[Greenbuilding] passive haus article

[Alan Abrams]

John, your points are well taken, but just to clarifiy: what was meant by
that is different heat flow rates to materials of different densities and
specific heat capacities.  the important question is then, does heat flow
at a greater rate to air at an average winter temperature of maybe 45d F,
or to the earth at maybe 55d F, per square foot.

and is it still air or windy?

it goes back to the original point, that there are some limits to rules of
thumb.


On Wed, Aug 21, 2013 at 8:50 AM, John F Straube <jfstraube at uwaterloo.ca>wrote:

> Heat loss through the slab is the same as heat loss through an airtight
> wall.  If someone told you that the physics are different they are not.
> The difference is, the ground temperature is much more stable, and never
> as extreme as air or roof surface temperatures.
>
> I recoil at the notion of large amounts of insulation (even 4" of foam is
> a lot) under the slab because it does not save much energy, unless you use
> some weird energy model that does not relate to measured sub- slab
> conditions. The soil simply does not get that cold below slabs on grade or
> below basement slabs and it IS the temperature difference between the soil
> and the indoor air that drives heat flow.
>
> I dont see much "complexity of adding more insulation to walls" as it
> involves the complexity of buying longer screws and extending the exterior
> window returns in wood frame buildings. As I have posted in the past,
> screws are not a problem or a major expense if you know how to do the
> shopping.  And if you dont want to use foam, you can use rock wool on the
> walls and roof. Once you are beyond about R20 (2" of insulated sheathing
> over 2x6) or so in the above grade wood frame walls, adding insulation is
> actually very cheap and simple.
>
> See for example drawing attached (PS. I would be tempted to either use a
> fully-adhered air-water barrier or seal the joints in the OSB to make a
> rigid air barrier, or use ZIP)
>
>
>
>
> On 2013-08-21, at 6:38 AM, Alan Abrams <alan at abramsdesignbuild.com>
>  wrote:
>
> re the GB advisor article:
>
> some people recoil at the notion of large amounts of sub slab insulation,
> but this is about heat loss via conduction, rather than convection
> loss through walls and roofs.  Heat loss to soil is different than heat
> loss to air.
>
> Conversely (assuming you are looking at net heat loss through the building
> envelope), increasing, say wall insulation, to compensate for heat
> loss under a slab, requires more and more complex assemblies.  It also
> requires either expanding the footprint--adding roof area, increasing
> tax assessments, etc--or reducing interior area.  So there can be some
> advantages to dumping some more insulation where it does not have a
> big impact on other building assemblies.
>
> the downside is that there seems to be no reasonable substitute for foam.
>  My present thinking (not yet implemented) is to put the slab on the ground
> where it belongs, and insulate on top of it, and cover with a floating
> floor system.  let point loads penetrate as they must.
>
>
> John F Straube
> jfstraube at uwaterloo.ca
> www.JohnStraube.com
>
>
>
>
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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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