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Thread: HVAC System Recommendation?

  1. #16
    Retired prof. engr. gator37's Avatar
    Join Date
    Nov 2009


    Your architect should be familiar with the code minimum requirements for insulation thickness (or max U values) and should be able to give you some indication on the cost for construction using different composit materials for walls, ceiling, etc. (Not sure what codes SC uses.)
    The only way I know that you can get an approximation of effective combinations for materials (energy savings) is going to be an iteration method using an HVAC energy analysis program that will provide you with an approximation of energy consumption based on materials of construction and HVAC types and efficiencies. Trane has such a program if you know somebody that will let you use it. At minimum it will give you some insight but is usually done on large commercial projects or government housing (people that can afford it, HAHA). I am sure there are many other programs but I am not familiar with them.
    There is another web page at Oak Ridge National Lab in Tn dealing with energy saving vs. construction that may make it easier if you have not already found it.

  2. #17
    DIY Junior Member Shooter45's Avatar
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    Mar 2011


    Quote Originally Posted by jimbo View Post
    Seems like a heck of a time to be making decisions like stud size, etc. 2x4 only gets you R-13 I think, in the walls. You will pay for that every year in heating costs.
    You can get R15 in 2x4 walls.

  3. #18
    In the trades Dana's Avatar
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    Jan 2009


    The difference in whole-wall R value (the average value after the framing shorts are factored in) between R15 cavity fill (any type) and R13 cavity fill (any type) of a 16" o.c. 2x4 wall is the difference between ~R10 and R11. The R3-4 of framing dominates the heat loss/gain in a typical 25% "framing factor". The framing only adds up to 25% of the surface area, but since it transfers 4x the heat per unit area the framing is transfering well over half the heat. The real performance difference of going with R15 batts is that the density of the batt makes it a better air-retarder than a low density batts, but that will only show up if the assembly hasn't been detailed to be air-tight (or nearly so.) Cellulose (at any density) is at least as air-retardent than R15 batts, and either wet-sprayed or dense-packed has far fewer gaps, and no compressions to allow thermal bypass convection to occur within the cavity. Sprayed/blown-in-bag high-density fiberglass such as Spider or Optima at 1.8lbs density has similar air-retardency to low-density cellulose and a slightly higher R-value, but is usually more expensive.

    The difference in whole-wall R-values between real-world installations of batts vs. blown R13 (rock wool, fiberglass, or cellulose) is the difference between ~R8 and ~R10. Blown/sprayed insulation is almost ALWAYS going to be nearly-perfect, where batts are only perfect in test-sample wall structures, never in the real world.

    Spray foam vs. blown/sprayed cellulose or fiberglass is only an issue if the sheathing (or other layer) isn't detailed as an air barrier. Foam won't allow convection within the cavities the way fiber insulation does (but even low-density cellulose allows only ~10% the convection of low or mid-density fiberglass batts), but isn't guaranteed to be air-tight on it's own without detailing other layers in the assembly. While foam makes air-tightness easier, it is by no means assured.

    Note, the amount of lumber in 2x4 16" o.c. construction is nearly the indentical amount of board-feet of 2x6" 24" o.c. With the 2x6 option you have a somewhat lower framing factor of R5-6 rather than R3-4, and with cheap ~R19 fiber fill you end up with a whole-wall R-value of about R14 as opposed to R10 for about the same amount of money. The insulation costs a bit more, the framing material cost is about the same, but the framing labor should drop slightly with fewer boards to cut.

    With either the 2x4 or 2x6 framing options, putting 1" of rigid XPS or iso on the exterior, and detailing both the structural sheathing and the rigid foam as air barriers (and lapping their seams) is well worthwhile from comfort, HVAC sizing, and longer term energy savings points of view. If 2x4 you end up at about R15-R16 whole-wall (as opposed to R10), in the 2x6 case you end up around R19-R20 (as opposed to R14.)

    The difference in comfort between an R10 whole-wall and R20 whole-wall value is something you can FEEL. Yes, the inch of foam is a cost adder, but it's a HUGE performance boost.

    For the CMU foundation and crawl, insulating the interior with 1.5-2" of closed cell spray foam (up over the foundation sill and band joist too) and putting a vapor retarder on the floor to protect against ground moisture/gases has huge advantages in places as humid as SC. CMU is inherently air-leaky (there are always cracks), and moisture-leaky. Deep subsoil temps are in the ~65-66F range, and with an air-conditioned space above, the summertime temperature in the crawlspace will spend MANY MANY hours below the dew point of the exterior air. If you ventilate with outdoor air, you're begging for a mold & rot problem. Closed cell foam on interior air-seals the CMU, foundation sill and rim joist, and at 1.5-2" will have an R value of R10-R12, effectively earth-coupling the house to fairly comfortable soil temps, making it unnecessary to insulate between the floor joists. (It's probably worth putting an inch of rigid XPS or EPS on the crawlspace floor if you're going to pour a slab- even a rat-slab down there.) With the joists and subfloor fully within the insulation boundary it's moisture content stays well-moderated by the HVAC, whereas if you instead insulated between the joists the thermal short of the joist from the air conditioned space into a ventilated crawl makes it the coldest spot in the crawlspace, concentrating the condensation on the wood.

    With the ducts in an insulated conditioned crawlspace the duct-insulation requirements fall to ~ R6 too. You'll still want the ducts to be mastic-sealed though. A very modest amount of HVAC air can be circulated in the crawl (required by code in some areas) to eliminate any air stagnation or gas buildup issues, but it doesn't need to be anywhere near the volume as in living space.

  4. #19
    DIY Junior Member tempritespokane's Avatar
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    May 2011


    Trane from our experience is a good choice. I would echo Tom. Make sure that you can get equipment and parts locally. That will be important.

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