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Community and Q&A

Vaulted ceiling insulation advice?

Travis Shepherd | Posted in Energy Efficiency and Durability on

I’m looking to do a re-roof on my home and want to improve the insulation at the same time.  After reading this article, I am leaning towards option 1 identified there:

How to Install Rigid Foam On Top of Roof Sheathing

Here are the details:
-Home located in climate zone 3, southern California.
-Saltbox style roof, but the front is 1 story, rear is 2 story
-Mostly vaulted ceilings with no attic, two small sections have attic space (details below)
-Soffit vents to ridge vent. There is a 4’x8′ skylight in one section of a vaulted ceiling
-2×6 rafters spaced @ 24″
-Very poor insulation (3″ batting labeled as R11 from Rockwool. Most appears detached from sheathing)
-1×6 solid board sheathing with small (1/4″) gaps – appears to be in good condition
-Front roof pitch is 7:12
-Back roof pitch is 4:12 or 5:12

Current roof layering (inside-out):
-1/2″ drywall
-Ancient 3″ Rockwool R-11 batting in a 2×6″ rafter cavity, mostly detached
-1×6″ solid board sheathing
-Underlayment
-2 layers of fiberglass composite shingles

Attics:
One 1st floor bedroom has an attic underneath. This attic space does has one soffit vent but doesn’t appear to connect to the ridge vent, nor does it have a gable vent.
Another small section of roofing over a 2nd floor shower has a tiny attic. This attic space is not accessible, but has a gable vent.  There is also a whirly bird on the rear section of roof that appears to be directly in a rafter cavity.

What I plan to do:
-Remove all shingles and underlayment
-Remove whirly bird and seal gable vent over small attic space
-Repair any damaged 1×6″ sheathing, but largely keep as-is
-Leave existing rafter insulation alone
-New airtight membrane
-7″ polyiso rigid foam with staggered and taped seams (two layers of 3.5″)
-OSB or plywood sheathing (no ventilation gap)
-Underlayment & new composite shingles

I would treat the roofing over the small attic areas the same as the rest of the house. Basically following what is in this video:
https://www.youtube.com/watch?v=rn0c4WDOlMQ

Questions:
-Does the above plan seem reasonable given the information?
-What type of air membrane is recommended?
-In my climate zone, is there any benefit to installing vents over the foam?
-How does the foam connect at the ridge? I assume there is no ridge vent in
this configuration
-Do I need to close off the existing soffit vents in the rafters? If so, what is the recommended way to do this?  Most of the house is stucco, and I would rather not have to cut into it.
-Are 8+” screws sufficient to hold this much foam and roof sheathing in place, particularly on a such a high pitch roof?
-Any special considerations that need to be made around the skylight?
-How does the above plan compare to installing a “Cool Roof”?  When I first started looking at this, I had a few roofing contractors estimate a reroof and none of them had used rigid foam.  They pushed installing a Cool Roof which reflects most of the infrared light but doesn’t do anything related to insulation.

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Replies

  1. User avatar Expert Member
    Dana Dorsett | | #1

    The 7" of polyiso is a bit overkill for your climate zone, and with a 3/4" nailer deck requires a lot of awkwardly long & expensive 9" pancake head timber screws, (not 8 inchers, and not bugle-heads- eg: https://www.diyhomecenter.com/fastenmaster-headlok-heavy-duty-wood-screws-250-bucket?screwlength=45 .)

    Even discounting the ratty tired batts to zero you could still hit IRC2018 code min on U-factor basis with only 5" of exterior polyiso (fiber faced or foil.)

    With 2x6 rafters you can drill from the exterior and blow them full of 2.5-3lb cellulose or 1.5-1.8lbs fiberglass over the ratty-batts which would hit about R20 in the rafter bays, then hit code-min on an R-value basis with just 3" of foil-faced polyiso or 3.5" of fiber faced roofing iso. With 3- 3.5" foam and 3/4" plywood for the nailer deck you can get there with 5" or 5.5" pancake head timber screws rather than the unwieldy 9"inchers you'd need for 7" foam. Five-inchers are about 55 cents each 5.5" about 60 cents, quite a bit cheaper than the 9", and you'll be using a lot of them:

    https://www.hunterpanels.com/technical-resources/fastening-pattern-guides/1371-h-shield-nb-steep-slope-wood-framing-24-oc-concrete/file

    https://www.hunterpanels.com/technical-resources/fastening-pattern-guides/1370-h-shield-nb-steep-slope-wood-framing-16-or-32-oc-concrete-steel/file

    Whether foam-only or filling the rafters, yes you have to seal off the soffit vents.

