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R-60 in cape cod attic

erniehart | Posted in Green Building Techniques on

I am in the process of having a cape cod style modular home built. The home will not come with the attic finished so I am planning on finishing it myself. I’m in Virginia and as far as I can tell code calls for r-60 insulation in the attic area. 

From what I’ve seen there’s basically two methods for insulating attic space, one where the knee walls are insulated and the other the roof rafters are insulated. I think I would prefer to insulate the rafters but in either case I am having a hard time finding suggestions on getting R-60 at a reasonable price. 

Would like to use fiberglass but how would I keep that much insulation fixed to the roof decking

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  1. relztes | | #1

    How much room do you have? Do you have a U factor alternative of 0.024? Maybe 1.5" polyiso on the interior with R-38C batts could reach a U factor of 0.024. That's still a thick cathedral ceiling: 1" vent + 10.25" fiberglass + 1.5" polyiso + .75" furring strips for drywall = 13.5". Maybe others will have better ideas.

    If you're short on space and hate spray foam, 1" vent space + R30 mineral wool (7.25") + 2" polyiso + 3/4" furring strips might just barely get to 0.024 including all the air films and drywall. The total thickness is 11".

  2. Expert Member
    DCcontrarian | | #2

    What you call "insulating the rafters" is properly called a cathedral ceiling. They are more complicated, you should start with this article:

    As a general note, the least expensive types of insulation are ones that use fibers to trap air, like cellulose or fiberglass, as opposed to foams. They are all going to give about R4 per inch, you're going to need about 15" of insulation to achieve R-60.

    Another note: insulation should be designed into a house, it's not something you can just throw in after the house is built. There needs to be space set aside for that 15". And if you read the article I linked to above you'll see that there are subtle difference in how the house is framed depending on the insulation strategy. Same thing with the walls, and it goes double for the basement.

  3. walta100 | | #3

    Consider abandoning the cape. I see it as a flawed design well all cathedral are, and you can’t do a cape without one. If you move the insulation to the roof line it almost forces you to use foam insulation expensive and ungreen.

    Consider keeping the square footages the same but doing a full 2 stories. This will give you a tighter envelope and let you fill the attic with lots of cheap fluffy insulation.


    1. jollygreenshortguy | | #5

      Think twice about abandoning a design model (the cape) because it may not optimize one design factor (energy efficiency). There are a lot of other factors involved in the design of a home, some of them highly subjective but entirely justified. After all, homes are NOT machines for living in. Yes, they must function at an objective level, but they must also satisfy emotional, cultural, and contextual requirements as well.

      Finding ways to achieve those latter requirements while meeting functional requirements is a large part of what good design is all about.

    2. Expert Member
      Michael Maines | | #7

      I keep hearing architects and builders say that it's impossible to do sloped ceilings without foam, but I've designed dozens of them without foam. I really don't understand what the problem is supposed to be, other than thinking very slightly outside the box. But no more so than adding rigid foam at the interior.

      1. jollygreenshortguy | | #11

        Michael, what is your preferred approach?
        Do you do a ventilated system, furring down on the inside to get adequate rafter depth for a fiber insulation? How would you handle a cape type design? If you've got a sketch I'd be interested to see it.
        By the way, I've learned a lot from your comments in the past and though I do specify various foams, I'm always looking for ways to reduce that without compromising other design elements.

        1. Expert Member
          Michael Maines | | #12

          I often do a gusseted rafter system like this: Or sometimes I use parallel chord trusses; occasionally scissor trusses. I like using a sarking roof system ( but have had a hard time getting builders to do it.

          1. jollygreenshortguy | | #13


          2. freyr_design | | #14

            My fear with a sarking membrane is safety. It seems like a prime way for someone to forget there is no backing and fall through while building. Is this the push back you get or some other reason?

  4. jollygreenshortguy | | #4

    First thing to find out is whether you are using the IRC 2018 or the IRC 2021 code.
    IRC 2018 has a lower requirement, R-49 or U-0.026. IRC 2021 requirements are, R-60 or U-0.024. I'm going to assume you need to follow 2021. If it turns out you follow 2018 then things will just get easier for you.

    If you want to simplify your installation and keep up front costs down then you'll want to use the minimum amount of insulation. I'll assume that's what you want to do. If you want to maximize your insulation then that's a different problem.

    You've basically got 2 options:
    1. Try to get all your insulation inside the rafter space.
    2. Put some of the insulation outside the roof sheathing.

    Your rafter depth controls how much insulation you can fit in it. If you do a ventilated roof then some of the rafter is lost to provide for that. In the 2 options I'm looking at there is no ventilation.

    To minimize this follow the code's U-value rather than R-value. That requires some calculation. With rafter framing at 24" on center, to minimize thermal bridging, you can conservatively use a "framing factor" of 0.92.
    (2x rafters at 24" spacing and a little extra for blocking: 22.5/24 = 0.9375)
    This means that 92% of your roof area is fully insulated and 8% is interrupted by framing.

