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Community and Q&A

Insulating a cathedral ceiling in the San Francisco Bay area

ganon | Posted in General Questions on

I live in the San Francisco Bay Area (San Mateo – Climate Zone 3). I currently have a low-pitched (2.68/12) cathedral ceiling with an asphalt shingle roof directly over the roof deck, no insulation.

It gets really hot inside during the summer months and even with A/C running all day I’m still not able to control the temperature (all large glass windows, few of which open). Even moderate temperature days can still require A/C and even late into the evening when it’s significantly cooler outside. Almost all windows are double pane and there is no insulation in the walls (what little there is of it that isn’t window). I’ve been told by several people that 75% of my problem is the roof.

I’m planning to add solar, so I figure if I’m going to re-roof it’s a good time to do it (even though the current roof is only 10 years old) and fix my heat problem (it does get a bit cool in the winter, but it’s still California so not a terrible problem). I’ve received quotes from several roofing and insulation companies all with different solutions.

1. Add more rafters under the roof, then insulate the ceiling 4.5″ with R30 spray foam and then sheet rock. (Not my preferred way since I’d lose the wood grain and bring down the ceiling.)
2. Overlay existing asphalt shingles with Safeguard 30 high performance hybrid underlayment, install batten system and batten style insulation system, then install Boral Steel stone coated roofing system.
3. Install low-e reflective insulation, OSB and shingles. (I don’t understand how low-e reflective layer will help without an air space).
4. Install 2″ foam board insulation, OSB and shingles.
5. Install 4″ foam board insulation, OSB and shingles.
6. Install 2″ foam board insulation, 2″ vented deck, ridge vent, OSB and shingles. (Also per my request, I have quotes for 3.2″ and 4″ insulation and adding the low-e layer to the roof deck prior to the vented deck.)

I’m confused by all the options and prices. Option #3/4 said the low-e (without air space) will help my problem enormously, but from my reading, I don’t see how. Option #6 said the R13 2″ polyiso will help a lot, and the vented deck will help even more, but from my reading here a vented deck doesn’t help cooling, just moisture. He said he also can use a single layer of 3.2″ foam board, which require large fascia boards, but get me R20.8. The 4″ would require 2 boards and be much more expensive. He also said that he didn’t recommend the low-e for the cost/benefit.

My reading about all of this mostly comes from:

As a Climate Zone 3, I’m supposed to have at least R25 roofing, which would be 4″. Is this necessary? Am I really going to notice a difference between R20.8 and R25?

Also Option #6 offered different types of shingles from just the regular ones (eg. GAF Timberline HD), their Reflector series (at the same cost) or the Cool Series (at a much higher cost – which he didn’t recommend).

I’m so confused with all my options, but I’m leaning towards 3.2″ R20.8 foam board insulation, 2″ vented deck, ridge vent, OSB and a light colored GAF Timberline HD Reflector Series shingles.

Any thoughts?

Thank you so much for any help!

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  1. user-6184358 | | #1

    It seems this will need a building permit- give that -option #6 with 4" of insulation is the best
    The 2 layers of foam board will allow the joint to be overlapped for a better final product.
    The vented deck air gap is needed to maintain the warranty on the Comp Shingle roof. Most won't warrant an unvented installation, and correct they don't do much for cooling.
    The cool roof is likely required as is a Class A rated roof system ( if you don't want a cool roof then the energy calc needs to be done to justify a non cool roof). Most all newer comp shingle roofs will last 25 plus years in the mild CA heat & winters.
    The roof decisions can be helped by having a title 24 energy calculation done to determine what is required. I see you have newer windows, that combined in the energy software will lead to the minimum code allowed insulation level for your roof. A Title 24 cost $200-$300 for a residential remodel, so money well spent and likely required to get a permit.
    Make sure who ever does your roof that you have it permitted & inspected by the building department.

    1. severaltypesofnerd | | #16

      Another advantage of 2 layers of 2 inch: it's more flexible, and will adapt to a somewhat bowed roof better than 4".

      Metal roofing lasts longer, a better value over a full lifecycle.
      The 25/50 year shingles seem to last 15-20 years in full sun, 30 years in part shade in this area. Moss has a way of growing in many areas.

