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An energy efficient insulation system for my area?

U8KmyrNTKL | Posted in Energy Efficiency and Durability on

I am building a new home in Sacramento (No. Cal) and would like it to be as efficient as possible. I’ve noticed that the design for an insultation system (air/vapor barrier, R value, etc.) depends on your climate. While that seems like an obvious concept I believe that is what is causing some of the confusion for a lay person like myself, particularly in regards to the vapor barrier discussion. Is there a resource available that can recommend/describe a standard shell “system” ( dry to interior or exterior?, would a hybrid system work in my climate, and if so open or closed cell?, etc.) for my area? Any and all guidance would be much appreciated. So far, I know we are using advanced framing techniques, (we are not using SIPS), and there will be foam board on the exterior but beyond that no decisions have been made. What options do I have to fill the cavities? Should I dry to interior or exterior? (I would like to shoot for R-30 walls and an R-50 roof.) Thank you.

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  1. Riversong | | #1

    Last try:

    Codes no longer require an interior vapor barrier, merely a class II vapor retarder (0.1 to 1 perm) or with exterior foam only a class III (1 to 10 perm).

    You've already eliminated drying to the exterior with your choice to use exterior foam, so your wall system has to be able to dry to the interior.

    You can use any insulation in the cavities, including batt or blown fiberglass, blown cellulose, batt cotton (recycled blue jeans) or mineral wool.

    If the exterior foam is taped to serve as the air barrier, then there's no advantage to using a hybrid system.

  2. user-626934 | | #2

    Why would like to shoot for R-30 walls and R-50 roof?

  3. user-869687 | | #3

    I am working on a house design that meets these R-value targets and has vapor-open assemblies. The basic structure uses 2x6 wall framing and 2x10 roof joists, insulated with cellulose in the cavities. Rather than adding plastic foam insulation on the exterior, there is overframing (2x4s for walls and 2x8s for the roof) with rockwool insulation in the outer cavities. Walls get 3" rockwool and the roof gets 5", leaving space for venting behind the cladding and the upper roof sheathing. The air barrier is the plywood sheathing over the inner framing. Adding the 2x8 overframing at the roof keeps that air barrier continuous, with the wall sheathing extending to the lower roof sheathing. The 2x8s frame the roof overhangs.

    What I like about rockwool is that unlike plastic foam it is (1) the opposite of a fire hazard, (2) an acoustic absorber, (3) vapor permeable and (4) resistant to termites and other pests.

    I'm not sure why this strategy is not more popular, given all the talk about moisture management. The discussion of exterior plastic foam ends in a standoff between those who say it doesn't matter that the sheathing can't dry to the exterior so long as it's protected from the cold, vs. those who say this is asking for trouble because incidental leaks through wall system are inevitable. Count me in the second camp.

  4. U8KmyrNTKL | | #4

    Thank you all for your advice. As an owner-builder, I know I need to bring someone on board who knows building science. The problem is, there aren't many folks out there who are willing to look beyond traditional practices and/or the importance of an integrated approach. I need to know enough to ask the right questions, and I think this forum is the best place for me to get there. Thanks again.

  5. U8KmyrNTKL | | #5


    Am I shooting too high? Are there more appropriate goals for my climate/region? In building an efficient house, I'm not sure how high to go...Again, as a lay person I think, more is better! Also, our local utility provider (SMUD) built a "home of the future" and the R-50, R-30 specs was what they recommended...

  6. U8KmyrNTKL | | #6

    Last question (for now!) - Given my climate is hot/dry - should I be drying to the interior, exterior or does it matter? (Let's ignore the exterior foam for now, as my husband is very concerned about the potential for mold with our new "tight" house.)

  7. user-869687 | | #7

    To achieve R-30 in a wall without plastic foam, you would either need to use rockwool as I described above use a double frame for a deeper stud cavity. You could use 2x10 studs but it wouldn't be particularly sensible. A double frame, e.g. two rows of 2x4 studs, is what many on this website recommend for high R-value walls. The deep stud cavity can be insulated entirely with cellulose for a cost effective, non-toxic and high performance assembly.

