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See if I have this correct (insulation order)

MedicineMan4040 | Posted in General Questions on

I’m a monthly paying member now=just too much information!
And I’ve been watching all of Matt Risinger’s videos (more information!)

I’m the guy who’ll build a retirement home in Homer Ak in a couple of years.

Shooting for an R-60 roof/R-40 walls.

Please see if this order of construction will work (outside in)
1. Metal roof (and metal walls )
    No ventilation. Modeling after this Risinger video-
2. Strapping/furring strips to hold metal roof on and provide ventilation
     for the metal
3. ClOSED cell foam board, alternating, seams taped/sealed) CONTINUOUS
     with walls and roof
4. House wrap (Tyvek or alternative) or self sticking water vapor membrane
5. OSB/plywood walls and roof continous
6. Inside 2/8 stud bays either rockwool (doubtful availability in Alaska), unfaced fiberglass, or cellulose
7. Wall covering (wood most likely).

I’m still trying to get my non-builder/non-contractor head around vapor movement. I do know the walls must be able to dry in at least one direction
and in the above scenario it will have to be inward.
I also know (thanks to this site!) that the closed cell foam on top must be thick enough (Zone 7=6 inches of closed cell if I’m correct in reading the tables) to prevent any cold penetrating of the OSB/plywood where condensation can occur.

Sincere thanks for your thoughts. I’m just trying to prevent a multi-thousand dollar mistake.


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Robert,

    Looks like you are on the right track. I'm not sure what type of rigid foam insulation you are referring to when you say "closed cell." The most common rigid foam insulation types are expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso).

    In any case, in Climate Zone 7, you can refer to the 2018 International Residential Code, which says that if you have R-10 or greater continuous insulation over 2x4 walls or R-15 or greater continuous insulation over 2x6 walls, you can use a class III interior vapor retarder (painted drywall). With a low-perm exterior assembly, this is advantageous to allow for inward drying.

    You can read the above paragraph with an emphasis on "or greater." The building codes require minimums. Most high performance builders do better.

    If it were my project, I'd consider plywood instead of OSB. Plywood handles wetting and drying better than OSB. I would detail the sheathing as the exterior air barrier and I'd consider a drainable housewrap instead of regular Tyvek, which for a minimal performance penalty, allows the WRB to perform as designed.

    If the metal siding profile creates an air gap, great. If not, consider furring strips on the walls too, for a vented rainscreen gap.

    If you are also installing continuous exterior rigid foam insulation on your roof, keep in mind that recommended minimum R-values there are different from walls. The nice thing about that assembly is that you can use more environmentally-friendly fibrous insulation inside the roof to reach your target R-value. I assume you have read this, if not, check it out: How to Build an Insulated Cathedral Ceiling

  2. Expert Member
    Dana Dorsett | | #2

    >"I also know (thanks to this site!) that the closed cell foam on top must be thick enough (Zone 7=6 inches of closed cell if I’m correct in reading the tables) to prevent any cold penetrating of the OSB/plywood where condensation can occur."

    The code prescriptive is R30, minimum exterior to the roof, but that's based on a total-R of R49. The average temperature at the roof deck is a function of the ratio of exterior R/total R. So when bumping up to R60 total the minimum on the exterior would become R30 x (R60/R49) = R36.7, which is more than 6" of any commonly available rigid foam.

    Standard 2lb roofing polyiso runs R5.7/inch at the labeled-R, but needs to be derated for temperature in colder climates. At the comparative temperate climate of Homer (compared to interior AK) derating to R5/inch would be about rigtht. So for an R49 roof it would indeed take R30/R5= 6" (minimum) But for an R60 roof it would be R36.7/R5= 7 - 3/8".

    If using Type VIII or Type II EPS instead of 2lb roofing polyiso it can be UPrated to about R4.5/inch, in which case you'd be looking at R30/R4.5= 6-3/4" for the R49 roof, or R36.7/R4.5= 8- 1/4"

    Bottom line, you'll almost certainly be looking at more than 6" of foam.

