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Manual J for an ICF house

BrandonA | Posted in Energy Efficiency and Durability on

After having paid a professional to do my manual J calcs I still wasn’t satisfied or convinced that they were correct as has been pointed out by a few of the pros here on this site.  I have been reading Martin’s “How to Perform a Heat-Loss Calculation” series and decided to try my own I=B=R spreadsheet.

I’m building a 4000 Sq. ft. home (2000 up and 2000 below grade) entirely ICF and triple pane casements (639 sq ft total), R50 attic insulation, and 3″ of EPS below the basement slab.  Nobody I can find in my area is familiar with heat loads for ICF. Dana had mentioned to use the 97th (18F) or even 95th percentile instead of the regular 99th (9F) to compensate for the air tightness of ICF.

My calculations led me to 24k btuh using the 97th percentile temperature. This does not include any internal gains or air infiltration.  What can I expect to add in terms of air leakage/btuh so I can get an accurate Manual J load calculation?  I can’t find any rule of thumb for super tight ICF construction.

Thanks in advance, anything helps!

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  1. Expert Member
    Dana Dorsett | | #1

    It's the thermal mass and time lag, not the air tightness that lowers the peak load. Some tools model this better than others. Using the 97th percentile is basically a fudge factor on what the actual peak load might be, but the climate, site factors and house design all affect that.

    There are no rules of thumb for air tightness, nor should there be. There are many ICF houses that failed even 7ACH/50 when tested. Building tight can be done with any wall type, and no wall type will air seal the attic or the slab, or the plumbing penetrations.

    Go ahead and assume its 100% air tight, but ventilated at ASHRAE 62.2 levels with an HRV at whatever return efficiency the model you're using has in the spec.

    1. BrandonA | | #4

      Thank you soo much, you answered my question exactly!

  2. Jon_R | | #2

    ICF walls will only reduce peak wall heat loss. So I'd only apply a fudge factor to the *wall* R/infiltration values - leaving all other losses (and percentiles and infiltration) alone.

    A WAG - add 16% to your actual wall R value. Make no other adjustments.

    1. Expert Member
      Dana Dorsett | | #3

      That's right. Manual-J only estimates peak loads, not average energy use.

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