[Greenbuilding] Which insulation? How to install?

Eli Talking elitalking at rockbridge.net
Sun May 27 14:05:21 CDT 2012


I do not have the answer.  I do have a similar situation.  I am designing a
renovation of an old farm house that has a crawl space with a rough ground
surface that tapers from 5' to a few inches. Basically, the grade of 
crawlspace follows the original grade of the hill. I have accepted the need 
to
excavate to at least 18 inches.  However, we are using mini-split heat pumps
to deliver heat directly to rooms, therefore do not need crawl space for air
ducts or mechanical equipment.  I was planning on the addition being built
over grade slab, but one day my contractor client got so enthusiastic with
her excavator that she assumed a crawlspace and excavated accordingly.  It
will be a crawlspace.  We are using closed cell foam for walls and below
roof deck.  We do have the budget to install what will work best.  I want to
insulate above the crawlspace along the floor plane.

We could insulate between joist with closed
cell foam.  However, this leaves the bottom of the joist exposed to
potentially high humidity.  However, with the tight insulation, the bottom 
surface temperature would be closer to the ambient, even in the summer, 
reducing the vulnerability to condensation on surface during active
cooling.

We could install a rigid insulation board thick enough to achieve
vapor tightness along the bottom of the joist.  This would also eliminate
thermal bridges.  Sealing the seams would become the critical execution
issue.  By installing the thermal barrier below the floor joist, the joist 
cavity could be available for wires and plumbing without all the sealing 
issues.  3" iso foam could provide R21+- continuous which is a vapor barrier 
that begins at both the top and bottom surface.  The crawl space is pretty 
open ventilated to the outside.  Warm air entering the shaded area would 
only be cooler because the ground in the crawlspace is not being heated by 
the sun.

Additional lower cost fiberglass batts could be installed to increase R 
value. For durability and to provide fire retardant barrier, I would bottom 
off with 7/16 osb (low cost).

I hope our list experts comment on my hunch that a floor or below thermal 
barrier that was tight on both exterior and interior side such that there is 
no flow through it above an ambient ventilated crawl space would keep the 
surface temperature above dew point to avoid the consequences of 
condensation.

Eli



-----Original Message----- 
From: Corwyn
Sent: Sunday, May 13, 2012 1:38 PM
To: Greenbuilding
Subject: [Greenbuilding] Which insulation? How to install?


Greetings,

What insulation would people recommend for a short crawl space?  This is
in a cold climate 7500 HDD, and the house is sitting on posts about 1'
above the ground (a few inches at some points).  There is a insulated
skirt around the perimeter with 2" of foam board insulation.  There is
incomplete vapor barrier on the ground.  Acknowledging that some digging
may need to be done, but budget concerns make serious work unaffordable.
  The budget is very small.  Is air entrained concrete a workable option?

Any thoughts?

Thank You Kindly,

Corwyn


-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.greenfret.com/
topher at greenfret.com
(207) 882-7652

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