure all the framing is sufficiently buried, flash any areas you suspect are possible air leaks, hit the worst/shallowest areas with a bit more foam. Do this instead of spraying a 1" layer everywhere. Targeted spray foam will lessen the environmental cost if you're worried about issue #2, and it will maximize the benefits you get from that cost.

          Note that my preferred spray foam contractor in my own area likes to have me go around with a measuring "tool" (bent piece of stiff, steel wire :-), check things, and mark anywhere I want him to hit again with some colored spray paint. He likes for me to do the checks over his lunch break, or at the end of the job before he's left. Once I'm done, he goes back and adds some more over all the spots I marked with paint. He does not charge extra for this, and considers it part of his service.


        2. Expert Member
          Dana Dorsett | | #12

          >"The extra energy savings you'd get putting in another 1" of spray foam over your existing insulation will probably not pay for the cost of the spray foam over the life of the house."
          >"The $ cost to us would be zero since this is a deficiency that the builder and insulator would be required to correct. "

          The cost to the PLANET of that much close cell foam is considerable, and there is nothing but NEGATIVE environmental "payback" on adding more:

          If you need to add more spray foam insulation just to hit some magic R value number, 3" of OPEN CELL foam adds twice the R of 1" of closed cell, using only 3/4 the amount of polymer.

          1. m854 | | #13

            How do you figure out where the break-even point is with insulation that's bad for the environment? Specifically, the amount of energy the insulation saves in use depends on many factors (fuel source, climate, etc), but regardless of those, thickness of insulation is a big factor. Every doubling of thickness roughly doubles the R-value and halves the heat lost. So the first few inches of insulation is relatively helpful, but adding many inches has diminishing returns. It looks like that document is assuming R-20? If so does that mean it's probably never worthwhile to use significant amounts of foam (assuming similar climate and fuel to whatever they are using)?

            Nevermind, I found some good info here.

          2. Expert Member
            Dana Dorsett | | #14

            >"It looks like that document is assuming R-20? "

            It wasn't ASSUMING R20, it was just comparing the IMPACT of R20. (They could have just as easily used R10 or R1, or R-pi. )

            >"If so does that mean it's probably never worthwhile to use significant amounts of foam (assuming similar climate and fuel to whatever they are using)?"

            It only means is that it's nicer to the planet not to use ONLY high-impact insulation when other options can deliver the targeted performance levels in a moisture safe way.

            In most new construction high impact foam can be designed out completely. In retrofits there is sometimes a rationale for some amount of closed cell foam in combination with lower impact (or carbon sequestering) insulation. Drawing the line at the minimum amount necessary for dew point control on the fiber insulated fraction of the assembly seems like a reasonable compromise to me. YMMV.

            Unlike energy use, which can and will likely come from a variety of sources with differing carbon content over the lifecycle of a house, the damage from high-impact foam insulation is baked into the cake on day-1. Even if the place is heated with a coal burner doesn't mean it's energy supply will still be a high-carb in 30 years, and highly unlikely in 100 years. The carbon footprint of the energy use is incremental, year on year, and there are choices. If heating and cooling the place with heat pumps powered by mostly wind, PV, and hydro means the environmental "pay off" on the greenhouse footprint of the foam is "never".

            In governor ( R- MA) Baker's State of the State speech this week he was crowing about the growing consensus toward dramatically lower (effectively eliminating) greenhouse emissions by 2050 for the whole state (all sectors, including agricultural, transportation & industrial emissions):

            ""I’m committing the commonwealth to achieving an ambitious climate goal: net-zero greenhouse gas emissions by 2050,"


            Baker's former Environment & Energy Secretary, Matt Beaton built and lived in an R50+ cellulose insulated PassiveHouse, one of the first PHIUS certified houses built in this state.

            But a crappy sub-code house built with lower impact materials in the year 2000 will likely end up with a dramatically lower lifecycle impact of a high performance house built with XPS insulating sheathing and closed cell foam sprayed under the roof deck. The timing of the impact matters- taking a high impact up front increases the amount of effort needed to get it under control by 2050 or 2100.

          3. tundracycle | | #15

            Thanks Dana. Agree. 3" of open cell might do a better job of reducing air flow between conjoined multi-ply trusses as well?

            I'm not too hung up on reaching a magic number but I do want this house to be reasonably energy efficient (genuinely energy efficient went out the window 3 years ago in a rather heated discussion but that's another issue) and perhaps most importantly to not have moisture or other problems.

          4. Expert Member
            BILL WICHERS | | #16

            Either type of spray foam will do a good job for air sealing. Usually a “flash” of only maybe 1/4-1/2” or so of closed cell spray foam will do a pretty much perfect job of air sealing. 3” of open cell is more than enough to do the job too if you want to go that route.


    2. m854 | | #9

      For my climate (around 5555 heating degree days in the Colorado front range), with a 1000 square foot roof the missing insulation would cost about an extra 5 therms of gas per year (assuming 90% efficient furnace). Gas is usually under $1 per therm here in the winter, so that's less than $5 a year. In case anyone wants to check my math, the calculation is:
      ((1000 [sf]/ [R]42*24[h/day]*5555 [HDD])-(1000 [sf]/ [R]49*24[h/day]*5555 [HDD]))/100000 [BTU/Therm]/0.9 [efficiency]

      It looks like you have about 7900 HDD, so around 7 therms of heat per year.

  6. Expert Member
    Michael Maines | | #8

    Did they use HFO-blown foam? If not, and they likely did not, it's not really R-42, more like R-38, and will eventually be about R-34 as air displaces the blowing agent. Conventional (HFC-blown) foam would have to be 8 to 9" thick to actually reach R-49, despite what the spray foam people will tell you. The environmental cost of installing more foam would not likely be recouped over the life of the house, though.

    1. MattJF | | #11

      I really hope it is HFO blown.

      From an environmental perspective, it would make a lot of sense for any additional foam to be open cell. At a lower $/R, you should be able to get more R value total in trade.

      Even just focusing on covering up the framing would make sense.

  7. Expert Member
    NICK KEENAN | | #17

    As a practical question, if it's code-required I assume it needs to be inspected? If it has been inspected and approved I can't imagine a contractor is going to be eager to redo the insulation and the drywall because a homeowner poking a piece of wire thinks it isn't thick enough.

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