  8. GBA Editor
    Martin Holladay | | #8

    You are too obsessed about the drying direction of your wall. In your climate, there's no reason to worry about vapor barriers. Relax.

    To be sure that any wind-driven rain that saturates your wall can dry quickly, you should include a ventilated rainscreen drainage gap between your siding and your sheathing. That helps your siding and sheathing dry to the exterior.

    In your climate, you don't really have to worry about wintertime condensation in your walls. It's not that cold. As I said, relax. You don't need to install interior polyethylene. Almost any wall in your climate should be able to dry to the interior if necessary, since no one in your area uses interior polyethylene.

    One final piece of advice: don't install vinyl wallpaper.

  9. Riversong | | #9


    If you're concerned about mold, then you need to plan both point source (kitchen & bath) and whole-house ventilation. Given your mild climate, a simple exhaust-only system, using the bathroom fans on a programmable timer, would be the most cost-effective. Passive make-up air inlets (like the Aldes Airlet 100) would balance the system.

    NOAA weather data shows only about 2500 HDD and around 700 CDD for Sacramento. That's a very mild climate and you may be aiming too high on your insulation levels.

    The simplest and most cost-effective wall system, which would breathe in both directions, would be a 2x6 frame (advanced framing is good) with 2x2 or 2x3 cross-hatching perpendicular to and inside of the studs, filled with dense-pack cellulose, no vapor barriers and some kind of breatheable exterior cladding (wood is good).

    You could use a rainscreen exterior, but given that most of your precipitation is in the winter when the drying direction is to the outside and summer months are arid, it may be unnecessary as long as siding is protected on all sides by a latex finish.

    Given the high percentage of sunshine, it would make more sense to incorporate passive solar design with good summertime shading (roof overhangs, e.g.) then to go to extremes in the thermal envelope. Also, wide roof overhangs and gutters does more to protect the exterior cladding than a rainscreen.

  10. J99aAMQzYo | | #10

    "The problem is, there aren't many folks out there who are willing to look beyond traditional practices and/or the importance of an integrated approach."
    Laura, I'm may be lobbing a grenade here that will set some of these guys off, but ... Since you are specifically having problems finding folks in your area, you may want to consider looking at the Passice House US site for folks in No. Cal. You can find the map and contact info here:

    And before everyone flips out ... I'm not recommending PH for this project. It was never conceived or designed for your climate. However, in my experience, all the PH folks I've met so far are also up to speed on building science in general and can help with non-PH design strategies. They are also typically much better versed than the average contractor / designer on your concerns such as energy efficiency, air exchange, mold, etc. It may not be a perfect solution, but it might be a good place for you to start looking.

  11. U8KmyrNTKL | | #11

    I very much appreciate your guidance. And based on what I've read, I think I will relax a bit. Thank you!

  12. user-626934 | | #12


    I don't know whether R-30 walls/R-50 attic is too much for your house. I haven't seen your drawings and I haven't calculated the heat loss/gain. My gut tells me it might be more than is optimal.

    I'd suggest hiring an energy consultant to help you optimize your envelope and mechanical systems. Ideally, this person/firm would also do your blower door and duct leakage testing, ensure Title 24 compliance, and help you navigate through the local, state and federal incentive structures for energy efficiency and renewable energy....oh, and they could bake you an apple pie, too!

  13. U8KmyrNTKL | | #13

    Hi Robert, still doing my research. how does your wall system differ from the Mooney wall? your cross hatching is inside the studs, any other differences?

  14. Riversong | | #14


    The cross-hatched wall system I suggested is neither "my wall" nor Mooney's wall. He simply took an old and time-tested method for increasing the thermal envelope thickness and decreasing thermal bridging, used for renovation, and added insulweb and dense-pack as a way to do new construction in a more energy-efficient manner.

    My wall system is an authentic innovation, known as the modified Larsen Truss (or Riversong Truss), created over 30 years of building super-insulated homes, that offers very high levels of insulation with negligible thermal bridging and a high level of resource-efficiency (where code allows). This is not the wall system I recommended for you, but is described and depicted at:

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