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Using for DIY Manual J calculation

keithhoffman22 | Posted in Mechanicals on

DIY here:

I recently discovered and am grooving on being able to more closely (I don’t think I’m a sizing fetishist … yet) model my structure for sizing of a planned HVAC replacement and possible future envelope modifications.  I swear I post about this annually (we keep pushing it off) but my knowledge base increases each time…

I really love having a clearer picture that I got from building the model myself to understanding where my load comes from.  I’d recommend any savvy (load calc fetishist) DIYer consider making their own model to understand their structure.

But there are a number of things I’m having trouble modeling that I think matter (not all differences from reality matter in a mode) that I could use some help with.  I have an older wrightsoft pro manual J that also punts on these items so getting more pro help might not make much difference. 

Here are my questions:
Assumptions by me:
exterior air film 0.17
Interior air film 0.68
Air gap behind foam board (varies but 0.75″ is common) 1.0
1″ R-5 XPS 4.0 (basically I’m assuming the xps is derated but the air gap makes up for it)
1/2″ fiberboard (stamped 1.3) 1.3
2×6 framing 5.25
2×4 framing 3.5
2×4 framing on flat (basement) 1.5
8″ concrete foundation walls 1.11
interior drywall 0.45
1/2″ plywood 0.62
Framing factor 25% (house has advanced corners and 24″ o.c. but also big framing around windows)

Stock 2×6 wall u factor:
In cool calc, my 2×6 R21 (actual R-23) walls with R-6 board insulation yields 0.049.  But if I spreadsheet it (admittedly I may be making a mistake), I get 0.041.  If I model my walls with R-23, I get 0.0.38.  I have to increase the framing factor to 50% to get 0.049.

So questions:
– Are my assumptions bad? My math?
– Anyone know how to trick coolcalc into using lower Rs than the dropdown menus or do I need to fudge the report values (or build my own spreadsheet)?

Double walls:
A few of my walls have interior double walls.  Exterior wall 24″ o.c., interior 16″.  My attempt to model this R-6.3+R23+R15 wall yields a U factor of 0.026.  In cool calc (and wrightsoft pro model), I can’t model this wall so it gets 0.049.

– I’m not sure what kind of framing factors to use for this type of wall. I resorted (in the spreadsheet) 25% FF in each wall.  Layout of 24″ vs 16″ likely results in 1 in 3 studs overlapping with full overlap at windows.  Is there a stock U factor that is used for double walls?  Is there a way to enter this type of wall effectively in a manual J?

Roof trusses: do these vary meaningful from roof joists in thermal bridging, assuming the between truss spaces are insulated?

I’ll stop there on the construction features as several of my questions are variant of how do I fake higher U factors in the software or a spreadsheet.

Internal loads:
Coolcalc is kicking out a 7600 BTU/hr internal load.  That seems high.  What’s reasonable here?

Overall load gut check:
Climate zone 5
HDD ~5500, CDD ~700 (not sure I believe that)
~3000 sf house
~36k btus/hr heating
~27k btus/hr cooling
~12 btus/hr/sf heating
~9 btus/hr/sf cooling

It’s a tight well insulated house with quite a bit of glazing but I’m hoping for a gut check on whether those are reasonable numbers.

I’m not just being a fetishist about this.  Aggressively accurate calculations are key to some cooling system sizing options (1 unit vs 2 units etc).

Any advice or assistance is appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    In a zone 5 climate the XPS will never drop below R4.2/inch (even when fully depleted of HFCs) at 75F average foam temp, and at the heating design temp would be performing at R4.5-R4.7 (when fully depleted) in that stackup.

    Advanced framing w/ 24" o.c. stud spacing usually comes in under 20% framing fraction, and can be as low as 15% if optimized. Typical 16" o.c. classic studwall construction with doubled up top plates & window headers etc is about 25%.

    The 7600 BTU/hr internal heat sources would be applied to the cooling load, but not deducted the heating load. Typical overnight 24/7 plug loads and mammalian body heat would usually add up to half that or less for a family of 4. Four sleeping humans are worth about 900-1000 BTU/hr, but awake and active it could be ~1500-2000 BTU/hr or more. Four teenagers having a video game party with multiple game computers and some lights running can add up to well above 7600BTU/hr of cooling load.

  2. qofmiwok | | #2

    I'm having the same problem. The maximum allowable external insulation is R5 and I have R8.4 (2 inches Rockwool.) In fact I saw the professional who did my Manual J with Wrightsoft did the same thing, so i'm thinking it's a limitation in the Manual J model. How that this be, in 2021? I found additional things that could be modeled as well. Such as 3-4' Heated crawlspace with R20 sub-slab insulation. Are there any programs that can model these higher efficiency features? Can wall R values be overwritten?

    BTW, is your house built yet? How does it compare in real life?

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