GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Attic insulation

Coopsdaddy | Posted in Energy Efficiency and Durability on

We are in NE Oklahoma (Zone 3). Looking for advice on insulation for the roof.

Planing on batts in the walls, with 3/4″ -1 inch foam on top of sheathing and Hardie siding.

The builder thinks we should foam the roof line.

The house is 24×42 rectangle with a 12/12 pitch roof, 9 ft ceiling both lower floors, with an option of a bonus room in the 3rd floor. Maybe put some mechanicals up there as well.

We will have a total electric central heat and air.

We are going to use a standing seam roof. What’s the best option for us? Does the roof need to vent? Do we use closed cell? What thickness? etc.

We want to do it right but also affordable. Thanks.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    In zone 3 if you put at least R5 above the roof deck, you can do the other R33 (to make the R38 code min) with cheap stuff below, tight to the roof deck, and go completely unvented:

    For long term performance and lower environmental impact it's better to do that with an inch of polyisocyanurate or 1.5" of EPS than with 1" XPS. Within the lifecycle of a metal roof XPS loses it's HFC blowing agent, and eventually setting in at about R4.2/inch. EPS and polyiso use pentane as the primary blowing agent, which is mostly gone by the time it's up on your roof, and it's labeled R is pretty much it's fully-depleted long term R.

    From an air-sealing point of view, with exterior rigid foam in place you're better off using cheaper and more effective open cell foam than with closed cell to seal up any soffits, but you may also be able to air seal the interior with the appropriate tapes and caulks more cheaply than with spray foam.

    If you have 2x10 rafters you can install high density R38s in there, or you could fill it with open cell foam and install air-tight sheet rock over the interior. Paint it with a standard latex paint and you're done. If you have 2x8 rafters you'd have to go with 1.5" polyiso (R9-ish) or 2" EPS (R8-ish) on the exterior, and high-density R30s (rock wool or HD fiberglass) in the rafter bays to hit the R38 code min.

    Avoid the temptation to install recessed lighting in an unvented roof stackup, since it creates a thin-spot and an inevitable air leak, which decreases performance an increases risk of mold/rot at the roof deck.

  2. GBA Editor
    Martin Holladay | | #2

    An insulated sloped roof assembly can be either vented or unvented. Either approach can work, but you have to get the details right. Here is a link to an article that explains all of your options: How to Build an Insulated Cathedral Ceiling.

  3. Coopsdaddy | | #3

    Thanks for the advise guys.
    Seems like attic truss will be 2x8 so looks like a max r30 batt is the max we can do and that will be packed touching roof deck,than if we put 1.5 inch or 2 inch polyiso on top of roof decking that should get us there,than sheetrock inside and paint with latex paint.
    Now we want to use a standing seam roof,so 7/16 sheathing,polyiso and 7/16 sheathing for roof system.Do we need to have any venting directly under the metal roof?
    Were do we put the felt or membrane,on top of first layer of sheathing,on top of second layer or both?
    We will have a stove pipe and two bath vents to plumb thru the roof.
    We got a quote on vented nail base and it was a 10000 doller add on,so that's out of the question.

  4. iLikeDirt | | #4

    Venting under the roof can reduce ice dams when it snows and slightly improve cooling performance in the summer. And if you install ridge-to-eave battens to facilitate this venting, you may be able to step down to potentially-less-expensive exposed fastener roofing since minor leaks at the screws will simply dribble down onto the roofing felt.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Do we need to have any venting under the metal roof?"

    A. No, although you can add vent channels if you want.

    Q. "Where do we put the felt or membrane: on top of first layer of sheathing, on top of second layer, or both?"

    A. The lower level of sheathing needs to be detailed as an air barrier (for example, by taping the seams with high quality tape), but you don't have to install roofing underlayment there if you don't want to.

    You definitely need roofing underlayment on top of the upper layer of roof sheathing.

    Q. "We will have a stove pipe and two bath vents to plumb thru the roof."

    A. I hope that you meant "approved metal chimney," not "stove pipe." It is illegal to penetrate a roof with stove pipe.

  6. Expert Member

    "you may be able to step down to potentially-less-expensive exposed fastener roofing since minor leaks at the screws will simply dribble down onto the roofing felt."

    Hey, hey now - that's not a good idea.

  7. GBA Editor
    Martin Holladay | | #7

    Malcolm and Nate,
    Needless to say, leaks at fasteners are unacceptable. However, it's fairly common for metal roofing to have condensation on the underside of the roofing under certain weather conditions. Properly detailed roofing underlayment allows dripping condensation to sit on the underlayment until the moisture evaporates.

  8. Expert Member
    Dana Dorsett | | #8

    "Seems like attic truss will be 2x8 so looks like a max r30 batt is the max we can do and that will be packed touching roof deck..."

    If it's a TRUSS rather than standard milled lumber it's almost impossible to get a good fit with a BATT, since batts aren't sized or sculpted to fit the webbing & chords without voids.

    With blowing mesh applied one side of the webbing in each truss bay it's possible to take a blown fiber approach, which would be more effective than mashing batts in there and hoping for the best.