  2. user-6184358 | | #2

    An additional item not on the list is improvement to the roof diaphragm to resist seismic forces. Installation of a structural wood panel diaphragm directly to the T&G deck board will stiffen up help to reduce damage in the big one or smaller big ones. Also it will give you a chance to install large structural screws to improve the roof to wall connection. This will prevent the wall & roof separating in a seismic event, or reducing the damage from earthquakes.

  3. ganon | | #3

    Thank you for your insights Tim.

    Going from 3.2" to 4" is quite a bit more expensive because of the additional layer and larger fascia board around the side of the house. I'm still thinking about that. Trying to decide if that's a better pay off than shingles with a higher SRI.

    I'm in a single family home in California Climate Zone 3. As far as I've been able to tell, Title 24 doesn't seem to have any requirements for my re-roof? I do want to be energy efficient, if it doesn't raise the costs too much.

    Yes, all the roofers I've talked to are pulling permits.

    A structural engineer did talk to me about adding metal strapping on the ridge for earthquake reinforcement. I need to figure out how to schedule those together.

    1. severaltypesofnerd | | #15

      Correct, Title 24 is not triggered until you increase the conditioned floor area.

  4. Expert Member
    Dana Dorsett | | #4

    It looks like you have 2x6 t & g roof decking(?). To make that air tight requires a fully-adhered WRB (eg Blueskin® PE200HT , or similar) or an over-layer of OSB/CDX or something.

    In climate zone 3 it only takes R5 of insulation above the roof deck to provided adequate dew point control. Foil faced polyiso provides a low-E facer if desired.

    R25 may be CA Title 24 code if the insulation is between rafters at a presumptive 7% framing fraction, but the IRC calls out R38., or U0.030 max (= R33.3 "whole-assembly R" ) on a U-factor basis. There is greater value to continuous layers of insulaion above the roof deck than high R/inch foam between rafters, so if going for more than the R5 above the roof deck you should be able to get there with less R between the rafters by calculating the U-factor:

    When calculating the U-factor all thermal bridging has to be factored in, but credit is also given for the R-values of the other elements such as the roof deck, roofing, interior & exterior air films, and the air films in the gap between the rooftop foam and nailer deck etc.

    For the 1.5" thick t & g roof deck you're allowed 1.5" x R1.25/inch= R1.88

    The 5/8" OSB nailer in option #6 is good for R0.74

    An asphalt shingle layup is good for another R0.44

    The interior ceiling air film on a low angle roof is good for R0.92, as is the air-gap under the nailer deck for a combined R1.84. The exterior air film is good for R0.25, bringing the air films total to R2.09

    Half-inch ceiling gypsum is good for R0.45.

    So before getting into the insulation layers there is already R5.6 to apply toward that R33.3 "whole-assembly-R", so only R27.7 is left.

    Using foil clad decking for the nailer deck and a foil faced foam for the above deck insulation adds another R1 or so, reducing the requirement to R26.7.

    A layer of 2" polyiso on the exterior would deliver R12, now it's down to R14.7.

    There is apparently 4.5" of space for insulation below the roof deck? That's enough for R17 of mid-density fiberglass (say, compressed R19s) or cellulose, which at a 7% framing fraction would get it there. The pretty ceiling would be lost, but could be regained by 1/2" t & g under your fiber insulation layer covering the sheet rock (or use a reinforced broad-sheet air barrier such as Intello Plus behind the half-inch t & g, skip the sheet rock.)

    Going for IRC code min on a U-factor basis with that R12 foam-over/ R17 fiber-under stackup would also meet/beat CA Title 24 performance on an R-value basis, and would outperform a code-min R25 batts between joist or rafter solution with margin.

    To hit IRC on a U-factor basis could also be achieved with just 5" of 2lb roofing polyiso above the roof deck, preserving the interior. That might even be easier and cheaper (a lot cheaper if using reclaimed roofing foam). That would require 1x6 facia boards, but wouldn't look too weird on your roof lines.

  5. ganon | | #5

    For cost reasons, I'm planning to stick with adding insulation above the roof deck.

    None of the roofers or insulation people I've talked to have said that I am actually required to add any insulation to my roof. When I asked one roofer, he said my city does not require it.

