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Community and Q&A

Preventing Moisture in Unvented attic in Coastal California

rfb | Posted in Green Building Techniques on

I have gone through a read a number of papers, and watched a fair number of videos on the pros and cons of vented vs unvented attics. We are building a house (new construction) that burned down in a fire in Northern California on the coast. The weather is hot and dry in the summer with cool nights and torrential downpours in the winter with snow once a year that melts away in a few days.  Coastal proximity means evenings cool down and can bring occasional dew. We will have both solar + batteries 

My partner and I are both engineers, and so understand a decent amount of what we are reading, but as Owner-Builders we lack a resource to help make a decision using local experience. Most of the literature out there on the pros/cons of unvented attics is referring to either extremely cold NE locations with ice dam prevention, or in hot/humid SE USA. We are struggling to put all those pros/cons into a decision.

-We have a heat pump but we have to have the blower unit (could be incorrect terminology) in the attic due to lack of spacing elsewhere.
– We live in a high fire prone area so have to comply with WUI (we understand there are wui approved vents), but given this and the fact a house already burned down once, we love the idea of an Unvented attic space to reduce ember intrusion.
– Similarly, we want to avoid soffit vents for the same reasons.
– Code requires us to have solar (we will have battery backup as well)
– The attic is not used for storage, just has the hvac in it and will not be accessible

-Given the solar and battery backup- is having the HVAC unit in a VENTED attic really that bad of a hit to us? I understand its vastly less energy efficient but cost wise it might just be a wash with the solar?
– My biggest concern with an unvented assembly is not noticing any leaks in the roofing. Things happen, and our neighbors rebuild has already had multiple roof leaks. I worry about lack of detection ability- particularly with lack of access due to solar panels.
– Are there things we can do to increase our ability to determine if there are leaks? For instance, one would assume that open cell spf would “leak” that moisture better and help one determine that there was a leak?
– I worry about the fire-proofness of spray foam and if that is only going to make our home more of a burn risk but drywalling it up seems excessive, are there other options to fireproofing it? I know there is the fire retardent paint but it seems not particularly effective..
– It seems like rigid foam board on the exterior is only really a thing on re-roofs or in cold climates for ice dams. Why is that not a thing in more moderate climates? Wouldnt that resolve both the fire risks and would allow you to see water intrusion?
– Similarly, I have read mixed things on the use of fiberlgass batts in unvented attics. Wouldnt fiberglass batts + a vapor diffusion vent resolve that as well? We seem to be in a border climate for this and I am unsure if it’s kosher to pursue this in our local district.

Thank you for any insight or help. This rebuild is our forever home and we want to ensure we do not screw this up too bad..

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  1. Expert Member


    A few related thoughts:

    - I'm unconvinced by the argument that open-cell foam allows enough water through it to be an effective way of finding leaks. Most roof leaks are episodic and of small volume, which are unlikely to be detected by the foam taking on the moisture.

    - The problem of HVAC in vented attics isn't just efficiency, the space and equipment are also prone to mold and moisture accumulation.

    - If you are still in Climate Zone 3 where vapour-diffusion ports allow permeable insulation to be used against the roof sheathing without problems. That would be my first choice.

    - My second choice would be create a service cavity below the attic using plenum trusses, or a floor joists (or trusses) with a subfloor used as the primary air barrier, leaving you free to insulate the attic floor above.

    - Last on my list would be a foamed cathedral ceiling for a wide variety of reasons, including the ones you have brought up.

    1. rfb | | #4

      Malcom, thank you for your time and thoughts. You have confirmed my suspicions that since we are in Climate Zone 3 (according to the maps at least, though I'm unconvinced our specific microclimate is #3), that batts + a vapour diffusion ports would be the top choice for simplicity purposes.

  2. Expert Member
    Akos | | #2

    Take a look at fig6 here for diffusion port in WUI area:

    There is lot of good info on their site for unvented ceilings with fluffy insulation in warmer climates.