    The "... OSB/plywood where condensation can occur..." is factually incorrect. OSB & plywood are hygroscopic materials, and will take up moisture in the form of adsorb (1 molecule thick films on the surface area of fibers throughout the wood) , not liquid condensation. Once the moisture content of the wood is above ~20% or so the risk of mold & rot begins to rise, which is what the insulation is trying to protect against. (At a peak moisture content north of 30% the risk becomes VERY high if it can't dry out fast enough when warmer weather arrives.) If the roof deck were made of a material that was not hygroscopic liquid moisture or frost could form on the cold surfaces, but wood needs to be at fairly extreme moisture content levels before that would happen.

  3. MedicineMan4040 | | #3

    Dana and Brian thanks!
    1. So I am correct in that I can have foam board on the exterior of the deck and 'loose' insulation on the interior.
    It would appear I'll have to do that to get the R-value desired.j

    2. Dana you are so far over my head it spins!
    'The code prescriptive is R30, minimum exterior to the roof, but that's based on a total-R of R49. The average temperature at the roof deck is a function of the ratio of exterior R/total R. So when bumping up to R60 total the minimum on the exterior would become R30 x (R60/R49) = R36.7, which is more than 6" of any commonly available rigid foam.'
    Please in laymans terms (I'm a pharmacist with precious little knowledge of building materials.
    Can R-60 be achieved with 6 inches of closed cell foam on the exterior (2x3" layers staggard) and 'loose' insulation in the stud bays?

    If not I will have to live with R-49, but according to you (my interpretation) R-49 roof will be sufficient for the Homer environ.

    3. I wish I could do all outsulation as per Risinger's video build but I cannot afford the thickness of closed cell foam for that! It must be a combination of the two.

    4. What do you all think of the concept presented by Risinger in the 'perfect wall' where the insulation of wall and roof is contiguous? Seems very smart to me, but what do I know :)

    1. GBA Editor
      Brian Pontolilo | | #5

      Yes. If you insulate above the roof deck appropriately, then you can use fibrous insulation in the rafter bays. I didn't mention it in my original response, but I do agree with what Akos says below. If you are not looking for a cathedral ceiling, a vented roof and insulated attic floor is a cost-effective and durable assembly.

      Matt's Perfect Wall house was based on the ideas presented here: From a durability perspective, it can make sense to put all of the control layers of a house outside of the framing, which is the idea behind the Perfect Wall. But that's not very typical construction, for a number of reasons. And your air and thermal barriers can still be continuous without all of the controls outside of the sheathing. I have an introductory article to the control layers coming in a couple weeks that you may find helpful.

  4. Expert Member
    Akos | | #4

    It is a common misconception that roofs need more insulation than walls. When it comes to heat loss in cold climate, the roof is just another wall. The reason we usually build with more insulation in the roof is that in case of attic with blown insulation, the extra bit of R value cost very little extra.

    If the roof and wall costs are the same, overall it is best to target all surfaces to be the same R value (ie a house with R60 roof R40 walls will use more energy than a house with R48 Roof and Walls).

    When it comes to cathedral ceilings, the cost of the extra insulation will never be saved in energy costs even in Alaska. Your best bang for your buck is to design for a vented mini attic/open trusses/deep TJI with a LOT of blown in insulation. If you want to go with unvented with rigid, then go with code minimum.

    You can also build an unveted roof with rafters and purlins with batts between the purlins instead of rigid foam. This works well for a retrofit, for new construction I would stick to simpler/cheaper assembly.

  5. Expert Member
    Dana Dorsett | | #6

    >"Can R-60 be achieved with 6 inches of closed cell foam on the exterior (2x3" layers staggard) and 'loose' insulation in the stud bays?"

    Nope. Can' t do it.

    You would need at least ~R37 performance out of the 6" of closed cell foam on the exterior.

