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Need for semi-conditioned attic in a new construction home with a semi-conditioned crawl space

AirstreamJake | Posted in Energy Efficiency and Durability on

I am building a new home that will have a semi-conditioned crawl space.  We have poured a 2″ slab floor (over a vapor barrier) and the crawl space walls will be insulated with rigid foam (as will the perimeter rim joists/beams).  Our air conditioning handler and all ductwork will be in the unvented crawl space.  We will have one A/C/heating vent into the crawl space and excess pressure will be vented outside of the crawl space.

My question is this – since I will not have any ductwork in the attic and will not be using it for storage, should I:

(1).  Consider a non-vented attic with rigid foam outside of the roof sheathing and open-cell spray foam between rafters?  Or,

(2).  Go the more traditional (and less expensive) route of venting the attic, air-sealing the ceiling tightly, and putting all my insulation directly on the ceiling between joists?

I live in Climate Zone 3 (far northeast Texas).

Thanks in advance for any help / advice!

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  1. BrianPontolilo | | #1


    If you will not be using the attic for anything, I would air seal and insulate the attic floor with a mountain of cellulose. You need to be diligent with the air sealing and know all of the places to pay attention to, but it is a pretty simple and cost-effective approach to a energy-smart lid for your home.

    Here are some articles that will help with your decision:

    1. AirstreamJake | | #4

      Hi Brian,

      Your reply was very helpful.

      I had initially planned a non-vented attic with rigid foam on the roof and spray foam between rafters, but then started thinking (and reading) about why I would insulate an area that is unused. I think with adequate soffit vents and a ridge vent, I'll have more than adequate ventilation, and with adequate air-sealing of the ceiling and top-of-ceiling insulation, I'll be good. Your confirmation of that helps.

      Why cellulose as opposed to fiberglass? Just curious. Also, is it possible to use open-cell spray foam directly on the ceiling between (and over) the ceiling joists. Advantages and disadvantages? It seems like spray foam, or at least just a little that is then covered by cellulose, would in itself help with air sealing.

      Thanks for the advice.

      1. BrianPontolilo | | #7

        Fiberglass would work. I guess you could used open cell foam although I am not sure about installing it deeper than the joist bays. You could also consider a flash coat of the foam, for air sealing, then the cellulose or fiberglass. All are possibilities. I recommended cellulose because it is much more friendly to work with and around. Also, the foam would make remodeling projects much more challenging (you may want to run cable for a three way switch or an additional ceiling light up there some day.)

        1. AirstreamJake | | #8

          Okay, Thanks Brian. All of that makes perfect sense to me. I agree that cellulose is much more user friendly, I was just wondering if there were other reasons for its use. I agree with the potential difficulty of remodeling with ceiling joists filled with sprayed foam. I like the idea of the flash coat merely to provide a good air seal, and then cellulose on top of that. All most helpful.

  2. Jon_R | | #2

    What do you expect your crawlspace humidity to be when it's 75F and humid outside and the AC hasn't run for weeks?

    1. AirstreamJake | | #5

      We have very few periods here in Texas where we aren't running either heat or A/C, and I NEVER open windows or leave a door open. I'm a firm believer in climate-control. We have screened porches if I want fresh air. So, I guess I'm not too worried about excess humidity. I also plan on installing a backup dehumidifier.

  3. walta100 | | #3

    A vented attic is the best possible solution so long as you are smart enough to keep your HVAC system out of your attic and it sounds like you are.

    Cellulose is a very good choice R60 would be a fine goal.

    Make sure you get blocking installed near the soffits extending 12 inches above the insulation this keeps the wind from blowing thru and around the insulation. Avoid 16 inch wide bits of cardboard installed with a few staples as they tend to fall down.


  4. AirstreamJake | | #6

    Hi Walter, thanks for your reply and the confirmation of what Brian was saying. I'll definitely do the blocking right, as you suggest.

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