    1. rfb | | #3

      Akos, That is a great resource you have linked, thank you for sharing! I want to make sure I understand the code correctly- In this case, in Zone 3 you can then do:
      - unvented attic with fiberglass batts on the inside of the roof
      - no exterior rigid foam
      - must include diffusion port (gypsum board version, per WUI)
      - must condition the space or provide air flow

      1. Expert Member
        Akos | | #6

        Yup. Unvented roof is much easier in your warmer climates.

        Condition does actually mean condition, the temperature and humidity there should be near house. Since you are heating and cooling a larger surface area, it does require a bit more energy but much smaller energy penalty than equipment in vented attic.

        Since the attic should be the same temperature as the house, insulation on the floor doesn't do anything. If anything it might get you into trouble if it turns the attic into this in-between space that is neither inside the house or outside it.

        Since you do have conditioned space up there now, I would look to use it as storage. Would be good to also put something over the batts to contain the fibers at least.

        The roof deck is now your main air barrier and should be detailed as such (ie taped seams or peel and stick over it) and it also needs to connect to your wall air barrier. There are many ways of doing this but make sure to figure it out as air leaks at the rafters can a big energy loss.

        1. rfb | | #8

          Do you have a recommendation on where to source CAD details that would describe this that I can provide our framer and put on the plans? I've tried searching the cad details on this site and but no luck finding cad details on batts + vapor diffusion ports or on how to do wall connections. I am fine paying for them, I just dont know what sites to use to find them.

          1. Expert Member
            Akos | | #15

            This shows it with exterior rigid on the walls, would be similar idea with just sheathing.

            The rafters will poke through this air barrier, you can try to seal around them with canned foam or with a layer of spray foam on the interior.


        2. rfb | | #23

          I have been thinking more about this- if one were to do rigid foam board on the exterior for the air impermeable layer + batts inside, but the exterior walls are normal house wrap + batts. To connect the wall air barrier to the roof air barrier, ie the rigid foam board to the house warp you would just use sealant I assume?

  3. rfb | | #5

    Follow up question- there is no major benefit to "double insulating" ie batts on roof and ceiling for unvented attics like one would see on vented attics is there?

    1. Expert Member
      MALCOLM TAYLOR | | #7


      You don't want to insulate on both the attic floor and slope in either a vented or un-vented roof. Those half-in half-out spaces peform unpredictably and can have moisture problems.

  4. jollygreenshortguy | | #9

    One caution regarding vapor diffusion ports - So often the "solution" to one problem creates an entirely new problem, in this case structural integrity.
    The structural roof sheathing is just that, structural. It plays an important and integral part to the lateral load (earthquake) resistance system of your home. Many of the diffusion port details I've seen (including GBA's) simply hold the sheathing back from the ridge a few inches, continuously along its entire length. This is very likely to wipe out the diaphragm strength of your roof sheathing, wiping out its ability to play its role in lateral load resistance.

    This may work in places with little to no seismic activity but should not be used where lateral loads are significant. Unfortunately GBA does not note this on the details.

    I was trained and worked in California for many years. I have a Masters Degree in structural engineering and did my thesis work on certain seismic issues. I would not use these details in any project in zones C or D without the involvement of a professional engineer.

    1. freyr_design | | #11

      Look at fig 6 of bsc guide

    2. Expert Member
      Akos | | #16

      I'm not in a seismic area, so not very familiar with those types of requirements.

      Our code allows for dens glass to be used for bracing walls, could it also not work to complete the roof diaphragm (assuming blocking under the seams and proper nail schedule)? Or maybe not have a continuous ridge vent and leave some panels fully blocked at the ridge to transfer shear?

      Of course the proper answer is talk to an engineer.

    3. rfb | | #24

      wouldnt it be the same structural hit as ridge vent ports?

  5. DamionL | | #10

    Are you open to a tile roof? Plywood underlayment, which is more vapor permeable than osb, a vapor permeable underlayment like tar paper, and a tile roof goes a long way in preventing condensation in zone 3. In fact in some California jurisdictions, code allows this setup without any vapor diffusion port or exterior insulation.

