[Greenbuilding] Slab heat loss calculation

[Clarke Olsen]

It sounds like putting 6" of course gravel under the insulation under the slab is the most cost effective move, both for reducing the heat transfer, and, if nicely leveled and tamped, reducing the thickness of the slab. There is a product for basement floors which is a plastic spacer of 1/2" bonded to 1/2" of OSB in 2' square pieces, giving a thermal break with the floor.
Clarke Olsen
clarkeolsendesign.com
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at taconic.net




On Aug 25, 2013, at 3:31 PM, John Salmen wrote:

> 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.
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> 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.  
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> in summary, PHPP tends to validate Norbert's vivid example.
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> 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.  
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> 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.
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> AA
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> AA
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> 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.
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> -------------previous message-------------------------
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> 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.
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> 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
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