[Greenbuilding] Slab heat loss calculation

[John Salmen]

In the same post you are discussing cost but also talking about net zero as
a goal.  I think that illustrates the dilemma – how do you establish a
‘rational’ system that covers 2 paradigms (jane Jacobs has an excellent book
‘systems of survival’ that explores this problem).  Jane argues you can’t –
and when you try you compromise both objectives by bending over backwards it
goes sideways (my experiences)

 

That said I think passivehaus is a good attempt at a standard and it raises
challenges that get us thinking. For example 9” of foam is no longer just a
thermal insulating material but is a structural material – at that point its
thickness is related less to insulation value than structural value.  4 and
6” lends itself easily to eliminating concrete slabs from buildings. Greater
thickness and/or density can easily allow us to eliminate concrete footings
and retaining walls and we have the technical data and infield applications
(geofoam)  to support this usage – eliminating concrete is a huge
achievement in reducing energy costs from a building. 

 

Can I do this for a client? I don’t know – my relationship with a client is
in the realm of commerce and the concept and designwork required for such
initiatives is in the realm of the environment – two different worlds, two
different areas of faith, understanding and codes of conduct.

 

 

 

 

 

 

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






_______________________________________________
Greenbuilding mailing list
to Send a Message to the list, use the email address
Greenbuilding at bioenergylists.org

to UNSUBSCRIBE or Change your List Settings use the web page
http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioener
gylists.org





 

-- 
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
 <mailto:alan at abramsdesignbuild.com> alan at abramsdesignbuild.com
 <http://www.abramsdesignbuild.com/> www.abramsdesignbuild.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20130825/9ba8675b/attachment.html>