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How to insulate a project started in Climate Zone 5A?

drewintoledo | Posted in Energy Efficiency and Durability on

Hello everyone. My girlfriend’s brother-in-law, Dave, decided to build a replica of a Victorian home and I’m hoping that you can help me help him.

He’s “old school” so to speak and told me that he was going to insulate using traditional batten rolls after I inquired. I do not know if he purchased the insulations rolls yet.

Being in zone 5a, I believe the goal would be to reach a wall insulated value of R30. But how to get there?

I asked him to explain to me how far along his project is. He replied that he installed and taped pink fanfold (value R1) over 1/2″ particle sheeting. This is all under vinyl siding the he just installed so getting to the exterior side can’t happen. I mentioned to him that he might want to look into spraying the cavities with and inch or two of closed cell or even look into the cut-and-cobble method (as I believe it’s called) before installing his fiberglass. There is one small bonus: His exterior walls are 2×6 studs.

I’d love to receive your informative feedback, prospective problems, or pass along any advice. I’m curious how others might insulate starting with the same situation.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm going to make some assumptions. I'm going to assume that "traditional batten rolls" means fiberglass batts. And I'm going to assume that "1/2-inch particle sheeting" means (I hope I've got this right) 1/2-inch OSB (oriented strandboard) sheathing.

    The first piece of detective work necessary to solving this problem is determining the vapor-permeance of the pink fanfold insulation. If it's relatively vapor-permeable, you have several options. If it is vapor-impermeable (that is, coated with a polyethylene facing), your options narrow considerably.

  2. drewintoledo | | #2

    You got it Correct Martin, fiberglass batts and 1/2 inch OSB.

    The product is Owens Corning underlayment and is perferorated to provide a permeance value of 1.

  3. GBA Editor
    Martin Holladay | | #3

    The best insulation method for Dave to pursue depends on Dave's goals.

    I hope that Dave sealed the seams of the OSB sheathing layer with tape. If not, perhaps Dave sealed the seams of the fanfold layer. Paying attention to airtightness is always a good idea.

    If Dave decides to fill his 2x6 stud bays with dense-packed cellulose insulation, he'll get better performance than if he fills the stud bays with fiberglass batts.

    If Dave wants a higher R-value, it's always possible to use the Mooney Wall approach. (This approach requires the installation of horizontal 2x4s, 16 inches o.c. or 24 inches o.c., on the interior side of the studs. If Dave does this, he'll end up with 7 inches of cellulose instead of 5.5 inches of cellulose. The Mooney Wall approach reduces thermal bridging through the studs.)

  4. drewintoledo | | #4

    Thanks Martin. I learned that he did not purchase insulation yet. I believe the fanfold underlayment is taped but I don't know how well. I remember him mentioning the use of gorilla tape and metal tape which he called it. Do you feel that the use of spray foam for better air sealing at this point is a sound investment or would you advise against it?

  5. GBA Editor
    Martin Holladay | | #5

    Nothing is "worth it" unless it aligns with Dave's goals.

    Trying to reduce the rate of air leakage is always a worthy goal, but many homeowners don't care as much about reducing their energy bills as the green builders who visit this web site.

    If Dave is interested in reducing his home's rate of air leakage, spray foam insulation can help -- but it's not the only way to achieve that goal. Here are links to four articles on the topic:

    Questions and Answers About Air Barriers

    Air Sealing With Sprayable Caulk

    Air Sealing an Attic

    Air Sealing a Basement

  6. Dana1 | | #6

    If the goal is to hit R30 (either at center-cavity, or "whole-wall" performance factoring in the thermal bridging of framing) it's going to take a lot. A 2x6/R20 fiberglass or cellulose wall with R1 fan-fold & vinyl siding comes in at between R16-R17 "whole wall", in which case it would take a continuous ~2" of foil-faced rigid polyisocyanurate on the interior (thermally breaking the framing fraction) to even come close.

    The "right" way to have done it would have been to put the continuous foam on the exterior, under the vinyl siding, both because it's easier to implement, and it keeps the structural wood warmer (= dryer), for much higher resilience against moisture.

    If it's a center-cavity R30 value it would still take 1.5" of continuous polyiso.

    You could also get to R30 center-cavity with 5" of closed cell polyurethane as cavity fill, but that would be a total waste of expensive foam, since the thermal bridging of the studs only raises the "whole-wall" (the performance that really counts), to about R19.

    A flash & fill with closed cell + batts isn't really "worth it" since the additional performance is miniscule. Air sealing the sheathing with caulk & can foam to the studs in every stud bay is good enough, after which damp sprayed or 3.2lbs per cubic foot (or higher) dense packed cellulose (or damp sprayed/dense packed 1.8lb density fiberglass) would be sufficient.

    Assuming they WON'T be installing interior side continuous foam...

    If they had skipped the fan-fold XPS siding underlayment or the underlayment is perforated to make it more vapor permeable it would not need an interior side vapor retarder since the sheathing could dry into the air behind the vinyl siding. Most fan-fold underlayment has thin facers reducing it's vapor permeance to less than 2 perms, impeding that drying. With an abundance of due caution it's worth installing air tight 2-mil nylon (Certainteed MemBrain) just behind the wallboard, which is less than 1 perm when the sheathing is cold and taking on moisture, but becomes vapor open when the sheathing warms up, releasing it's accumulated wintertime moisture burden. With a nylon vapor retarder an standard latex paint it will have 3-5 perms of drying capacity toward the interior when it needs to dry, but will be pretty vapor tight when the moisture drive is from the interior toward cold sheathing, limiting the peak & average moisture content.