    Stick with 6" of roofing polyiso (labeled R34, but more like R30-ish performance), and R21-R23 fiberglass or rock wool batts snugged up to the underside of the roof deck. That doesn't meet code on an R-value basis, but it BEATS code on a U-factor basis, because the continuous foam on the exterior isn't being defeated by far more thermally conductive wood rafters passing through the layers.

  6. Expert Member
    Akos | | #7

    For budget cathedraled roof:

    -11 7/8" TJI with vent baffles stapled to the bottom of the top flange
    -cross strap 12" OC with 2x4 on edge (spacing needs to be tight to prevent the drywall from sagging)

    With the cavity dense packed with cellulose you end up with a R52 whole assembly.

    This works well for any mono slope or gabled cathedral ceiling. You want longer clear spans, go with 14" or 16" TJI and get an even higher R value (~R60 and R68).

    Much cheaper than 6"+ of foam never mind dealing with hitting studs with 9" screws.

  7. MedicineMan4040 | | #8

    Someone please offer a link to image depicting
    mini attic/open trusses/deep TJI
    As is often the case I'll be being not for me but the she-boss of the house.
    There will be a substantial view we'd like to enjoy of Kachemak Bay and the mountains/glaciers
    on the other side. My guess is the open truss/deep TJI might be a compromise worth looking at over the pure cathedral ceiling.
    And as always thanks for helping in this.
    Meanwhile I'll be googling U-factor.

    1. Expert Member
      Akos | | #9

      These are just different structural ways of building a roof. They make insulating with low cost blown fiber insulation easier and have much less thermal bridging than 2x lumber.

      mini attic ( raised heel scissor truss):

      open webb trusses:

      TJI roof:

      Any of these options let you build a cathedral ceiling, which one you go with depends on your clear span and local material availability.

  8. MedicineMan4040 | | #10

    Akos fantastic information to study.
    So the wife can have her cathedral ceiling, the large array of windows for the view and we can enjoy a super insulated roof as well! Thanks so much. Just good to know options are available. Now to see what is available in the area and what the construction companies are willing to work with.
    Thanks so much again.

  9. MedicineMan4040 | | #11

    To belabor the point and confirm a few things.
    We can use, for example, 16 inch TJI, even on raised heels (for more insulation/ventilation clearance) and still have a cathedral ceiling AND get R-50+ roof.

    Now, let's say the bays of the TJI rafter are full of dense pack cellulose can I ALSO on the interior place open cell foam board against bottom of the rafters and then sheetrock. Me thinking I can get a better air seal with the large pieces of foam board.

    So from the inside out is-
    sheet rock
    open cell foam board and/or something like MemBrain)
    TJI rafter full of cellulose
    roof deck
    peel-n-stick WRB membrane
    spacers/furring strips/strapping
    metal roof

    If this is OK with you guys then I now have a game plan!

  10. Expert Member
    Akos | | #12

    With TJI you don't need a raised heel as the insulation is the same thinkess all the way along. This doesn't change even if you lower the celing slightly for the rigid insulation.

    You can't use 16" TJIs with celluose and hang a ceiling off it directly. The cellulose weighs too much and it will bulge the ceiling. You can dense pack with fiberglass though.

    If you want to use celluose, you would have to strap out the ceiling with a tighter spacing to support the drywall.

    The layer of foam underneath works well, the permeability of the foam doesn't matter in cold climate, you can use foil faced polyiso as it is higher R value and easier to tape. With a vented roof, Membrane doesn't do much, not worth the cost.

    The full peel and stick on the roof is not buying you much, only needed for low slope+shingles. Save some money, install two rows above the gutters and use regular underlayment elsewhere.

    The rest should work.

    When designing the structure of the roof, if you need a beam (especially steel) to hold up the rafters, try to have that inside the house or wrap it in a couple of extra layers of foam. Large beams can be a big thermal bridge in an R60+ roof.

