[Greenbuilding] passive haus article

[John F Straube]

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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