  9. Coopsdaddy | | #9

    Draftsman's said bottom chord on truss is 2x8.
    So blow in tight to the ceeling is what would be better?
    I'm waiting to here back from the roofing company,after the quote,if we don't use the vented nail base I'm curious what he will recommend, on venting under the metal.

  10. Coopsdaddy | | #10

    I have researched to death,I want to do a rain screen because we are using hardie so I that helps the paint live longer just that will be worth it,looks like using 1x4 and cor-a-vents sv5 or cor-a-vents sturdi strips(3/8) and sv3 will be my options.Im a little concerened with there bug mesh though.
    Our walls are 2x6 24 on center,same in knee wall attic,we are still confused on insulation.
    We don't want to use open or closed cell so options are fiberglass,or blown in with netting,dense pack cellulose,loose cellulose or damp cellulose I guess.
    For the attic looks like a cold roof is what we want,I just don't want to spend the money on a nice standing seam roof and have issues.But we are still not sure of the best insulation combo.Looking at the cor-a-vent soffit vent for venting from eave to ridge.

    Would our wall benifit from 3/4 or 1 inch foam?
    Here is my order for wall
    3.(vapor perm or imper?)
    6. 7/16 osb (Taped?)
    8.3/4 or 1 inch polyiso (foil on one side,tapped?)
    9.furring strips
    10.hardie lap siding

    Roof from inside out
    3.vapor barrier (perm or imperm?)
    4.insulation?2x8 bottom chord on knee wall attic truss
    5.7/16 osb sheathing
    6.vapor perm or imperm? Felt,solitex mento etc ,taped seams?
    7.1.5 inch polyiso
    8.1x4 furring strips
    9.7/16 osb
    10.membrane ,titanium etc-perm or imperm?
    11.standing seam
    Please advise on how bad I'm screwing this up,this isn't normal building practice in our area so I hope the contractor is open minded.would blow in cellulose get me to were I need to be in attic?What about the walls,we want it right but affordable.
    Man this site is great and thanks to all that make it possable!

  11. Expert Member
    Dana Dorsett | | #11

    The wall stackup should NOT include a vapor barrier on line 3, especially with the foil faced polyiso on the exterior. Making the OSB air-tight to the framing in every stud bay would allow you to get good performance out of damp sprayed cellulose of Spider without dense-packing (but dense packing would still be good), or perfectly installed R21-R23 batts.

    Open cell foam in the 2x6 cavities is another viable option. Closed cell is too vapor tight and adds very little to the performance due to the thermal bridging. Adding just 1/4" to the polyiso thickness would be enough to make up for the performance difference between R6.5/inch closed cell and R3.7/inch open cell foam.

    The exterior foam is still "worth it" in zone 3A on a lifecycle basis or if you intend to be "Net Zero Capable". In incremental cost of 1" vs. 3/4" is worth it, since the labor cost is the same.

    On the roof assembly again, no interior side vapor barriers, only air-tight gypsum. A vapor impermeable membrane the right choice unless there's an air space for the roof deck to dry into. With 1.5" polyiso between the roof deck and the roofing, there's no drying path for the roof deck except toward the interior, so keep the interior relatively vapor-open and it'll be fine. The R-value of the polyiso is more than sufficient for wintertime dew point control.

  12. Coopsdaddy | | #12

    We plan on the drying path to be between the metal and the top layer of sheathing via (1x4 strips),so there should be a wrb on the top layer of sheathing that the furring strips are attached to?

  13. GBA Editor
    Martin Holladay | | #13

    You don't need an interior vapor barrier on your walls or ceiling. Paying attention to airtightness is much more important than worrying about vapor diffusion. For more information on this issue, see Do I Need a Vapor Retarder?

    As long as you don't include an interior vapor barrier, your plan for your walls sounds fine.

    Your plan for ceiling insulation is hard to understand. Are you planning for the insulation to follow the roof slope (in other words, to create a cathedral ceiling)? Or will some or all of your insulation be on the attic floor?

    If your insulation will follow the roof slope, you have two basic choices: a vented assembly or an unvented assembly. This article explains how to proceed: How to Build an Insulated Cathedral Ceiling.

    It sounds as if you want to combine rigid foam (your comment refers to "1.5 inch polyiso") with some type of air-permeable insulation. If you want to combine rigid insulation above the roof sheathing with air-permeable insulation under the roof sheathing, you have to make sure that your rigid foam is thick enough to keep the roof sheathing above the dew point and to keep your building inspector happy (because the building code specifies minimum R-values for the foam layer). In your climate zone, the rigid foam in this type of assembly must have a minimum R-value of R-15. That means 3 inches of polyiso, not 1.5 inches of polyiso.