  11. MedicineMan4040 | | #13

    Akos thanks!!!!!!!
    Speaking of gutters. Do I have to have them.
    On a cabin we here here in the Appalachians both long gutters drop the water
    into equally long French drains that are lined with pond liner and filled with big gravels.
    I hate gutters, but if I have to I have to.

  12. MedicineMan4040 | | #14

    Well since I'm continuing my learning and posting here I thought I'd show you guys what I do and why retirement in Alaska is our dream-
    Might be worth a one minute scroll.

    1. GBA Editor
      Brian Pontolilo | | #17

      Amazing photos, Medicine Man! Thanks for sharing.

  13. MedicineMan4040 | | #15

    Akos can you tell me more about this you said 'ou can't use 16" TJIs with celluose and hang a ceiling off it directly. The cellulose weighs too much and it will bulge the ceiling. You can dense pack with fiberglass though.

    If you want to use celluose, you would have to strap out the ceiling with a tighter spacing to support the drywall.'

    I think I'd prefer cellulose over fiberglass given those two choices. What does 'strap out' mean.
    Running 1x4's 90 connecting to the bottoms of all the rafters maybe?

    1. Expert Member
      Akos | | #16

      1.3 psf for 24OC and 2.2 psf for 16OC.

      Dense pack cellulose is ~3.0lb/ft3, so 14.5" of it 3.6 psf, well above the limit even at 16OC. You generally want your TJIs at 24 OC for roof (cost) so the weight limit is even less.

      That is what I meant by strapping out the ceiling, you might want to go with 2x3 on flat (less likely to bulge from cellulose). As a bonus, you can also use it as a mini service cavity for running your electrical.

      The only thing I'm not sure is where to put the rigid insulation. Maybe install the rigid insulation, blow in the cellulose through it and then plug the holes. Something to talk to the insulation contractor about.

      P.S. That is some pretty amazing pictures.

  14. MedicineMan4040 | | #18

    Akos, thanks so much again. Really helping me.
    The 2x3 on flat, that is the 1x4's I was guessing at, running 90 degrees to the TJI', screw to the bottom of them. Ok on that.
    Now the weight issue of the cellulose. It looks like the easy solution is dense pack fiberglass, I even read it is less itchy.
    The bonus service channel will be needed, each side of the cathedral ceiling will get two large ceiling fans and elsewhere LED track lighting, so win win on that.
    Oh, since also using rigid foam below the TJI filled bays I can decrease the thickness of the insulation in the bays being off-set by the rigid below. Just making sure you agree.
    Again thanks for the kind words. Photography is our life.

    1. Expert Member
      Akos | | #19

      Blown insulation is always cheaper than anything else, usually it is best to design with as much of it as possible, saving an inch or two on it doesn't change the price much. I would talk to the insulation installer first, they might not want to dense pack through foam. If they want use OSB instead, it makes more sense to use the OSB as the air barrier and skip the foam.

      I would figure out what is available in your area first. There is no point in designing for dense pack if there aren't installers that can do it. In my area, unlike loose fill, dense packing is not very common. For the R31 ceilings required here, it is cheaper to design with batts than dense pack.

  15. MedicineMan4040 | | #20

    Akos please help me get a game plan for the walls (now I have a concept of the roof/celing).
    After this I promise to leave you alone for a while!

    1. I wonder is there any reason why I cannot build walls using 2x8's to get a super insulated wall system. From inside to outside these layers-
    1. sheet rock/or other covering
    2. 5 inches RockWool
    3. 2 inches closed cell foam
    total r-value so far roughly (open cell+rockwool) 29.5
    4. OSB/plywood
    5. 2 inch OPEN cell foam boards. R-value=7.4
    Total wall r=36.9

    I'd hoped to get over 40.
    So why the open cell on the outside? Me thinking the OSB/plywood can dry 'through' open cell where it couldn't if I put closed cell on the outside and inside (bad to sandwhich sheathing between closed cell).
    6. Strapping
    7. Metal siding

    Can you suggest other alternative to get me over r-40 walls please.
    I'd prefer not to to the double wall method if possible.