    Grace Ice & Water Shield is a pretty good air barrier membrane that self-heals over any nail or screw penetrations.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Travis,
    Dana gave you good advice. There are two ways to seal the soffit vents: either remove the soffits and seal all openings with spray foam from the soffit area, or pull up the sheathing at the bottom of your roof when the roof is opened up, and seal the bottoms of the ventilation channels from above with spray foam.

    You will be sealing the ridge vent when you install the rigid foam above the sheathing -- the two slopes meet at the ridge, sealing any ventilation openings. Pay attention to airtightness when installing the rigid foam.

    If you don't want the hassle of installing rigid foam and a separate layer of OSB, you can simply install nailbase. If you don't know what nailbase is, you can Google it.

    Unless your skylight already has a very high curb -- that's unlikely -- you'll have to remove the skylight. You can either reinstall it in a new curb, or you can take the opportunity to upgrade to a new skylight with better glazing specs.

    In your climate, you don't need ventilation channels above the rigid foam, unless you care about asphalt shingle warranties. If you want vents, you can get nailbase with built-in vent channels.

    The "cool roofing" discussion has nothing to do with insulation. Whether or not you want "cool roofing" is up to you.

    1. User avatar Expert Member
      Dana Dorsett | | #3

      >"If you don't want the hassle of installing rigid foam and a separate layer of OSB, you can simply install nailbase. If you don't know what nailbase is, you can Google it."

      The fastener spacing templates I linked to were from Hunter Panels, a manufacturerer of polyisocyanurate nailbase panels.

      In high labor-cost markets nailbase panels can be cost c0mpetitive with separate layers of polyiso + nailer deck. But if using reclaimed roofing polyiso it's almost always (a lot) cheaper. Insulation Depot has a facility that usually has large inventories of reclaimed polyiso in Las Vegas, but there may be reclaimers closer than that to you. Got a ZIP code?

  3. Travis Shepherd | | #4

    Thanks Dana/Martin for the advice - this is very helpful.

    @Dana, How do you get R-38 from 5" polyiso? At best I thought it was around R-6/inch. I am generally trying to avoid packing new insulation into the rafters to reduce complexity of install, though will need to weigh that option against the added complexity/cost of drilling longer screws. I am located in Orange, CA 92869 - reusing reclaimed polyiso would be ideal.

    @Martin, Is there a particular type/brand of spray foam you would recommend for sealing the soffits? Related to nailbase, I would prefer to use multiple layers of foam compared to nailbase to keep a tighter seal. I assumed the cool roof topic wasn't related to this, but it does make me wonder what the benefit would be in addition to insulation. Taking a step back the main problem I am trying to solve (beyond replacing a deteriorating roof that is probably 25+ years old) is to reduce excess heat in the summer. We often have to run our AC for long periods of time and the upstairs area of our house is always significantly warmer than the rest (some of that is to be expected but it . When the AC cuts off, it does not take long for the house to heat again. I blame the poor insulation (of likely the entire building envelope, but primarily the roof), but feel the roof is the easiest to remedy given the need for a reroof.

    1. User avatar Expert Member
      Dana Dorsett | | #7

      >@Dana, How do you get R-38 from 5" polyiso?

      It's not R38, but it hit's code min because it's continuous, with no thermal bridging. As stated:

      "...you could still hit IRC2018 code min on U-factor basis with only 5" of exterior polyiso...".

      The key phrase is ".. on U-factor basis...".

      The IRC code-max U-factor for zone 3 per Table 1102.1.4 is U0.030, which is (1/U=) R33 "whole assembly". You have ~R30 of polyiso @ 5", but you get credit for the thermal performance of the other materials in the stackup, as well as any air films. The roof decking and ceiling gypsum add up to about R1, bringing it to R31, the interior and exterior air films add up to another R1 for R32, but the dead air space in the cavities is good for R1, so you're already above R33 wh0le-assembly (=below U0.030) even before applying any credit for the existing batts.