    Of 4 roofers (for above roof insulation) and 3 insulation (for under the roof insulation) people, only one of them actually discussed venting my roof deck. 1 roofer and 1 insulation person, when asked, said that venting wasn't necessary and they weren't concerned about moisture.

    None of roofers discussed waterproofing the insulation layer. They're just going to sandwich the insulation between OSB. The guy that is going to vent the roof deck said that the venting would take care of moisture and water sealing isn't necessary. Is that normal?

  6. ganon | | #6

    Does a vapor barrier need to be installed if adding insulation only above the roof decking and adding a vented deck above that? No one seems to have added this to their estimates, but I'm unsure if it's needed.

    Their plan is:
    - tear of existing roof, remove and place dry rot
    - install 2 layers of 30# felt paper
    - install 2-5.5" of rigid polyiso insulation
    - install vented deck - 2" air gap, 2" holes on eves, ridge vent, OSB

    I've read vapor barriers are needed for unvented roofs, but that shouldn't be a problem in this case then?

  7. GBA Editor
    Brian Pontolilo | | #7

    Hi Ganon.

    You are correct. Unvented roofs need to be able to dry inward, so a vapor retarder is not necessary. Even though you will have venting above the insulation, what's unvented is the rafter bays. That assembly, the rafters and sheathing is what will need to be able to dry to the interior. You should do everything you can to air seal the ceiling below the roof which will help the new insulated assembly to perform and keep it healthy.

    1. ganon | | #8

      Thanks Brian. I have a tongue and groove ceiling, which I assume is not going to be air sealable without covering it up with sheet rock or something. Is there anything that can be done to preserve the T&G?

      1. GBA Editor
        Brian Pontolilo | | #9

        Hi Ganon.

        In your assembly, where the roof deck will be kept at or near the indoor air temperatures by the exterior rigid foam, the main reason to air seal the ceiling is to keep conditioned air inside the house for the sake of efficiency.

        There's no good way to seal a tongue and groove ceiling. You could take it down and install drywall and then reinstall the tongue and groove boards. Most people are reluctant to do that.

        What would also be helpful is to air seal the roof deck. It's actually considered best practice with an assembly like this. If the deck is OSB or plywood, this can be done by taping or using a fluid applied product on the seams. If it is boards, you could use a peel-and-stick underlayment instead of the felt. This won't stop conditioned indoor air from escaping in other places around the perimeter of the roof, but it will help.

        This is a good assembly and if you don't do that interior air sealing now, I don't think there will be any problems, just some loss of efficiency.

        1. ganon | | #11

          Ah, okay, that's really helpful. So nothing to worry about if I don't have anything more than the felt layers under the foam boards, but air sealing would be nice to have for A/C efficiency. Thank you!

          1. GBA Editor
            Brian Pontolilo | | #12

            That's not what I meant. Sorry if I was not clear. You definitely want to keep indoor air out of that roof assembly, particularly for when you are heating the house. It is best practice to air seal the roof deck and I'd say that should be done one way or another. If you can't air seal the ceiling, that just becomes more important. Have you read this? Insulating Low-Slope Residential Roofs

  8. user-6184358 | | #10

    You could use Henry Blueskin VP100 a house wrap product , a peel & stick that is vapor permeable direct to the t&g prior to the felt. The hard part is sealing the v notches, both at the top of the boards and from the ceiling level out to the world. Sealing the v-notches could be done with a thick fluid applied to seal the notches to the peel & stick

    1. ganon | | #13

      Thanks! I'll ask my roofer about that.

  9. sn7 | | #14

    Hi Ganon,
    I am in a similar situation. What route did you go with - any suggestions that would be helpful to me?

  10. Chicken_Soup | | #17

    Hi Ganon, sn7, and the clearly informed GBA contributors!

    I am a fellow San Francisco homeowner with a cathedral ceiling with a reroof need.

    This thread is really relevant to me and I am looking to hear the latest on best practices and recommended San Francisco roofers.

    Our old roof was failing due to age (thermal and wind damage).

    The existing roof assembly was just decking, felt and shingle. No ventilation. Poorly insulated batts between rafters. The rooms below the ceiling get very cold in winter and baking inl summer heat.

    Hi Ganon and sn7. Could you kindly share what roof assemblies you decided to install and which roofer you worked with?

    Thank you!


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