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Tell Me If My Building Strategy For NE Texas Is Sound

rw7810 | Posted in Energy Efficiency and Durability on

I’ve been studying the different building and advanced framing articles and opinions here and where ever I can for some time now. We want to build an old style raised foundation (pier & beam) 1.5 story farmhouse style home in NE Texas, near Tyler. This is considered climate zone 3A Warm & Humid. Here is my plan which is keyed on being both cost efficient and building a relatively air tight and efficient home.

1. Raised foundation approximately 42 inches, vented crawlspace. 6mil vapor barrier ground.
2. 3 inches of open cell foam, R11.7, under 1st floor joists for insulation
3. 2×6 on 24″ centers exterior walls with R20 cellulose, 1/2 sheathing, house wrap (taped), 1/2 foam (taped) as a thermal barrier, furring strips covered in LP Smart Lap Siding.
4. Roof deck would get 5.5″ of R21.5 Open Cell Foam.
5. I’d love to use a standing metal roof, but due to budget will have to go with composite.

The house is designed to be oriented North and South with the attached garage facing to the West. There are eight and ten foot wide (deep) porches running along the front and back of the house.

Am I making any mistakes here ?? Any recommendations ?

Bob W.

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  1. homedesign | | #1

    HI Robert, I am near Dallas
    I am prejudiced ... I do not like crawlspaces
    Dormers are not-so-energy efficient or durable
    R-21.5 between the rafters is not enough R-value... and does not address thermal bridging
    you have not mentioned an air Barrier for the wall
    Attached Garages are Not-so-beautiful and not-so-green
    That is a lot of covered porch area and will be not-so-cheap

    does the front face North...or do you plan to "flip" the floor plan?

  2. homedesign | | #2

    Here is my Not-So-Big "Texas Farmhouse"
    If I had a do-over
    I would build a One Story with Dbl Stud, Airtight Drywall & Airtight Sheathing Walls (Cellulose)
    An Airtight Drywall Ceiling
    And Cellulose Galore on the Attic Floor

  3. Expert Member
    ARMANDO COBO | | #3

    Bob… just like John, I also work near Dallas.
    1. Ventilated crawl spaces are a huge mistake, and sealed crawl spaces are a lot of hard work to do them right. It’s building science ignorance to believe that it is the best foundation available. Look for post-tension slab or SlabTek suspended foundation depending on your soil test.
    2. Conditioned attics with O.C. foam ONLY are against code (read 2009 IRC 806.4), you would need 2” rigid foam on top of the roof decking for thermal bridging and condensation control, or 1" rigid foam on top of the roof deck and 7.5" OCfoam under the roof decking. Bottom line, you need R30 minimum, and I would go R38 minimum, especially to the west side.
    3. Your wall assembly is good; however, I would increase the thickness of the rigid foam on the west side, if not the whole house, to 1”.
    4. Is the long axis for the house running N-S direction? Then am I assuming the site development did not allow design of the house for easier and less expensive passive solar design? I would suggest buying the best west windows with the lowest U-value & SHGC available.
    5. If you can find a way to install metal roof over notched battens, do it. It’ll save on your bills and probably the increase in mortgage will be offset by lower bills; do a life-cost analysis to know the answer.

  4. homedesign | | #4

    Armando, I like your idea about increasing R-Value on the West
    I am also hearing a lot of good "buzz" about SlabTek

  5. GBA Editor
    Martin Holladay | | #5

    I think the weakest link is your foundation plan. I'm not a fan of ventilated crawl spaces, and it makes no sense to choose a vapor-permeable insulation over a damp crawl space in a climate where the home is likely to be air conditioned. That combination can lead to damp subfloors.

  6. Expert Member
    ARMANDO COBO | | #6

    I'm recommending SlabTek on all my designed jobs when soil test are expansive or have a high PVR. It's a bit higher than a post-tension, less than P&B, but I don't have to deal with crawl spaces. Check them out.

  7. rw7810 | | #7

    Thanks for all the comments.

    John & Armando - The soil test said PVR on my property is 0, therefore, not much chance of movement and we like the idea of a raised foundation vs slab, aesthetics, feel, etc.

    John - 1. You say R21 is not enough, but AJ Builder in an adjacent post says, " I know R 20 works and here Brooks, actual use in zone 6. And what is your actual use in zone 4-6 are you referencimg, any?" 2. Air barrier was listed as house wrap, probably Tyvex or an equivalent
    Armando - The house will be oriented N-S with the front facing north, 8 foot porch. The back has a 10 foot porch. Garage faces West. and should help shield the house.
    Martin - If I am committed to such a design what would you recommend to alleviate the problem, a vapor retardant paint over the foam ??

  8. wjrobinson | | #8

    Robert, I didn't post here because I have no Texas experience and was hoping John and Armando would post which they have.

    As to my R factor, it is with some continuous insulation and our climates are not the same so stick with the other two fine gents.

  9. homedesign | | #9

    Hi Robert,
    Martin is on Vacation
    I noticed that he did not mention the R-value of your Roof.
    I think this blog may express how he feels about "Less than Code Minimum" R-value

  10. wjrobinson | | #10

    Ok Robert, I'm here my two cents worth;

    Closed cell might be better for under your home. (Martin mentioned this)

    Above per me, John, if you are using trusses, your Sprayfoam can be continuous like I have done by encapsulating the truss members.

    Walls, for a minor added cost I would use thicker rigid foam on your walls.

    The key to all this is a builder doing what he already does well and not "teaching" him as the owner.

    And most important, fly all this by your permit people, design team, contractor, subs, tradespeople!

    Oh, and ASHRAE 62.2, and a modern HVAC plan done by experienced pros.

    Edit: As John and Martin recommend, R 38 beats R 20. Just that in spray foam, High R does get expensive. Cellulose in attics would cost less though air sealing and other details will then cost extra. So, always trade offs. It is very important to build a home that was successfully built before! Experimenting for the first time with too many first time details can get one in deep trouble. See Don's thread!!!!!

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