      Even mis-installed and degraded the /R11 batts good for at least R5-R6, and so are the rafters, so the existing insulation adds at least an other R5 to the "whole-assembly R", bringing it to a bit over R38 whole-assembly, or U0.026, which would be good enough to (barely) meet code in zones 4 & up higher.

      Even with 5" of the worlds crummiest R5.5/inch roofing iso you'd meet code on a U-factor basis in zone 3.

      https://codes.iccsafe.org/content/IRC2015/chapter-11-re-energy-efficiency?site_type=public

      1. Travis Shepherd | | #11

        @Dana, thanks I totally forgot about the rest of the assembly. Do you know of any reclaimers in my area?

        1. User avatar Expert Member
          Dana Dorsett | | #12

          I'm not sure how many smaller scale locals there are in Orange County. Running this search usually turns up one or two, but in your case not so much (though Insulation Depot in Las Vegas shows up.)

          https://orangecounty.craigslist.org/search/sss?query=rigid+insulation

      2. Travis Shepherd | | #16

        @Dana, I realized that the rafters on the rear of my house are actually 2x8 (instead of 2x6 like the front). I am now reconsidering adding insulation inside the rafters. Instead of blown-in insulation, would it be viable to remove 1-2 of the 1x6 sheathing boards every 4 feet and insert 2'x4' mineral wool batts? This would give me R30 on the back of the house and R23 on the front. At this point, is the additional rigid insulation even worth it in zone 3?

        If I did go further with polyiso, I would install 2 layers of 1" polyiso on the back and 2 layers of 1.5" polyiso on the front. Do you have any concerns with the dew point with this configuration? It seems fine based on the table here (13%+ from rigid foam): https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation

        For the air membrane, given the limited rainfall in southern California would something like Solitex Mento be a viable alternative to Grace Ice and Water shield? It is an air barrier, but vapor open and less than half the cost: https://foursevenfive.com/solitex-mento-1000/

        And lastly, what underlayment would be recommended for the upper layer of sheathing over the foam? I assume this does not need to be an air barrier, and can by any sort of typical shingle underlayment.

  4. User avatar GBA Editor
    Martin Holladay | | #5

    Travis,
    If you are going to be sealing all of the ventilation openings near your soffits, you will need either (a) a spray foam contractor, or (b) one or more large two-component spray foam kits. Most two-component spray foam kits are similar -- I don't have a favorite brand.

    "Cool roofing" can help reduce temperatures in an attic, and that can matter if the ducts are located in the attic. If your roof assembly is well designed, however, and if you have at least code-minimum levels of insulation, and if you don't have any ducts in an unconditioned attic, there really isn't any reason to install "cool roofing," unless you want to do your part to reduce the urban heat island effect.

    1. Travis Shepherd | | #8

      @Martin, Luckily I have no ducting along the roofing or in the small attic spaces.

      I'm pretty clear on the front/rear edge of the roof (cut the current roof back to the soffits, fold air membrane over bottom sheathing onto soffit area, new top sheathing above foam overhangs to create new eaves).

      What needs to be done for the sloped/side edges of the roof? The edge rafter and cantilevered support beams currently appear to be embedded in the stucco wall. Should I just leave this as-is and add a short overhang on top of the new upper sheathing (over the foam) to extend the facia? Should I open the two edge rafter bays and spray foam in to seal these better as well?

  5. User avatar
    Jon R | | #6

    > main problem ... is to reduce excess heat in the summer.

    I'd start with some blower door directed air sealing.

  6. User avatar GBA Editor
    Martin Holladay | | #9

    Travis,
    Q. "What needs to be done for the sloped/side edges of the roof?"

    A. I think you are talking about the rake of the roof. (A roof has one or more planes. Each plane of the roof has four sides: the eave, the ridge, and two rakes.)

    I don't really understand your question, but I think you are asking, "How do I trim the rake?" If you are adding thick rigid foam above the existing roof sheathing, you will usually need 2x lumber at the perimeter of the roof to enclose the rigid foam. For example, if you want to install rigid foam that is 5.5 inches thick, you need a 2x6 at the perimeter of the roof. Then you would trim the 2x6 with coil stock (metal flashing) or the trim board of your choice. Fancy molding to create shadow lines is possible but optional.