    Flash & batt is one approach. Spray a closed cell spray foam against the underside of your roof sheathing and then supplement this with some batt insulation. To get good vapor control you need to get the ratio of foam to batt insulation correct. I'll assume you're in climate zone 6 (parts of northern Maine are in 7). You want at least 42% of your insulation to be foam.

    By my calculation you can accomplish this with 4" of spray foam (R26) and an R-20 batt (5.5" thick). This easily fits in a 2x12 rafter. You can also make this work in a 2x10 rafter despite the 1/4" of compression of the insulation batt.

    Spray foam can be an expensive proposition. It MAY be cheaper to put foam sheets above the roof sheathing, but this complicates the roofing installation. So you would want to check with your builder to do a cost comparison on that. Your insulation package would certainly be cheaper but the savings might be lost to a more expensive roofing installation.

    By my calculations, if you put R20 rigid foam sheets above the roof sheathing and R25 batts between the rafters the numbers work.

    I've attached 2 images showing my calculations. I welcome other commenters checking my numbers as I am not perfect (despite appearances).

    The flash & batt approach keeps the rest of your building assemblies conventional. Putting rigid foam above the roof sheathing requires thoughtful detailing of roofing, flashing, eave and gable rake details.

    Best of luck with your project. Please share your progress, including some photos. Thanks.

  5. Expert Member
    DCcontrarian | | #6

    "If you want to simplify your installation and keep up front costs down then you'll want to use the minimum amount of insulation."

    That's not going to win you many friends on a green building website!

    1. Expert Member
      Michael Maines | | #8

      I don't know about that--I'm pro-insulation, but when using foam, with it's outsized negative impact on our environment, I use the least I can get away with. And even I think the R-60 requirement for roofs is too much, at least for zones warmer than 6 and maybe 5.

      1. jollygreenshortguy | | #10

        Michael, I agree. At a certain point I think there are diminishing returns on more insulation and it's better to put the money into getting a tighter shell, better windows, and/or more efficient mechanicals.

        Back when I started doing this R19 in the roof was the best you could expect. I would specify R30 and contractors would tell clients behind my back that I was wasting their money.

    2. jollygreenshortguy | | #9

      It's just a statement of fact. I'm not advocating any position. I'm trying to respond as closely as I can to the specific issues raised by erniehart. A significant part of that are the constraints posed by his framing, in particular, the rafter depth. There are certainly more complicated and costly ways to increase his insulation if that's what he wants to do. But from the tone of his comment, I don't think that's what he wants to do.

  6. walta100 | | #15

    “By my calculations, if you put R20 rigid foam sheets above the roof sheathing and R25 batts between the rafters the numbers work.”

    I like this plan but getting a modular home builder to put R20 on the exterior of the roof under the factory installed shingles seem like something very unlikely to happen.


    1. Expert Member
      DCcontrarian | | #16

      Yeah, I think the question we maybe should be asking is what does the manufacturer want to do? Aren't they supposed to deliver a finished house?

      1. erniehart | | #18

        They are delivering a finished house, but the second floor will not be finished. They will be insulating the floor between the first and second floors

    2. jollygreenshortguy | | #24

      reply to walta100 - Yes, I totally agree. But I wanted to present at least one alternative to flash & batt. I thought about suggesting other alternatives as well, such as furring the ceiling down with gussets to get more depth between the rafters. But my comment was already too long. So I decided to stick to 2 options that didn't involve changes to framing and ceiling heights.

      If I were building a cape from scratch I might go either the gusseted rafter route or the foam on top of roof deck route. I'd consider the flash & batt only if I was tied into doing flash & batt in the walls already.

  7. erniehart | | #17

    Thanks everyone for the information thus far.

    I have done some research on a gusset approach and really like it but want to make sure I understand it and ask some clarifying questions

    first I would install baffles below the roof decking into the eaves and soffit area

    then I would basically get a couple sheets of probably 1/2 inch osb or similar and cut it into strips 4-6 inches wide and as long as I would need them to fit the thickness of insulation required about 20 for fiberglass or 16 for rock wool and I think 17 for cellulose. then nail or otherwise attach them to the sides of the rafters and nail a 2x4 to the other end of the piece of osb.

    once that's done I'd basically install batts as normal, just two layers.

    I think I would go with fiberglass as that seems like the cheapest and easiest install, anything blown in seems like a lot more work imo.

    Would I be able to install drywall directly to the 2x4 at the end of the gusset?

    pretty sure this is a yes but would the baffles be required if I were to use rock wool?

    would there be any kind of barrier required on the inside of the insulation, if so could I use one layer of non-faced batts and another layer of faced batts?

    do I need to fill the gaps between the 2x4 and the rafters with insulation? I assume so, any reason I couldn't just stuff some more fiberglass between there

    how high do the baffles need to go? just above the insulation above the ceiling?

    sorry about all these questions, first time doing this kind of a project and really didn't realize how complicated insulation could be

    I am in Virginia and afaik we moved to the 2021 IRC the beginning of this year. I also believe the whole state is zone 4

  8. Expert Member
    DCcontrarian | | #19

    "They are delivering a finished house, but the second floor will not be finished. They will be insulating the floor between the first and second floors."

    OK, that changes things. If it's going to be code-compliant it's going to be delivered with a vented attic, there will be vents in the soffits and the ridge.