    Check with your structural engineer about drilling holes in each rafter bay at the ridge to achieve the 1/150 diffusion port area requirement, install a vapor diffusion port with the addition of what I described above, air seal the attic floor and the eaves to prevent conditioned and exterior air from entering the attic and you should be okay to install batts on the underside of the sheathing.

    You can also consider 1.5” exterior rockwool rigid insulation which will provide you with condensation control without the risk of using foam in a fire prone area.

    1. rfb | | #25

      i am thinking we want to keep the attic conditioned since it has all the hvac in it, though i could be convinced otherwise since even though we have cold wet winter our summers are hot and dry.

      unfortunately a tile roof is far outside the scope of our budget. our framer hasnt had experience with vapor diffusion ports so for our roof line I am leaning towards rigid foam exterior + batts interior since cathedral ceilings are pretty common here so they have experience with that. The question then becomes how to seal the transition from the rigid foam exterior on the roof to the house wrap on the walls (no rigid foam). i am thinking blocking + a bead of sealant? i dont see any details showing precisely this make up so I am winging it a bit with our change order submittal packet on the details.

      1. DamionL | | #26

        Blocking, sealant, and/or spray foam should take care of the eaves air sealing details. Consider rockwool rigid insulation instead of foam if you’re in a fire prone area, although I’m not sure how much it matters since it will be sandwiched between plywood.

  6. walta100 | | #12

    First let’s look at your questions title.
    It pushes my buttons. The “unvented attic” is not a code approved thing. The approve options are vented or conditioned. When I read unvented or encapsulated attic, I see it as a big red flag for I am unwilling to pay for and operate the equipment necessary to full condition the attic! Some people do build the unvented attic and leave the old insulation on the attic floor and put more insulation at the roof line. The dew point of the air in those attics tends to be about the same as the temp so stuff gets wet and mold and rot often happens. It sounds like you live in a desert so mold and rot are less likely than for most. I am not recommending you playing that game.

    To my way of thinking HVAC in the attic is a very poor choice and the smart move is to undo that decision but that lead balloon is not going to fly.

    The way I see it a conditioned attic full of HVAC equipment is only marginally better than the current situation at best.

    Engineers like math lets do some. What is the surface area of the conditioned attic vs the attic floor. Note be sure the count the area of any gable end walls. If you have a few decorative dormer windows and a steep roof 250% seems likely.

    Moving the insulation to the roof line all but forces you to use closed cell spray foam the most expensive, ungreen and risky insulation sold today. Let’s guess your CCSF cost $2.90 a board foot or $0.415 for 1 sqf of R1. Cellulose come in at $0.03 for 1 sqf of R1. So, you need to cover 2.5 times more space with stuff that costs 13 times as much. That is crazy and you still lose 2.5 times as much energy. In fact, the numbers say CCSF has no return on investment over about R25 because it costs so much.

    As bad as the conditioned attic is the current situation of leaky HVAC equipment in the hot vented attic is likely worse. In that 20% of you precious conditioned air is likely leaking into the vented attic and lost. 100% of that air gets replaced with unconditioned out door air pulled into the home somewhere at a tremendous cost.

    If the HVAC must remain in the attic consider new tight ductwork buried under a thick layers of cheap 100% post consumer recycled cellulose insulation on the attic floor.


    1. Expert Member
      MALCOLM TAYLOR | | #13


      "Moving the insulation to the roof line all but forces you to used closed cell spray foam the most expensive, ungreen and risky insulation sold today."

      But somehow I've managed a dozen plus cathedral roof buildings without it. I understand you don't favour them, but you always omit viable alternative methods to make your point. Spray foam becomes the only option when the type of insulation gets chosen when the design is already complete.

      1. walta100 | | #20

        I agree, I am a poor advocate for your position on this topic.

        I do wish this forum had about 10 “sticky” topics like every other forum so we do not have too repeatedly answer almost identical questions daily.


