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Insulation advice please

Dog_Smith | Posted in Green Building Techniques on

I’m somewhat confused on the whole insulation issue. I’m working with an engineer on a 2,500 sq. ft. home in Truckee, Ca (Sierra Nevada, 6,000′ elevation, 210 lbs psf snow load: Wall construction will be 2×6 on 16″ centers. This is a cold climate:Zone 5, I think. Mostly dry in the summer, very little rain, precipitation mostly snow.

We are still in the planning stage. I want the house (my primary residence) to be well insulated: Walls R-24/R-25, Attic R-49, Sub-floor R-38 are my targets.

The more I read the more confused I get. Trying to get to the most cost-effective, proven solution. We have gotten prices on a couple of alternatives, one for $3,000 for R-21 in the wall cavity, the other for $6,000 for R-23 cellulose (JM Spider ?), again in the wall cavity. Is this the best use of $3,000 (going from R-21 batts to R-23 cellulose)? If not, what is the most cost-effective solution?

I also want the house to be air-tight, is spray-in-foam the only way to ensure the house is air-tight? Or should I look at air-tight drywall?

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Replies

  1. davidmeiland | | #1

    You should look at rigid foam over the exterior sheathing, under the siding, AND probably cellulose in the stud bays. Fiberglass batts should be out of the question. You should research the various air-sealing strategies and put them on the plans for the builder to follow. I would go beyond R-49 in the attic and I would not use recessed lights.

    What's the heating system?

  2. kimcrow | | #2
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  4. Dog_Smith | | #4

    Thank you all...the heating will be hydronic radiant floor heating
    I will look at all the links provided.

  5. davidmeiland | | #5

    Are you the builder, or are you hiring a builder?

  6. Dog_Smith | | #6

    I will be hiring a builder. I hope to find someone who has experience building an air-tight, well-insulated home.

  7. davidmeiland | | #7

    Earl, I believe that's the right thing to do. Hiring someone who has no experience with the energy details can get you a beautiful house, but it may be very costly to heat and lack in comfort.

    Seems like there are two options, and I'm not sure which is better. You can make yourself fully familiar with the necessary details and put them on the plans. If your research is good and your details are correct, then it should work. With this method you risk offending someone who already has their own details that they've worked out and gotten comfortable with. Or, you can ask the builder to treat the energy details as design/build, and ask him to create drawings and specs to be added to the house plans. If he has experience and knows how to get it right, this will also work.

    Whoever said "trust but verify" was onto something....

  8. Dog_Smith | | #8

    David, I appreciate your advise, I think I will try the 2nd option...

  9. DickRussell | | #9

    Earl, I would argue the first route, self-education to the point where you can specify the exterior wall design yourself and provide the details to the engineer who will be producing the required stamped drawings. Then find a builder who isn't necessarily experienced in building tight and very well insulated houses, but who is eager to let you be his "building science expert" on your project and build the house to your specifications, so he can add it to his resume. It would certainly help if you will be able to be onsite very often during the build, to follow progress and catch any missed things before it's too late.

    I would also advocate upgrading the whole-wall R expectations. Just an insulation-filled 2x6 cavity won't get you mid-20s for R value when you factor in the thermal bridging of the framing. Either external foam or a simple double wall design will be a surprisingly inexpensive addition to the design and get you upwards of R30 whole-wall.

    If you build a really good shell, then you might rethink that radiant floor heating, an expensive heat distribution option with questionable comfort benefits in a very well insulated house. The house you (should) want will have a very low rate of heat loss, and your radiant floor circulation water would have to be so close to room temperature, to avoid overheating, that you wouldn't get that "warm feet" feeling. Where and how you introduce heat to a very well insulated house becomes les and less important as the shell gets better.

  10. Dog_Smith | | #10

    Dick, When I first started researching this subject I got some recommended insulation levels of R-49/R-25/R-30 for the attic/walls/crawlspace. These values are higher than current code requirements, but I figured that increased insulation requirements are coming (January 2014?) and I didn't see the point of building a house with minimal insulation. Since I have not found a cost-effective way of getting R-25 inside the wall cavity, the engineer and I have agreed on R-21 inside the cavity and R-4 insulation sheathing minimum. I also want the builder to use "Air-tight Drywall Approach" to reduce infiltration as much as practicable.
    But you're point about the radiant floor heating and it's questionable value is something I had not thought about.

  11. davidmeiland | | #11

    I would still consider the radiant floor, even if your loads are lower. What are your utility choices and rates? Natural gas/propane? What is the electric rate?

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