[Greenbuilding] passive haus article

[John Salmen]

I used to use 6" foam (but as part of a foam on grade layout with no slab).
It seemed to be that part of the curve where added resistance flattened out
with additional thickness (actually about 4" with minimal benefit to 6" then
negligible).  Foam slabs at 6" are nice to handle (lay flat and effectively
bridge an uneven gravel or sand base). Also seemed less potential for
vibration over time causing shifts in the sub base.  Since then have cut
back to 4" with better specs for how the subbase is prepared. 4" can still
span about 16" (if needed). 

 

It is funny how materials get used excessively based on what we think they
are doing as opposed to how they work - most footings can easily be 4" x 14"
- most concrete walls could be reduced significantly with proper
reinforcement design. I would happily eliminate concrete in a foundation
entirely and float a building on foam - seismically much more responsive.
Unfortunately eng working in geofoam applications are not too interested in
simple residential.

 

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of John F Straube
Sent: August-21-13 5:51 AM
To: Green Building
Subject: Re: [Greenbuilding] passive haus article

 

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.

 

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