  14. Coopsdaddy | | #14

    We want insulation to follow roof line,we will use the knee wall area for some duct work and the center for a bonus room.
    Can we not put 1.5 inch polyiso on top of roof deck and the rest of the insulation under the roof deck(roof line) to get r38?
    From my under standing we can use open cell foam or cellulose or fiberglass to get there?
    If so what makes the most sence,we really don't want foam because of price but if that's the best route to take than we will look at it.
    We only have so much room in half story truss so no space for vent channels,and the ones you buy are very cheap looking.So we are thinking unvented attic,that's the reason for 1.5 inch polyiso on top.
    The metal roof does not have to be vented from my understanding but after some research we decided to do a cold roof,1x4s on top of roof insulation than another layer of osb,we will have a ridge vent and a cor-a-vent soffit vent.
    Like the pic but the venting will be between the top layer of sheathing and polyiso.
    Is this not right?

  15. GBA Editor
    Martin Holladay | | #15

    The article that I linked to explains all of the different ways you can insulate this type of roof.

    If you want to install an air-permeable insulation like fiberglass, cellulose, or mineral wool under the roof sheathing, the building code requires that you install rigid foam on top of the roof sheathing, and that the rigid foam layer have a minimum R-value of R-15. Your plan to install only 1.5 inch of polyiso doesn't meet code. It's a code violation.

    However, if you choose to install spray foam insulation under the sheathing, you don't have to meet the R-15 minimum requirement for above-the-sheathing insulation. That's because spray foam insulation is not air-permeable. So if you want to, you can install 1.5 inch of polyiso above the roof sheathing, and open-cell spray foam insulation under the roof sheathing.

  16. Coopsdaddy | | #16

    I thought it said r5 for zone 3?

  17. Dana1 | | #17

    Yup- R5 makes it in zone 3A:

    But there's no drying path for the roof deck through 1.5" of foil faced polyiso, which is extremely vapor retardent, a true vapor barrier. Even fiber faced polyiso is under 1 perm, a Class-II vapor retarder. Latex ceiling paint is 3-5 perms, and more than 4x the typical permeance of fiber face polyiso, and about 200-300x the vapor permeance of foil faced polyiso, which renders the vent channel under the standing seam moot from the roof-deck's drying capacity point of view.

    But having the channel relieves the mechanical stresses between the steel & wood with temperature due to differences in coefficients of expansion, and allows the exterior facer of the polyiso to behave as a radiant barrier, adding an effective R1-R2 to the total performance of the stackup, and allows the steel to convection-cool slightly better under a blazing sun (though it already has ample radiation & convection cooling on the exterior side of the steel at your roof pitch.)

  18. Coopsdaddy | | #18

    Would you suggest not using foil faced and just 1.5 inch non foil faced?

  19. GBA Editor
    Martin Holladay | | #19

    Sorry -- my mistake. I was keeping track of several threads in my head, and for some reason thought you were in Climate Zone 4.

    R-5 is all you need.

  20. Coopsdaddy | | #20

    No problem,thanks for the advise!
    Rmax has a non foil polyiso called multi max fa3

  21. Expert Member
    Dana Dorsett | | #21

    Keeping the air gap and using foil-faced polyiso is fine and gives a modest thermal performance benefit for both the roof and wall assemblies. If the roofing were going to be tight to the foam without the air gap you wouldn't get the thermal benefit of the foil facer, but you also wouldn't get any drying benefit of a more permeable product.

    The majority of the drying capacity for just about ANY roof deck in an unvented assembly is going to be toward the interior no matter what. With ~R9 above the roof deck for dew point control you won't have any wintertime moisture accumulation of concern from interior-air moisture drives.

  22. Coopsdaddy | | #22

    Going over this and it guess I'm just worried my roof sheathing will have moisture problems if we don't vent under the metal but don't want all the extra work of venting if it's not really needed.
    If we have 2x8 bottom cord on the trusses(maybe have them changed to 2x10)than use open cell foam or blow in insulation tight to roof deck,than put 1.5 inch polyiso nailbase instead of vented nailbase above roof deck than standing seam would that work.That way we can leave out the venting of the facia/soffit area and ridge vent.If so how do we handle the gap at the top of the ridge created by the 1.5 nailbase,or just cover it with a ridge cap?
    How much differance is the blow in vs open cell on average?

  23. GBA Editor
    Martin Holladay | | #23

    Q. "If we have a 2x8 bottom chord on the trusses (maybe have them changed to 2x10), then use open-cell foam or blow-in insulation tight to the roof deck, then put 1.5-inch polyiso nailbase (instead of vented nailbase) above the roof deck, then standing-seam metal roofing, would that work?"

    A. Yes, assuming that you can achieve a total R-value of R-38 for the assembly. If we call the polyiso about R-8 or R-9, that means that you need to achieve R-29 or R-30 under the roof sheathing. You need a little over 8 inches of open-cell spray foam or blown-in fiberglass or cellulose to achieve that.

    Q. "How do we handle the gap at the top of the ridge created by the 1.5 nailbase -- or just cover it with a ridge cap?"

    A. If the nailbase is cut with a bevel rather than a 90-degree cut, you won't have a gap at the ridge. Whether your installer knows how to create a bevel or not, you will always need a ridge cap.

  24. Coopsdaddy | | #24

    I will check on how to cut a bevel,thanks.
    My biggest hang up is to put nailbase and lay metal on top of osb with no gap vs vented nailbase with some kind of soffit /facia vent,because that will add alot to the design and build.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |