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Can you feel the difference between R-15 vs R-25?

smgb | Posted in Pretty Good House on

I have a drafty house built during the 1960’s.  The siding was done 2 years ago.  I am going to improve the insulation and was wondering, all other things being equal, can you really feel the difference between a wall that’s R-15 vs R-20 or R-25?

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  1. gstan | | #1

    Depends on where you live. It's mostly a cold weather thing. If you
    live in Fairbanks, Alaska or Yellowknife, Canada, or Gunnison,
    Colorado in the winter then YES you will feel it - warmer climates,
    or summer probably not. A really tight house (no air leaks) and good
    windows will generally have more effect on comfort than insulation

  2. Expert Member
    ARMANDO COBO | | #2

    Ha! Spoken like a Yankee 😁 Just see your energy bill double or triple in STX or AZ when the temperature is 100°F+++.
    Do an energy analysis to find out how much it affects you. It also makes a huge difference how tight your building envelope is (probably more so than the R-value of your wall/attic) , how energy efficient your windows are, ect., ect.

    1. gstan | | #3

      You're right, but he asked if you could feel it! I assumed he meant
      physically not in his wallet.

    2. Expert Member
      MALCOLM TAYLOR | | #4


      What was your summer like? Did you get toasted down there this year?

    3. Expert Member
      ARMANDO COBO | | #8

      @Malcolm - According to the National Weather Service, DFW had somewhere in the neighborhood of 53+ days where the temps were over 100°F, 4th longest or so. I did go to New Mexico for a couple of weeks to cool my kohones off and feel no humidity... that was nice! Plus I got to stuff my face with green chili!
      Next two weeks suppose to be in the 80s and 90s, but I'm going to IL and MI for another two weeks... 60s and 70s will be welcomed for awhile.

      1. Expert Member
        MALCOLM TAYLOR | | #10


        I don't know how you do it. If it doesn't cool down to 60F here at night I complain.

  3. monkeyman9 | | #5

    Im in Massachusetts. My walls were originally R11 fiberglass. Ive redone with R15 mineral wool and there's a big difference in the thermal cam. Granted all plywood on the mineral wool walls is taped now.
    Now my close cell spray foamed, large bumpout bathroom is around R25 and you can feel the difference. Was rhe coldest room in rhe house by far and is now the warmest. We've shut off the baseboard heaters.

    I wish I close cell sprayed foamed each side of the house as I went. But the DIY kits are so pricey when you do a whole side if a colonial.
    With all the time I spend air sealing each bay...I prob shoulda spent the money in retrospect.

  4. user-5946022 | | #6

    You note you have a drafty house.
    Then you note you are going to improve the insulation and ask if you can feel an R value differential.

    While improving the insulation is good, and while you may be able to fell the differential in some climates, and will feel it in your bank account in all climates.

    You WILL FEEL improved air sealing. Air Sealing is VERY different than improving insulation.
    The siding redo would have been a great time to address some air sealing issues. However if you are going to open walls for improved insulation, your first order of business needs to be air sealing. If you are not going to open walls, there are also MANY other air sealing measures you can take for SIGNIFICANT improved comfort.

    At my previous home, LONG before air sealing was common, I applied sealant. My intent was to seal between the slab and bottom plate, as I was able to confirm significant drafts there once I removed the carpet. In trying to get the sealant in there, I also ended up sealing the gap between the gyp. and the bottom plate.
    The increase in comfort was unbelievably enormous!
    Do air sealing!!

  5. smgb | | #7

    I’m near Boston MA

    Agreed! I’ll be air sealing for sure. Once I remove the drywall I will determine the steps required. I was asking whether I can physically feel the R-value differential to determine if it’s worth it to increase the wall thickness on the inside (via rigid foam) and lose interior living volume.

    I should have done more when the siding was done but what’s done is done.

    I posed in another section of this forum this question: what’s the feedback on this plan?

    Can’t change: Hardie Board > Hardie WRB > wood plank boards with gaps >

    Open to options: Henry Blue skin on the back of the board sheathing and spray-can-foam to air seal the top and bottom plates as required > 3.5” of R-15 mineral wool > Vapor Barrier > air tight drywall method > primer and paint

    I don’t want to use spray foam for health reasons.

    I have cinder block foundation and will try to get the hollow centers filled. I know they allow air in.

    Here’s the board siding before Hardie WRB and Hardie boards went on.

    1. Expert Member
      BILL WICHERS | | #12

      I think the "can you feel a difference between R15 and R11" is sometimes also because the mineral wool insulation is probably better installed than the old low-density R11 fiberglass was. I recently (last fall) replaced old R11 batts in a wall on my home with R15 mineral wool, and I noticed a MUCH bigger improvement than I was expecting. I attribute this to not just the R value increase, but also the more careful job installing the batts, which avoided all the depressions and seperation issues the old R11 batts had.

      Air sealing is huge too -- don't skimp on that. Once you have the old batts out, that's a good time to do air sealing work, then install the new batts. You are pretty much guaranteed to see an improvement after completing things.


  6. walta100 | | #9

    First you have two very different questions is sounds like you have a leaky house with R15 walls.
    Adding insulation generally will not fix drafty as most will allow air to blow right thru.
    Before you insulate one most air seal and block the drafts before the insulation can do its job.
    If you are starting with an R15 wall that is a pretty high starting point and jumping up to R25 is less than doubling and you would need to double to make a noticeable change. Look at the attached chart.
    When I had cellulose blown into my empty walls, I went from R4 to R19 the house did feel different but not that much different. There was one wall cavity that did not get filled and when it was below zero if you put your hand on that wall yes you could feel the difference.
    You may find this article interesting.


  7. Expert Member
    DCcontrarian | | #11

    I recently worked on a house in Rhode Island, zone 5. The master bedroom is over an unheated garage, so the floor is insulated. The ceiling of the garage had never been properly air sealed, which is what I was working on. But I noticed that the last joist at the end of the garage was closer than the rest, and the joist bay had never been insulated. Nothing in it. And it wasn't air sealed from the bottom, and looking up I could see where holes had been drilled for electrical and never fire-stopped. So basically that end of the bedroom was exposed to outdoor air.

    I explained this to the homeowner, and their response was, "hmm, I never noticed it was cold."

    "Never noticed a draft?"


    The radiator was along that wall. So even though cold air was gushing in all winter, they never noticed it.

    This is actually a big problem in building science. Most people aren't terribly curious about how their house works, and so long as they're not uncomfortable they don't care much about efficiency.

  8. smgb | | #13

    Thanks for all the responses.

    I will definitely air seal as much as possible when the drywall is removed. I’m pretty convinced we’ll stick with R-15 mineral wool and not build out the wall towards the interior of the house.

    Please let me know if there is any flaw to this plan, particularly applying Blue Skin to the interior of the stud bays (as opposed to the exterior of the sheathing, which is not an option at this point). I don’t know if Blue Skin’s permeability only goes “one way”, where it will trap moisture in my walls if applied as I suggest.

    Can’t change: Hardie Board > Hardie WRB > wood plank boards with gaps >

    Open to options: Henry Blue skin on the back of the board sheathing and spray-can-foam to air seal the top and bottom plates as required > 3.5” of R-15 mineral wool > Vapor Barrier > air tight drywall method > primer and paint

  9. Expert Member
    Michael Maines | | #14

    Once air sealing is taken care of, what you feel as the difference between different assemblies is the mean radiant surface temperature. Your skin loses heat directly to cold objects, just like wood stoves and the sun heat exposed skin directly, through radiant heat flow.

    You can use thermal modeling to estimate what the interior surface temperature will be, but once you have a few R's in the cavity, the difference in surface temperature won't be huge. In cold weather you can tell the difference between R-1 glass and R-10 glass, but it's harder to tell the difference between R-1 glass and R-3 glass, or R-3 to R-10. The same is true for walls. I doubt you would notice much difference over R-10.

    What does change is the rate of heat loss, so your equipment will need to work harder to keep up, which costs money and pollutes the environment, but it also usually means noise and blowing air, which are also comfort issues.

    In Passivhausland, they say that people can feel a difference of about 6-7°F in mean radiant surface temperatures. PH practitioners use that fact to get windows within 6-7° of walls. The windows might be R-10 at best, and the walls are often R-40, so that's a clue as to how much difference quadrupling the insulation has on occupant comfort due to radiant heat loss.

    1. Expert Member
      DCcontrarian | | #16

      What I have found useful to estimate wall temperature is to assume the wall has an air film with an R-value of about 0.7. So let's say the wall is R-40, it's 10F outside and 70F inside. Total R-value of the wall with air film is 40.7, temperature delta is 60F, 58.9 degrees of the delta is in the wall and 1.1 in the air film, so wall surface temperature is 1.1 below room temperature, or 68.9.

      With a window that's R10, total R-value is 10.7, you'd get 56 degrees in the window and 4 in the air film, so surface temperature is 66F.

      To the OP's question of R15 vs R25, I get surface temperature of 68.4F with R25 and 67.3F with R15 under the same conditions.

      1. Expert Member
        Michael Maines | | #17

        I hadn't thought of using air films for that calculation but it makes perfect sense. Just yesterday I was describing to an architect friend how to do a quick dewpoint analysis which is a similar process. I think we used air films for calculations in Passive House training but I haven't used them since then.

        1. Expert Member
          DCcontrarian | | #18

          It's the only analytical way I can think of for explaining that the surface of the wall isn't at room temperature.

          The problem is that it's very sensitive to the number you pick for the R-value of the air film. My recollection is that measured r-value of air films is different for walls, ceilings and floors.

  10. smgb | | #15

    I’ll stick with 3.5” mineral wool for a R-15 value.

    Any problems with applying Blue Skin to the back of the sheathing boards (in the stud bays)?

  11. walta100 | | #19

    Is there another reason to remove the drywall?

    With the water-resistant barrier, you could blow the walls full of cellulose and get almost the same R value and better draft blocking at much lower cost.


    1. smgb | | #20

      The walls currently have insulation and paper from the 1960s. I don’t think blown-in insulation can be uniformly installed unless I remove all of the drywall and clean out what’s already in the bays.

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