[Greenbuilding] Slab on Grade Insulation

[elitalking]

I am moving away from using crawlspaces by avoiding ducted heating systems 
that need them.  I am planning a house using a similar approach the rigid 
insulation of solar home in Massachusetts.



http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/MAZeroEnergy.htm



http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/Montague%20Powerhouse%20As-Built%20Sheet%202.pdf



In a mixed climate of Appalachian Virginia, I am adding an addition to the 
uphill side of a house by filling in up to the existing floor level with 
compacted gravel, rigid insulation and slab.  My client is not as motivated 
by super high performance as MA residents. I am proposing 2x6 framed walls 
with flash coat of closed cell to achieve tightness with balance filled with 
un-faced fiberglass insulation.  The slab edge is critical.  If I turn up 
concrete edge to transfer load from framing, I will need at least 4 inches 
horizontal, leaving only 2 1/2" for insulation. Another possibility is to 
put on a double band joist (although there are not joist) to transfer load 
of walls to footing.  This would give me 3 inches for slab edge insulation. 
The band could be used as the form for slab.  Our climate is not nearly as 
cold as MA.  In that house, they also insulated down to footing on interior 
side of foundation in addition to below 6"xps slab insulation. Is that good 
value, considering that the 6" XPS significantly reduces the heat flow into 
the ground?  I prefer the under slab insulation approach over the foundation 
edge approach because it completes the loop of the thermal envelope.  I am 
proposing 2 layers 2" xps with a poly layer between to insure tightness. 
However, we could also stagger the joints and seal.  I like the completeness 
of the poly without fasteners.  However, how much draft will come through 
compacted gravel?  This results in an R20 which I can sell to my client. 
Conventional fiberglass is R19.  Never mind the infiltration flaws of 
fiberglass.



This document from buildingscience.com describes both slab edge and 
underneath approach.  The MA house referenced uses both.



http://www.buildingscience.com/documents/information-sheets/slab-edge-insulation/files/bscinfo_513_slab_edge_insulation.pdf



I would enjoy reading suggestions from designers who have used this 
approach.



Eli