    Thanks if you can, and thanks if you can't :)

    1. Expert Member
      Akos | | #22

      Again, this will come down to what your builder is comfortable with.

      If you can get the ceiling dense packed, it makes the most sense to dense pack the walls. If the ceiling ends up being loose fill or batts then you should design you walls with batts as well.

      Overall, I try to design out spray foam. Spray foam is very expensive, an extra layer of exterior rigid foam is much cheaper and gets you more R value as there is no thermal briding from the studs.

      For budget cold climate wall, I would say aim for:
      -dense pack double stud
      -double stud with batts
      -2x6 on 24"OC with exterior rigid insulation

      The double stud could be "standard" double stud construction or Larsen truss setup. In very cold climate it might make sense to have a vapour barrier in the middle of a double stud wall or an interior smart vapour retarder.

      The key to long term durability in cold climate is a continuous air tight envelope (ie taped sheathing), decent warm side air barrier (air tight drywall) and vented rain screen siding.

      1. MedicineMan4040 | | #24

        Akos thanks as always.
        See below the link to the USA wall. Using all mineral wool.
        I like the concept.
        My builder will build whatever I pay for and happily mineral wool is found in Anchorage, only 4 hours away. That might sound bad but in Alaska that's not bad.

  16. Expert Member
    RICHARD EVANS | | #21


    Your wall detail sounds a lot like a high performance wall used by Steve Baczek, a highly regarded architect here in New England.


    He specs Zip-r sheathing however rather than exterior foam. I'm not sure what you meant by "open cell foam boards" but I presume you are referring to type I EPS which is a bit vapor open (3 perms/inch?) 2 inches would be half that. It would probably be fine but I'd probably opt for the zip r instead just to avoid the vapor sandwich that you described.

    The materials choice (spray foam, roxul, and rigid foam) are far worse for the environment than cellulose and even fiberglass, etc. The ideal wall keeps you warm while minimizing the impact on the environment.

    I would consider a double stud wall or research the "New USA wall" designs by Gregory La Vardera.

  17. MedicineMan4040 | | #23

    Thanks for that link. and the correction in my thinking---I thought that the Zip-r insulated panels used a closed cell foam :( and with my sprayed in the stud cavities on the inside with closed cell would have created a foam sandwhich.
    The 'Best' of the Best USA wall's (bottom of page you linked to) would surely work for my purposes (greater than r 40 wall, and no foam sandwhich).
    But I didn't see what the insulation material is for the insulated wiring chase at the bottom of the page...I assume mineral wool? In fact I think he's using mineral wool for all the insulation layers=a very good thing for fire protection too.
    Going this route will def. avoid any foam sandwhiches, and honestly I have no love of foam...just convenience....and the loss of r value over time (a foam problem) is something also avoided with rockwool.

  18. Expert Member
    Akos | | #25

    Before you go too far down the road with rigid mineral wool, get some pricing.

    Rigid panels are essentially batts squished down 4x, so with 2" rigid mineral wool, you are paying for the equivalent of R32 of material but getting R8 worth of insulation.

    They are also still slightly compressible, worst so unevenly so (panels will have soft spots in them), so it takes a lot of monkeying to get the strapping even and the siding flat.

    If you don't want use foam, go with a double stud construction or cross strap the walls on the outiside with 2x3/2x4/2x6 on edge and use regular batts. Simpler and significantly cheaper in materials.

    For example a 2x6 wall cross strapped with 2x4 (R24+R14 batts) you end up with an ~R37 wall, pretty close to your target with "standard" materials.

    For higher R value, a double stud, 2x6 interior wall 3.5" gap 2x4 exterior wall, all filled with batts (R24+R14+R14), ~R44.

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