[Greenbuilding] Passive solar home--concrete overlay rethermalmass

[KTOT (g)]

I missed a few emails from a few days ago. The slab is elevated, not on grade. The engineering analysis and calculations have been done and the subfloor is more than adequate to resist deflection and everything else. And the house has shown zero signs of settling (per an expert engineer who inspected the house, as well as others). So the floor is not deflecting at all.

What aspects of “placement of the slab?” in the comment below by Alan Abrams could cause problems, please?

From: Alan Abrams 
Sent: Tuesday, May 14, 2013 7:59 AM
To: Green Building 
Subject: Re: [Greenbuilding] Passive solar home--concrete overlay rethermalmass

coupla things-- 

1. I assumed the slab was on grade
2. I was not referring to removing insulation, I was talking about removing the slab if it were known that there was not sufficient insulation beneath it.

that said, if the slab is over a frame floor and it is failing, then the first task is to diagnose the cause of failure.  Is the frame adequate to resist deflection?  or was the failure due to the reinforcement, mixture, and/or placement of the slab?  Or both?  

if the floor is deflecting (more than, say, L/480), all bets are off, and whatever is placed over the existing slab would be bound to fail as well.

AA


On Tue, May 14, 2013 at 8:02 AM, KTOT (g) <ktottotc at gmail.com> wrote:

  Thank you for the comments.

  However, I’m not completely sure what you’re saying about insulation. Removing the insulation provides the opportunity to install what? There is a very good layer of expensive blow-in insulation under the slab, between the two floors, and the ceiling downstairs is finished. No way is that insulation coming out without destroying much of my house. 

  The point on more mass furthering the cycle is good. The slab is already thicker than my designer planned for the house so already probably is somewhat retarded.

  From: Alan Abrams 
  Sent: Tuesday, May 14, 2013 5:18 AM
  To: Green Building 
  Subject: Re: [Greenbuilding] Passive solar home--concrete overlay re thermalmass

  one consideration here is that capping an existing slab in a passive solar system that presently works "extremely well"-- that is, adding more mass--is not necessarily more better.  more mass will retard the cycle of heating and cooling, changing the character of thermal comfort.  if the cap is thin enough, maybe it won't be noticed--but this is all quantifiable.

  it all raises many questions--about the climate, about the substrate under the slab, about the details of the envelope of the house--all factors that come into play in this decision.  maybe there is not much insulation under the slab, so that removing it provides the opportunity to install it.  if there is adequate insulation under the slab, then maybe a thin cap is in order.  

  on the subject of thermal mass and heat transfer, I think RT's earlier reply was definitive.  thermal mass is simply a function of density or specific gravity, and an acrylic admixture and/or thin film of bonding compound between original and new slabs would neither change the density of the slab or retard conductive heat exchange between the two in any significant way.

  some time ago on this list serve, back in the pleistocene era, IIRC, there was a discussion on high strength, super thin slabs, akin to ferro-cement boat hulls--which might have application here.  someone with even more time than I have to waste could look it up. 

  AA

  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
  6411 Orchard Avenue Suite 102
  Takoma Park, MD 20912
  office  301-270-NET- ZERO (301-270-6380)  
  fax      301-270-1466      
  cell     202-437-8583
  alan at abramsdesignbuild.com
  www.abramsdesignbuild.com





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