[Greenbuilding] Insulating shallow crawl spaces

elitalking elitalking at rockbridge.net
Thu Apr 12 09:04:09 CDT 2012


Thanks John Bone for responding to my query. 

 

I thought of insulation above the floor.  For me, I could live with a lower ceiling height.  However, I am tasked with satisfying my client.  It is only an 8' ceiling to begin with.  Since below the floor would be ventilated with outdoor air, it would have a lower Winter temperature.  Though the first inch would achieve the air tightening benefit, the R value would still be low.  Using Isocyanurate, 2" could give only an R14+-.   I use the above the floor strategy for insulating existing concrete slabs with new wood floor above.  In that case, the ground temp is much higher in the winter than outdoor air.  

 

If this is how it is commonly done in UK, I have these questions about your experience.  

 

In hot weather do you cool your houses with AC?  If so, does the shade and cooler surface of the bottom of the floor result in pulling in the warmer air into the crawl space?  Does this ever result in lowering the air temp to dew point?  You say the ventilation is sufficient to keep wood dry.  I would like to understand why.  My impression of UK is that the climate is fairly maritime (humid).  

 

Eli 



----- Original Message ----- 
  From: john bone 
  To: 'Green Building' 
  Sent: Wednesday, April 11, 2012 11:41 AM
  Subject: Re: [Greenbuilding] Insulating shallow crawl spaces


  Hi,
  The English (UK) solution to this not uncommon situation would be to insulate above the floor, leaving the timber to age in a ventilated space, which will protect it from damp build up.

  John Bone, MBEng, BSc Hons
  UK Building Code Official (1974-2009)
  UK Fire Risk Assessor (2009-ongoing)

   


------------------------------------------------------------------------------

  From: greenbuilding-bounces at lists.bioenergylists.org [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of elitalking
  Sent: 11 April 2012 16:14
  To: Green Building
  Subject: [Greenbuilding] Insulating shallow crawl spaces

   

  I have a deep energy retrofit project on an old framed house where there was no excavation for the crawls space.  The result is that the front, downhill side of the house has adequate room for installing insulation.  However, much of the house has less (18"-24").  The floor condition is good enough that we are not considering removing the floor.  The room height is insufficient to consider adding insulation to the top of the floor.  How do I insulate the floor.  

   

  For the rest of the house, we are planning on installing new ceiling rafters parallel to the roof pitch to define 14" space to fill with open cell foam.  We are installing furring strips installed with 2" gap in front of old wood siding.  We are adding continuous 2.5" closed cell foam to fill the gap.  The old plaster is gutted.  We can fill with adhered cellulose or open cell foam in the 2x4 framed walls.  On another similar job, I came to the inside of the foundation to continue closed cell foam thermal barrier and came across the ground with closed cell foam.  All of that was covered with the high cost spray ignition barrier.  In hind site, with that job, I would have preferred to use rigid board insulation such as XPS.  I would create a ground plane with combination of digging high spots and filling low spots with crush and run (small gravel), lay one layer of extruded board to provide puncture protection, a layer of poly for continuous vapor barrier and another foam layer.  I put osb on top for cheap ignition barrier that protects the foam from traffic damage.  The whole house is provided with HRV for fresh air.  

   

  However, this house does not have adequate space to work in.  

   

  Do I need to hand excavate to a minimum clearance for the work? If we did that, I could install either of the approaches describe above.   I was wondering if I could fill the whole space with open cell foam.  I realize that 1/2 # open cell foam is not a vapor barrier, therefore likely not a good idea.  

   

  I am encouraging my client to use a mini split heat pump to avoid the need for ductwork in the crawl space.  Therefore, I can define the thermal barrier at the floor plane instead of the ground.  Because we would still have moisture issues below, I am leaning towards ground insulation.  However, a closed cell foam application to the bottom of the framing could provide the vapor barrier to protect the wood.  Below that the stone and concrete surfaces are not vulnerable to humidity. 

   

  Right now, I consider the hand excavation to be the most likely scenario. Because the mini split will cool the house, I am against the traditional ventilated crawl spaces with fiberglass batts in floor framing because the cool shadow will pull in hot humid air and cool it to dew point creating humidity problems.    

   

  This is a common situation with old house.  I hope our list can offer some good ideas for cost affective ways of upgrading to a high standard.  

   

  Eli 

  www.conservationarchitect.net



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