    That would be pretty much a code-minimum wall, maybe 5% better due to the R1 XPS.

  7. drewintoledo | | #7

    I was using R30 based on feedback from my own project which is in the same climatezone so I thought that might be a goal to strive for, but I believe Dave will be stuck with best effort which won't even be close to R30.

    Does this represent your idea, Dana?

    -Scratch the expanding foam.
    -Siding|fanfold|OSB w/caulked seams|2x6 foamed all edges|densepack fill|MemBrain|drywall

    Am I accurate calculating an R value of about R23? ([3.8/inch dense pack * 5.5 inches = about R21] + [fanfold =R1] + [OSB+DRYWALL = R1]) I'm learning that the thermal bridging is going to kill here.

    The only thing I'm thrown on is this: "If...the underlayment is perforated to make it more vapor permeable "
    The fanfold is definately perforated, with a perm rating of 1. It's the Owens-corninig 4'x50' pink stuff. Would you still recommend the interior MemBrain knowing this?

    You guys rock!

  8. Expert Member
    Dana Dorsett | | #8

    The "whole wall R" factors in the 25% framing fraction (typical of 2x6 framing) of ~R6.5 wood. Between all the studs, plates headers the framing edges add up to about 25% of the face area of a wall. With 1/4 of the face being ~R6.5, and 3/4 of the face covered by ~R20 insulation, the wood is conducting as much of the heat as the insulation, which reduces the average performance considerably. When you do the parallel heat flow math for the framing fraction & cavitiy insulation, add up the R values of wallboard & OSB, the R1 XPS, add in the R-value of the interior & exterior air films you're at about R16.15 for a 25% framing fraction. With 24" o.c. spacing and minimizing doubled-up plates & headers etc you can usually get down to 20% without a lot of compromise, in which case it would rise to about R17.1 whole-wall. That's it. (To hit Net Zero Energy with a solar array that still fits on the roof in a zone 5 climate typically takes R25-R30 whole-wall.)

    For rough estimations, assume a typical 2x6 wall with fluffy fill is an R15 wall, to which you can add the R-value of any continuous insulation. If you spend the money (and destroy the planet) with 5.25" of closed cell foam cavity fill it starts at about R17 + continuous insulation.

    When people are talking loosely about center-cavity R, only the insulating layers get added, not the air films, sheathing, wall board. Code prescriptive R values only count the insulation layers, unless you are reaching compliance on U-factor basis.

    R3.8/inch is a bit optimistic for dense packed cellulose. The R/inch for cellulose peaks at about 2.6-2.8lbs density, above which it starts to fall off slowly. Dense pack starts at 3lbs (some say 3.2lbs). At 3.5lbs you can count on R3.6-R3.7- call it R20, not R21, but it's air retardency is is much improved over 2.8lb density. Not that it matters much.

    Perforated fan-fold XPS is typically more like 5 perms. The 1 perm stuff is smooth, with thin polyolefin/other facers both sides. At vapor permeance of 1 perm it meets the definition of a Class-II vapor retarder (and meets the Canadian code definition of "vapour barrier"). If yours is 1 perm XPS you will definitely want to use MemBrain or Intello Plus on the interior side.

  9. drewintoledo | | #9

    Thanks for the input. I will suggest dense pack with membrain or intello plus.

    Martin - thank you for the reference material. In one of the articles, I found another link that states OSB isn't considered a good air barrier. ugh! This fella is going to have quite a battle sealing this place up.

  10. user-2310254 | | #10

    OSB is fine if you tape or caulk the seams and cover it with house wrap that is similarly detailed.

    What is Dave's plan for the insulating at the roof line?

  11. drewintoledo | | #11

    Hi Steve. Thanks for chiming in.

    I haven't spoke with him about the roof. I'll relay any advice you might have. He doesn't have a computer or access to the internet nor any desire for either.

    No chance of house wrap since the siding is already up. He didn't tape the OSB, just the fanfold. I'm going to tell him to attempt a best effort to seal the seams with caulk and "trim" around the studs and such with expanding foam from the inside of the house before adding insulation. Martin referenced an article with sprayed sealant products so a contractor be something for us to start researching in the area.

  12. Expert Member
    Dana Dorsett | | #12

    Since this is in zone 5, the Mass Save Deep Energy Retrofit Guide would be a good reference (even on new construction)

    This is a good reference for any high-R house, though some of it is zone-5 specific (such as the R-ratios of unvented roofs, etc). There are lots of detail drawings on how to deal with vented & unvented roofs, as well as many other aspects of taking a sub-code house up to reasonably high-R cost-effectively. If the siding is already up it's a certainty that the roofing has already been installed too, which may limit changes at this point.

    In new construction some things are more cost effective when implemented from the get-go, but with an unfinished new construction that already has the roof and siding up it's pretty much the same as a retrofit.

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