        1. Expert Member
          MALCOLM TAYLOR | | #22


          I'm not asking for you to advocate using un-vented attics or cathedral ceilings, I just wish you wouldn't say they invariably need spray foam, which isn't true.

    2. freyr_design | | #14

      The “unvented attic” is not a code approved thing.

      This is not true, diffusion vent with insulation at ceiling height is code approved.

      1. walta100 | | #17

        “diffusion vent with insulation at ceiling height is code approved.”

        Sound like a conditioned attic to me.


        1. freyr_design | | #18

          It’s not, see figure 2 in bsc guide. It is unconditioned and unvented. This assembly is not thermally connected to the house. There is verbiage in code that supports this installation.

          1. walta100 | | #19

            If you want to post the photo you are describing we can talk about it.
            If all the insulation is at the roofline that puts the attic inside the conditioned space.

          2. freyr_design | | #21

            As I said it’s not at the roofline it is at the ceiling. If you want to click on the link at the top of this thread that akos linked you can educate yourself.

  7. walta100 | | #27

    The original poster rejected soffit vent.

    One can’t really say Fig 2 is unvented while it requires soffit vents.

    1. freyr_design | | #28

      I’m not sure where you are seeing a soffit vent… anyways here is the code verbiage in case it helps clear things up. You do not need soffit vents.

      In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:
      An approved vapor diffusion port shall be installed not more than 12 inches (305 mm) from the highest point of the roof, measured vertically from the highest point of the roof to the lower edge of the port.
      The port area shall be greater than or equal to 1:600 of the ceiling area. Where there are multiple ports in the attic, the sum of the port areas shall be greater than or equal to the area requirement.
      The vapor-permeable membrane in the vapor diffusion port shall have a vapor permeance rating of greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96.
      The vapor diffusion port shall serve as an air barrier between the attic and the exterior of the building.
      The vapor diffusion port shall protect the attic against the entrance of rain and snow.
      Framing members and blocking shall not block the free flow of water vapor to the port. Not less than a 2-inch (51 mm) space shall be provided between any blocking and the roof sheathing. Air-permeable insulation shall be permitted within that space.
      The roof slope shall be greater than or equal to 3:12 (vertical/horizontal).
      Where only air-permeable insulation is used, it shall be installed directly below the structural roof sheathing, on top of the attic floor, or on top of the ceiling.
      Air-impermeable insulation, where used in conjunction with air-permeable insulation, shall be directly above or below the structural roof sheathing and is not required to meet the R-value in Table R806.5. Where directly below the structural roof sheathing, there shall be no space between the air-impermeable insulation and air-permeable insulation.
      Where air-permeable insulation is used and is installed directly below the roof structural sheathing, air shall be supplied at a flow rate greater than or equal to 50 CFM (23.6 L/s) per 1,000 square feet (93 m2) of ceiling. The air shall be supplied from ductwork providing supply air to the occupiable space when the conditioning system is operating. Alternatively, the air shall be supplied by a supply fan when the conditioning system is operating.
      Where both air-impermeable and air-permeable insulation are used, and the R-value in Table 806.5 is met, air supply to the attic is not required.
      Where only air-permeable insulation is used and is installed on top of the attic floor, or on top of the ceiling, air supply to the attic is not required.“

      1. Expert Member
        MALCOLM TAYLOR | | #29


        It's also supported by the text directly below the image Walta posted:

        " Unvented attics can be insulated on the bottom of a roof deck (conditioned attic) and attics can be insulated on the top of ceilings or attic floors (unconditioned attic) using fiberglass and mineral wool insulation. The approaches constructing vapor diffusion ports are different for each approach.

        Installing a “vapor diffusion port” at the upper part of attic spaces and sloping rafter roof assemblies allows moisture (in the vapor phase) to exit the attic and roof assembly. In practice, this involves eliminating the lower soffit attic/roof vents, while installing standard roof vents near the ridge, but covering the vent opening in the roof deck with an airtight but vapor open layer."

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