[Greenbuilding] Slab heat loss calculation

[Alan Abrams]

to confirm or confound Norbert's calculations, I ran some hypothetical
changes through PHPP.

the Current Project calls for 9" of subslab geofoam rated at R-4.2 per
inch, and clocks in with a specific space heat demand of 4.75 Btus/SF/year.
 The house contains 1735 SF of *treated* floor area (which is the actual
interior floor area, not including partitions and other unusable areas).

changing to 2" of foam increases the SSHD to 5.90 Btus/SF/year, requiring
about 20 additional therms of natural gas to make up the difference.  Local
cost for the first 45 therms per month runs $0.3853/therm.

doubling down to 4" of foam drops the SSHD to 5.28 Btus/SF/year, saving
about 10 therms.

in summary, PHPP tends to validate Norbert's vivid example.

the bigger question is of course, does this exercise in itself invalidate
the PH requirement to hit the magic 4.75 target?  It's worth noting, and
perhaps no surprise that the cooling load is higher with the thicker
subslab insulation.

This all goes back to my original point, which seems to have gotten lost
under some slab, that rules of thumb-- such as, that two, or four, or
however many inches of subslab foam is "enough"--only gets us so far, and
that if we are to achieve a performance standard, or a means of achieving
net zero, or carbon neutral, or whatever measure--some rational system of
putting this all in a greater context is necessary.

AA



AA



On Sat, Aug 24, 2013 at 7:42 AM, Norbert Senf <mheat at mha-net.org> wrote:

> Or, you could invest that $1350 in a US 30 yr bond @3.8% instead of
> styrofoam and get $51.00 per year instead of $4.00.
>
> -------------previous message-----------------------**--
>
>
> A 5 lb piece of wood burned at 70% efficiency gives about 32,000 BTU. That
> would cover the slab heat loss for 7 days. For a 5 month heating season, it
> would need 100 lbs of wood. Doubling the foam thickness to R20 would save
> about 50 lbs of wood per year. At $240 per cord, that's a saving of about
> $4.00 per year.
>
> At $1.50/sq ft for the extra foam (not counting labour), 900 sq ft would
> cost $1350, for a simple payback of 337 years..........N
>
>
> At 05:44 AM 8/24/2013 -0400, you wrote:
>
>> RT <archilogic at yahoo.ca> writes:
>>
>>  ... In putting our pants back on frontwards, someone noticed that we had
>>> all received tattoos onto our backsides:
>>>
>>> ? ? ? ? ? "? q? =? A [ dT ( K/s + hc ) +? ( ? * T^4 ) ]? "
>>>
>> (snip)
>>
>
> ------------------------------**-------------
> Norbert Senf---------- mheat(at)heatkit.com
> Masonry Stove Builders
> 25 Brouse Rd.
> RR 5, Shawville------- www.heatkit.com
> Québec J0X 2Y0-------- fax:-----819.647.6082
> ---------------------- voice:---819.647.5092
>
>
>
>
>
>
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-- 
Alan Abrams*
**certified professional building designer, AIBD
certified passive house consultant, PHIUS*
*certified passive house builder, PHIUS**
*Abrams Design Build LLC
*sustainable design for intentional living*
cell     202-437-8583
alan at abramsdesignbuild.com
www.abramsdesignbuild.com
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