    I think your best bet then is to keep the venting but move the insulation to under the roof. This means running a channel from the soffit to the ridge so that air can flow freely under the sheathing. Then your R60 of insulation, then a vapor barrier and air barrier (most likely drywall).

    Do you know how the roof will be constructed? Specifically, the size of the rafters? That will affect your strategy somewhat.

    Also, will the joists that make up the floor of the attic be strong enough to support a floor in living space? When the space is unfinished they only have to be strong enough to support the ceiling, which is a lot less weight.

    1. erniehart | | #20

      "will the joists that make up the floor of the attic be strong enough to support a floor in living space?" - I very much assume so, they do run utilities up to the unfinished space, not sure why they would do that if it wasn't designed to be living space

      "Do you know how the roof will be constructed? Specifically, the size of the rafters? That will affect your strategy somewhat." I do not know much, the literature I have only indicates that the rafters will be 24" oc

      Will there be any need to do anything with the insulation between the floors. I think I have heard things about air sealing below the knee wall.

      1. Expert Member
        DCcontrarian | | #21

        You can't assume so. I would assume the opposite, that a modular company has figured out how to squeeze every nickel out of the design. Thick joists are expensive.

        And it's not designed to be living space. It's outside the building envelope.

        1. Expert Member
          Akos | | #25

          Yup, I can pretty much guarantee the ceiling joists are not sized for a floor load. I would find out if they can do it otherwise this idea is DOA. Retrofitting structural beams and posts or new ceiling joists in a finished place is possible but not simple or cheap. While talking to them, I would see if you can get the rafters bumped up to 2x12 and build per post #1.

          Also make sure your foundation can handle the extra floor load.

          Stair openings need structural support, make sure you have a plan for it. The framing won't be there from the factory so you have support it from bellow, this means some posts and extra foundation there.

          With batt insulation baffles are really only needed by the soffit. If you do make them full length make sure they are permeable otherwise you can have issues with condensation on the baffle.

  9. Expert Member
    DCcontrarian | | #22

    " I think I have heard things about air sealing below the knee wall."

    What you want to do is insulate the roof and then build the knee wall, so the knee wall isn't part of the building envelope. It makes everything easier.

  10. Expert Member
    DCcontrarian | | #23

    Serious question: have you priced what it would cost just to build a two-story house from the get-go?

    Finishing this space is going to be more than the cost of insulation and drywall. It needs electrical and HVAC. It needs stairs going up to it. Those are expensive.

    1. erniehart | | #27

      It will already have stairs in place, I am paying about 12k to have them increase the roof pitch from I think 5/12 to 12/12. Everyone here is right that I should make sure that it can handle the loads of adding living space up there but being as I am paying this much more I’m thinking that’s part of what I’m paying for, part of that is running the utilities up to that space that’s not normally something they do unless you get the cape upgrade package

  11. walta100 | | #26

    My guess is if you only count the floor space with 7 feet of head room after you fer down the sloped ceiling to make room for the insulation and subtract the space the stair way will occupies what do you get 300 sqf maybe?

    1. erniehart | | #28

      The roof pitch is going to be 12/12, according to the modular sales person I should get about 900 sq ft, I’m sure the gussets will take some headroom away but it will still be over 8’

      1. Expert Member
        DCcontrarian | | #29

        I think of a typical cape as about 24x40. On a 12/12 roof to have a 4' knee wall you have to come in 4', let's say it's 5' by the time you finish everything. So that gives you a finished width of 14'. To get to a ceiling height of 7' you have to come in another 3' on each side, which is another 6', or a finished width of 8'. So that gives 320 square feet of area where the ceiling is 7' or higher.

        If the ship hasn't sailed, you really should look into just doing a second floor with the same vented roof and attic as the original. You get the full 960 square feet of the footprint. My suspicion is it might be around the same money.

        1. Expert Member
          Michael Maines | | #30

          For a full second story, you have to add the cost of exterior cladding and windows, which can get expensive. It's not that hard to detail a Cape-style roof properly; it's just rarely done.

        2. erniehart | | #31

          It sounds like what you’re describing is a room with the walls and ceiling perpendicular, I am not planning on doing that. Was planning on putting drywall directly on the rafters till I got to a height that’s good for a ceiling. On that note I think I get an exception for the r-60 requirement there where I can do up to 500sq ft of r-30.

          The home sales place has their office set up just like this and while I didn’t measure anything it definitely felt much bigger then 300 some sq ft

          1. Expert Member
            DCcontrarian | | #32

            What I'm describing is a knee wall that is vertical until a ceiling height of 4', then a sloped ceiling that follows the roofline until a heigh of 8', then horizontal.

            You don't want to follow the roofline all the way to the floor, you end up with an awkward space that's not really usable. If you make it 4' high you can put a bed or desk against the wall. If you insulate the roofline all the way to the floor, then the space behind the kneewall is insulated and you can use it for storage, or even as a kid's play fort or a reading nook.

            To answer your earlier question, you can insulated the roofline to the ceiling and then insulate across the ceiling. It's easier, uses less material and gives better venting.

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