    1. Travis Shepherd | | #10

      My question was more how do I ensure the air barrier is best sealed around the rafters near the rake? For the soffits, the membrane will fold over the roof deck and spray foam will fill any gaps between the soffit and top plate. On my roof the rafters near the rake are fully embedded in stucco. The stucco itself is probably a good seal, but the sheathing will be right on top of it. Would applying a caulk to the rafter top when placing the sheathing in this area be a sufficient seal?

  7. User avatar GBA Editor
    Martin Holladay | | #13

    Travis,
    Q. "Would applying a caulk to the rafter top when placing the sheathing in this area be a sufficient seal?"

    A. That's what I would do. It's imperfect, perhaps, but it's the best way to proceed.

  8. Expert Member
    Zephyr7 | | #14

    Be really careful with the skylight. I just completed a similar project to yours and found that the flashing kit wasn’t as universal as we thought. We sealed with a neoprene roof sealer (a pour-on material similar to tar, but much cleaner to work with, expensive though) to seal the entire area around the perimeter of the skylight. This is similar to how skylights are installed on commercial flat roofs. You need to make sure water doesn’t get into your foam layers since polyiso can wick moisture.

    You’ll probably need to build up a curb from the interior framing up through all your insulation layers too to secure the skylight. Sometimes you can install a bit of foam in that assembly to minimize energy loss through the sides, but I don’t think there is any way to avoid essentially less effective insulation around the edges of the skylight.

    Bill

  9. Travis Shepherd | | #15

    Thanks everyone - I think I am almost ready to finalize my plan but had one last question. I plan to use Grace Ice and Water shield as an air barrier on the lower layer of sheathing. Should I also use it on the upper layer of sheathing beneath the shingles, or a more conventional felt product? I'm concerned with the two layers of barrier preventing any moisture from exiting (though none should ever get in).

  10. User avatar GBA Editor
    Martin Holladay | | #17

    Travis,
    You probably don't have to worry too much about ice dams in Southern California, so you don't need Ice & Water Shield under your asphalt shingles. Ordinary asphalt felt is fine.

    1. Travis Shepherd | | #18

      Hi Martin,

      I asked Dana above, but would like your input as well. I realized that the rafters on the rear of my house are actually 2x8 (instead of 2x6 like the front). I am now reconsidering adding insulation inside the rafters. Instead of blown-in insulation, would it be viable to remove 1-2 of the 1x6 sheathing boards every 4 feet and insert 2'x4' mineral wool batts? I'm not sure how easily a batt would slide into the rafter cavity, but if it works I could avoid removing all of the solid sheathing and it would give me R30 on the back of the house and R23 on the front. At this point, is the additional rigid insulation worth it in zone 3?

      For the air membrane, given the limited rainfall in southern California would something like Solitex Mento be a viable alternative to Grace Ice and Water shield? It is an air barrier, but vapor open and less than half the cost: https://foursevenfive.com/solitex-mento-1000/

  11. User avatar Expert Member
    Dana Dorsett | | #19

    >"...would something like Solitex Mento be a viable alternative to Grace Ice and Water shield?"

    Solitex Mento would be fine under inherently back ventilated tile, slate or wood shingle roofs, or metal roofing on purlins, but it's not AT ALL clear if it would cut it under a composite shingle roof, which has much lower vapor permeance and can trap/wick moisture up under the shingles

    Tried & true #30 felt would be cheaper, but not an air barrier. If you tape the seams of the roof decking and detail all the roof penetrations to be air tight it should be good enough.

  12. Travis Shepherd | | #20

    Ok I'll stick with the Ice & Water. Did you have any thoughts on the mineral wool batting and if rigid foam would be worth it?

  13. Travis Shepherd | | #21

    Finally moving forward with my reroof and had one final question related to attaching shingles properly to an over roof.

    I have 3/4” OSB as a structural deck, two layers of 1.5” polyiso, then another 3/4” OSB which I believe is considered non-structural since it is the upper layer. When attaching shingles to this layer, do the nails need to penetrate the upper deck, foam, and into the lower deck, or just the upper deck where they will be attached?

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